Abstract:
A control system for a toaster oven incorporating a conveyor driven by an inexpensive ac motor, where the control system allows the entire range of toasting demands to be met while also compensating for variations in the line voltage. The heart of the toasting control is the reliance on the total dwell time of the bread products within the toasting zone of the oven, rather than on the speed of the motor. The control system also provides for a convection fan of varying speed without the need to replace the inexpensive muffin fan with more elaborate devices. Among the advantages offered by the dwell time approach is on-the-fly adjustment upward or downward of the degree of toasting, there being no need to await for the toasting zone to heat up or cool down to achieve this.

Description:
[0001]    This application claims benefit under 35 USC §119(e) of the Provisional Application No. 60/380,563 filed May 14, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to toaster ovens and particularly to toaster ovens that transport bread products through a toasting chamber during the cooking process. More particularly, the invention relates to a microprocessor-based method and device for controlling such toaster ovens so as to adjust the degree of toasting, to adapt to different types of bread products, to compensate for line-voltage fluctuations, to enable power-conservation, and to effect other desired oven characteristics.  
           [0004]    2. Description of the Prior Art  
           [0005]    Conveyor toasters are popular convection ovens in which items of food stuff, usually bread-related, are transported on a motor-driven conveyor. Such ovens range from small counter-top bread/bagel/sandwich toasters to the large commercial ovens common at pizzerias. Typically, the conveyor&#39;s food-bearing surface moves along a substantially horizontal plane through a toasting chamber positioned between upper and lower heating elements. The toasting is effected by a combination of radiative and convective heat generated by upper and lower heating elements, either working in concert to provide uniform toasting on both sides of the (generally planar) food item, or alone, for those products that need be toasted on only one side. Because of the range of what people consider to be a proper degree of toasting, and because different types of bread products have different toasting susceptibilities, there must be some means available to the operator of the toaster for adjusting the “toasting energy” to which an item is exposed. The toasting energy (the quantify of heat) received by an item will be jointly dependent on the heat flux it exposed to (where heat flux will be dependent on the air temperature within the oven and the level of radiation from the heater, both determined by the heater temperature) and the length of time for which it is exposed. This being the case, one can vary the degree of toasting by changing the heater temperature, the conveyor speed, the rate at which the oven exchanges air with the outside, or a combination of all three. Only the heater temperature and the conveyor speed will be addressed here, though it is noted that the convection fan with which the typical toaster oven is equipped is far from a passive element in the operation.  
           [0006]    Consider first changing the heater temperature while maintaining the conveyor speed constant. Presuming that the desired mode of operation is such that the throughput of toasted products is maintained at its maximum rate, this approach would require that the highest heater temperature be used for the products, such as bagels, that require the most toasting energy. For items, such as slices of white bread, requiring the lowest toasting energy, the lowest heater temperatures would be used. Typically the heating elements are energized by the mains electric power (the “line voltage”), either via a single-phase 117 Vac or 234 Vac line, by a 208 Vac three-phase-line, or, rarely, by a three-phase 480 Vac line. Since the typical heating element presents just a passive resistance (R) to the applied voltage (V), the current through the elements will be directly proportional to the applied voltage, and heating of the heating elements occurs through simple Joule heating, varying as the square of the heater-element current or, equivalently, as the square of the voltage (V) applied across the heater element. (The Joule power dissipated per cycle will be proportional to the mean square of that applied voltage.) For maximum temperature on the heater elements and hence in the toasting zone, the full line voltage would be placed across the heating element(s) all of the time. Reducing the mean voltage across the elements lowers their temperature. Although the electrical power dissipated by an element is directly proportional to the mean square of the voltage, the temperature response is more complicated, because of the manner in which heat is transferred from the elements into the toasting zone, by radiation, by convection, and by conduction. It is noted that once one is positioned to vary the heater-element temperature, by whatever means, one has the capacity to compensate for one of the banes of commercial toaster ovens: line voltage fluctuation and drift (drift being just a long-period fluctuation, “fluctuation” will generally be taken to refer to both short-term fluctuation and to drift).  
           [0007]    Controlling the mean voltage across a heater element is commonly done by placing a phase controller in series with the line voltage to the heater element to be controlled. The phase controller is tantamount to a fast switch that is “on” for an adjustable fraction of each cycle of the line voltage and “off” for the rest of the time, so as to vary the mean square voltage across the heater element. The usual phase controller incorporates a triac and a circuit that will gate the triac at a determinable point in the line voltage cycle. The triac is the switchable component placed in series with the heater element; specifically, it is switched by voltage input to its gate electrode. In order to avoid a dc component to the voltage placed across the heater element output in this technique, equivalent portions of the negative- and positive-going halves of the cycle are applied to the element. With no gate input, the triac presents a blocking resistance to the ac voltage, but when a small dc voltage is applied to the gate, the triac freely passes the ac line voltage. Because of its high input impedance, the triac gate draws negligible power.  
           [0008]    The triac is essentially a pair of SCRs wired in an antiparallel configuration and their gates tied together. That is, the “forward” direction of current for one of SCRs is in the opposite direction from the forward direction for the other. When a gate voltage is applied to switch them “on,” the line current will pass through one of them for the first half of the ac cycle and through the other for the other half. The nature of the SCR is that once turned “on” (so as to pass forward current) by a voltage pulse to the gate it will remain “on” as long as forward current is flowing. With a 60 Hz ac voltage, the forward voltage will fall to zero every {fraction (1/120)}th of a second. The zero-crossing point of the forward current will be delayed by an interval determined by the reactive component of the load. For a purely resistive load, such as is represented by a heater element, the line current phase is the same as the live voltage phase. Because of this, the phase controller switches the triac “on” during the second half of either the positive- or negative-going cycle, with a short voltage pulse to the gate. When the voltage falls through zero (the zero-crossing point), the triac switches off and current ceases. Then the gate is pulsed “on” again at the analogous position in the other half of the 60-Hz cycle. Adjusting the mean voltage in this manner involves setting the “delay angle” on the phase controller. As the delay angle is varied from zero to 180, the mean voltage applied to the heater element goes from full to zero.  
           [0009]    This simple system of maintaining and/or adjusting the heater-element temperature provides a simple way of compensating for the inconstancy of the line voltage, the magnitude of which can typically varying over time by up to ±10% without being considered out of spec or as violating any performance standards. One means of compensating for this drift in phase-controller-based temperature regulation is to install within the oven an electronic temperature sensor coupled into an error-signal generator. The error signal is then fed into the phase controller so as to periodically adjust the delay angle so as to maintain a set temperature regardless of the variation in line voltage and other environmental conditions. The phase controller can thus be made into a very sensitive thermostat for the oven, without the need to resort to anything other than well-known circuit elements and sensors.  
           [0010]    More typically, because of the lower cost, a relatively insensitive bimetallic-based temperature sensor is used to control an on-off switch in the line leading to the heater element, with the result that the voltage across the heater element cycles between full on and full off. That, combined with the bimetallic sensor&#39;s requirement of a relatively large temperature deviation from the set point for it to respond, leads to a relatively large oscillation in the oven temperature about that set point, much greater than the triac-based methods and other electronic approaches allow. Under the right circumstances, the performance advantage of the electronic approach and in particular the approach using the phase controller in combination with the electronic temperature sensor more than offsets the cost advantage of the bimetallic-switch control. For obvious reasons, the methods that rely on a feedback signal being generated by direct temperature measurement are referred to as “closed-loop” systems.  
           [0011]    Regardless of the approach used, there are some serious disadvantages to controlling the toasting energy delivered to an item solely by varying the oven temperature. Most seriously it reduces the overall rate at which a distributed range of products can be toasted, since it requires the conveyor speed to be set so as to ensure that those items requiring the most toasting energy (e.g., bagels) are properly toasted with full power applied to the heater elements. This means that for the other food items, which will constitute the majority of the food types toasted in establishments not specializing in bagels, the heater power will have to be cut back so that they do not get burned during the long transit time through the toasting chamber. Obviously, the optimum production rate across all items will be attained by always maintaining the heating elements at their highest temperature and varying the time that the items spend in the toasting chamber (though this will not in general result in the lowest per-item cooking cost). Traditionally, this time control has been accomplished by varying the conveyor speed, the highest speed being used for the products that toast most readily and the lowest speeds for items like bagels.  
           [0012]    Cost of manufacture is one of the paramount considerations going into the design of toaster ovens, especially those that will be used in large numbers in commercial establishments. This consideration underlies the decisions made about many of the oven components. For example, a typical toaster oven conveyor is driven by an inexpensive universal motor to which it is coupled through a gearbox and chain linkage. This setup permits the conveyor speed to be varied mechanically by changing the gear ratio, even as the simple motor continues to operate at a fixed speed. Of course, it is much more convenient to provide for electric or electronic control of the conveyor speed, an approach that is also more readily and flexibly automated than is the mechanical approach. The simplest method of controlling the conveyor speed electrically is to vary the voltage to the motor and hence the motor speed; this is the approach in many of the existing systems. Unfortunately, the inexpensive motors traditionally used with conveyor-based toasters do not function well at speeds that differ significantly from their synchronous speed. (For a line frequency of 60 Hz, the synchronous speed for a two-pole universal motor is 3600 RPM.)  
           [0013]    For those systems that vary the ac motor speed in order to vary conveyor speed, the motor-voltage control is typically exercised through a variable resistance in series with the motor or by a phase controller (as discussed above) in series with the motor. Both the series resistance and the series phase controller approaches are relatively simple and low in cost, though the series resistance results in wasted Joule heating when the motor is being operated at any but the highest speeds. Both approaches falter when very low motor speeds are attempted, for the reason set out above. At motor speeds lower than the synchronous speed, the torque produced by the motor falls off, and at speeds significantly lower than the synchronous speed, the torque falls to the point where variable frictional forces in the motor&#39;s mechanical load become significant, and erratic operation of the motor (and the conveyor) can result. For example, the motor may stop completely even when a non-zero speed is desired and the corresponding non-zero voltage applied to the motor. Furthermore, as discussed elsewhere, the line voltage available in most facilities can fluctuate over a considerable range and still be considered “normal,” meaning that even if the motor is running at a speed high enough to ensure continuous operation, a drop in the line voltage may cause it to stall—and the toast to burn.  
           [0014]    An alternative to the above approach is to use a dc motor in place of the universal ac motor. This gets away from the constraints on low-speed operation, though at a higher cost. Rosenbrock et al. (U.S. Pat. No. 5,197,375; issued Mar. 30, 1993, and U.S. Pat. No. 5,253,564; issued Oct. 19, 1993) discloses an advanced system incorporating a dc motor to drive the conveyor. Incorporating a microprocessor and various sensor/feedback loops, the Rosenbrock et al. system reportedly maintains the conveyor speed and the oven temperature at operator-selected levels, even as the line voltage varies irregularly. Conveyor speed is maintained in the system taught by Rosenbrock et al. through a control loop incorporating a motor-speed-monitoring sensor (optical-based or otherwise). This sensor generates an error signal whenever the conveyor speed begins to deviate from the speed selected, an error signal that causes an increase or decrease in voltage applied to the motor so as to counter an unwanted decrease or increase, respectively, in the conveyor speed. In this manner, all external influences, including line voltage fluctuation, tending to vary conveyor speed are compensated for. The voltage to the dc motor in the Rosenbrock et al. system comes from a power supply energized ultimately by the ac line voltage. The conveyor speed is controlled by toggling this power-supply-generated dc voltage to the motor on and off, so as to produce a train of similar voltage pulses at the motor input. Each pulse has a height corresponding to the full dc voltage and a width that is adjustable; the average voltage input to the motor is then varied by varying the pulse width in this Pulse Width Modulation (PWM) speed control. In Rosenbrock et al. the interval is never so long that the motor stops. For that matter, the circuitry in Rosenbrock et al. appears to be such that, because of induction and other mechanisms, the motor speed for a given PWM is essentially constant, not responding to the discreteness of the individual pulses. This method of maintaining conveyor speed through PWM and a speed-sensing feedback loop has the potential to provide very close control of the oven operation. However, it is very expensive compared to the traditional means of varying conveyor speed in toaster ovens. The cost is increased in part because of the additional feedback circuitry, including the sensor network, and the fact that dc motors of the type incorporated in the Rosenbrock et al. system are significantly more expensive than the universal motors traditionally-used in the industry. In general, the closed-loop toaster-oven systems of the prior art are capable of providing good temperature and conveyor speed control, but at a high monetary cost compared to the prior-art open-loop systems.  
           [0015]    With the line voltage available at the toaster oven allowed to vary as much as ten percent about its nominal level, nominal 117 Vac single-phase line voltage can be as high as 129 Vac or as low as 105 Vac and still be acceptable under the rules governing the local utility responsible for delivering electricity. Since the heater elements are normally just wires or bars of resistance R, a simple expression gives the rate at which the elements give off (dissipate) energy, namely the Joule heating expression (V 2 /R). This means that the power dumped into the heating chamber increases by 21% when the line voltage increases by 10%. Although, as mentioned above, the temperature does not follow the power-dissipation level directly, a 21% increase in power dissipated increases the temperature in the oven significantly. The power radiated by the heaters varies with the fourth power of the heater temperature. Thus the temperature of the heating element has to increase only by 5% to increase radiative power by 22%.  
           [0016]    Line voltage variations may be short lasting, but they may also endure for hours, reflecting demand elsewhere in the power network supplying the toaster-oven site. Thus, a large demand for air-conditioning may result in the line voltage at the toaster oven being reduced by as much as 10% for the entire afternoon. It is normally at the onset of the change in the line voltage that the most mischief is wrought with toaster ovens. A line-voltage reduction can result in untoasted bread emerging from the oven, and a line-voltage increase can result in carbonized toast and a smoke-filled eating establishment.  
           [0017]    An important, though often slighted, component in the toaster oven is the convection fan used to circulate air within the oven. In a properly designed system, the convection fan increases the efficiency with which energy dissipated at the heater elements is delivered to the products being toasted. Historically, this function has been served by the simple, inexpensive muffin fan.  
           [0018]    Traditionally, toaster-oven muffin fans have alternated between operating at full speed and not operating at all, as the voltage applied to them cycles between the full line voltage and zero voltage. Finer control over the fan speed would offer several advantages. For example, during inactive periods, when the heaters are placed on reduced-power standby mode, it would be useful to also reduce the fan speed to a lower, yet non-zero, level. Also it would be beneficial to maintain fan speed at its set level regardless of voltage fluctuation. Although there are ways that this can be achieved by the use of dc fans, it is again desirable for reasons of economy to achieve this control with the inexpensive ac muffin fans.  
           [0019]    In addition to the quantitative improvements in toaster ovens that are described above, certain qualitative departures from traditional operation are also desirable. All of these improvements can be achieved by the use of microprocessors for toaster oven control. For example, one can set up the control unit at the beginning of each day or at the start of the season, or at the manufacturing plant, with criteria for placing the oven on stand-by operation. This may be as simple is having the shift to stand-by to occur at the same time or times each day, based on historical information regarding slow periods in the business. Alternatively, the shift to stand-by may be triggered whenever no toasting has taken place for a pre-selected time interval. The advantage of this approach is that it is automatic, with no need for a conscious decision on the part of the operator every time the shift to energy-conserving stand-by is made. Similarly, various start-up and shut-down modes may be built into the oven control and, with the flexibility provided by microprocessors, the toaster operator can easily set the various triggering criteria to meet the conditions at the specific establishment where the toaster is located, conditions that may vary throughout the year. As a yet-further improvement, the microprocessor-based control system could enable the operator to introduce a slight increase or slight decrease in the degree of toasting (the darkness) for a few items out of a large number that receive the “default” degree of toasting. The challenge is to introduce the numerous microprocessor-mediated improvements in a manner that avoids significant increases in the cost of manufacturing the toaster ovens.  
           [0020]    Therefore, what is needed is a toaster-oven controller that permits easy operator-mediated control over the level of toasting energy supplied to the items to be toasted, while providing the capacity of maximizing the average throughout of a variety of items. What is further needed is such a controller that permits efficient control of the speed of the convection fans used with toaster ovens. What is yet further needed is a method of using such a controller so as to implement a wider range of start-up, shut-down, and stand-by protocols than have been used previously. What is still further needed is such a controller and method that can provide toasting processes impervious against short- or long-term line voltage variation. Moreover, what is needed is that the controller and method achieving these objectives do not add significantly to the cost of production of toaster ovens.  
         SUMMARY OF THE INVENTION  
         [0021]    An objective of the invention is to provide a control system for conveyor-based toaster ovens that guards such ovens and the products they produce against adverse effects from line-voltage fluctuations. Another objective is to provide such a control system that permits easy adjustment in the degree to which a given item in the oven is toasted, and that permits the accommodation of a large variety of food items with their concomitant range in toasting-energy requirements. It is a further objective that this control system provide the oven operator the ability to make these choices and also to make the choice between maximizing product throughput, on the one hand, and minimizing operating cost, on the other. Yet another objective is to introduce to such ovens&#39; operation improved power-up, shut-down, and shift-to-standby protocols. An overriding objective is that all these objectives be achieved without significantly increasing the complexity nor the cost of the individual components of the control system.  
           [0022]    The present invention meets the stated objectives by introducing particular microprocessor-based oven-control circuitry that does not incorporate the complexity of closed-loop systems. The invention also introduces a new philosophy regarding conveyor speed, one that emphasizes not conveyor speed per se but rather the dwell time within the toasting chamber of the items to be toasted. By such an emphasis, the present invention is able to avoid completely the problem of operating conveyor motors at very low speeds, a problem that the prior art has addressed by going to more expensive motors and/or to complex, expensive circuitry. Once it is realized—that for a toasting chamber presenting heating characteristics that are either uniform or slowly varying as a function of location, it can be seen that the exact nature of the transit of the bread items through the chamber is not important. In particular, since the degree of toasting is basically a function of the total toasting energy that the item receives, an item can move through in a stop/start fashion with no detriment to the final product, providing that the item&#39;s total dwell time is commensurate with its toasting-energy needs and the distribution of convective and radiative heating within the chamber. With this approach, there is clearly no upper limit to the dwell time in the cooking region, that is no lower limit to the average speed with which the conveyor moves. In an extreme example, the conveyor can mimic the toasting procedure of the simple home toaster, by moving the food item into the toasting chamber, halting the conveyor motor for the duration of time required for the item to be toasted, and then moving the item on out the exit. However, as a practical matter, the demands on the typical commercial conveyor toaster are such that a quasi-continuous transit of the food through the chamber is required. The word “microprocessor” is used throughout this discussion as a concise reference to any digital processing and control device or collection of devices, including without limitation those devices sometimes referred to as “microprocessors” or “microcontrollers.” Further, the term should be taken to encompass as well the support circuitry necessary for carrying out various peripheral functions including, but not necessarily limited to, analog-to-digital conversion, digital-to-analog conversion, timing, memory, digital input and output, watchdog, and reset or initialization. In summary, the use of the label “microprocessor” should not be taken as limiting in any way the range of embodiments of the invention described and claimed herein.  
           [0023]    By permitting stop and start motion of the conveyor, one can retain the economic advantage provided by the inexpensive universal motors without having to be concerned with the erratic behavior of such motors under low-speed operation. In the present invention the motor receives either full line voltage or no voltage at all. Apart from the transient periods while the motor is getting up to speed and coasting to a stop, respectively, the motor (and conveyor) either operates at maximum speed or is at rest. The system controlling the motor must provide a duty cycle to the motor reflecting the dwell time required by whatever food item is then being toasted.  
           [0024]    The on/off sequencing that the invention uses is based on the stable 60 Hz line frequency provided in the U.S. and most countries. Corresponding to this frequency is a period of one-sixtieth of a second (approximately 0.016 sec). In its Preferred Embodiment, the invention ensures the proper dwell time by alternating between applying the full ac voltage (or substantially the full voltage) for an integral number of such periods and completely removing the voltage from the motor for an integral number of periods. The motor sees bursts of ac line voltage. The shorter the bursts are (i.e., the fewer cycles for which the voltage is applied) the shorter distance the conveyor advances during each “on” interval. Similarly, as the time between bursts is made longer the conveyor is stopped for a longer time between bursts. The toasting operation depends on the conveyor&#39;s average speed while the item-to-be-toasted is within the toasting chamber, a joint function of the duration of the “on” intervals and the length of time between the “on” intervals. At one extreme, the controller applies the full ac line voltage to the motor without any interruption as the item travels through the oven. This defines the maximum conveyor speed for that line voltage, and the minimum dwell time (toasting time). In contrast, the controller may apply the voltage for a very small number of complete periods (say  5 ) and then wait for a large number of cycles (say  120 ) before applying the voltage again. In this case, the conveyor will move forward a small distance, say one-quarter inch, then pause for two seconds before moving another quarter-inch. It is easily understood that with this control scheme there is no lower limit to the average conveyor speed nor, consequently, no upper limit to the dwell time.  
           [0025]    There are various ways in which the dwell-time approach of this invention can be implemented. This is particularly true for embodiments that devote one or more microprocessors to translating the independent parameter—total dwell time—to the on/off sequencing of the conveyor. Furthermore, with a microprocessor-based control unit, it is straightforward to increase or decrease the dwell time from its default value so as to comply with the customer&#39;s taste and also to adjust the dwell time so as to compensate for environmental factors, in particular line-voltage fluctuations. The latter function will require in one way or another a comparison of the line voltage magnitude with some fixed voltage reference. The invention depends on automatic electronic shifting of the on/off sequence to compensate immediately for a detected deviation of the line voltage. This might be a deviation up or down (usually down) from the nominal value or a shift from an erstwhile voltage level back to the nominal value for the line voltage. Alternatively, the measured change in line voltage can be compensated for by adjusting current through the heating elements so as to ensure that their temperature does not change, thus eliminating the need to adjust the dwell time in response to a change in the line voltage. Finally, some combination of dwell-time adjustment and heater current adjustment may be made to compensate for the change in line voltage. The present, microprocessor-based, invention provides the flexibility needed to select whichever combination is most beneficial, whether it is to minimize operating costs or to maximize production rates. If the establishment operating the oven is extremely busy, the emphasis will presumably always be maximizing output. Under those circumstances, for example, an increase in line voltage will be seized upon as an opportunity to cut down on dwell time. Of course, when the voltage dips, the only response can be to increase the dwell time, that is to decrease output.  
           [0026]    Once a microprocessor has been introduced into the system, the other objectives of the present invention can be achieved by properly programming the microprocessor so as to provide the protocols for turning on the oven, for shutting down the oven, and for putting the oven on a pre-programmed stand-by mode. These protocols relate primarily to powering the heating elements, the fan motor, and the conveyor motor. For the purposes of controlling the fan, one can exercise the same type of start-and-stop operation as used with the conveyor motor, thus permitting the oven to use the traditional inexpensive muffin fan. In the alternative, if there is not a need to operate the fan at very low speeds, the traditional voltage-lowering can be used to vary fan speed. The most important aspect of the invention is the line-voltage-fluctuation compensation achieved by adjusting conveyor speed, heat settings, and/or fan speed. In practice the compensation is based on providing the microprocessor information gained by monitoring the line voltage or surrogate for the line voltage. Here the surrogate can be derived from the line voltage itself through any combination of discrete or integrated passive and active semiconductor devices (including without limitation, resistors, capacitors, inductors, diodes, transistors, and semiconductor controlled rectifiers). The surrogate itself can take the form of a voltage, current, frequency, phase angle, pulse width, pulse position, temperature, resistance, reactance, and so on, indeed any physical parameter detectable by the microprocessor. The choice made in the Preferred Embodiment is to generate for the surrogate a voltage derived directly from the line voltage by a step-down, isolating transformer. That approach also entails isolating the entire control circuit from the line voltage, the front end by a power transformer and the output end by optical-isolation devices. Alternatively, one could establish insulating barriers between the control circuit and the user, in which case there would be no need for isolation in the line-voltage surrogate generation path.  
           [0027]    The commentary of the previous paragraph is provided so as to emphasize the many embodiments that a person skilled in the field and art can devise once the details of the present invention are known and the fact that the invention claimed is far broader than any particular detailed embodiment described in this document.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a block diagram of the invention&#39;s Preferred Embodiment, exclusive of the conveyor and the circuit that switches the conveyor motor on and off.  
         [0029]    [0029]FIG. 2 is a diagram of the circuit used to turn the conveyor motor on and off in response to the on/off signals generated by the control unit logic in the Preferred Embodiment, as well as a block diagram of the conveyor motor and equipment linking the motor to the conveyor.  
         [0030]    [0030]FIG. 3 depicts waveform representations illustrating various aspects and consequences of turning the conveyor motor voltage on and off.  
         [0031]    [0031]FIG. 4 is a schematic depiction of the Preferred Embodiment keypad by which the operator selects the toaster operations desired and causes the system to deviate from any default values built into it. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    The following discussion is best followed with reference to FIG. 1, which is a block diagram of a number of the invention&#39;s components that are configured by long-known techniques and circuits. The invention is designed to use standard ac line voltage such as is available in most homes and business establishments. The Preferred Embodiment is configured in particular to be energized by single-phase ac power such as is available in the United States and Canada (referred to alternately as ac line voltage, ac line power, or simply line voltage). In the Preferred Embodiment, the line voltage is introduced over a first ac line L 1  and a second ac line L 2  to several input points in the system. These input points are isolated from one another, and the line voltage is isolated from ground. In particular, and most importantly, the line voltage is isolated from the low-voltage dc voltages produced within the system as control signals.  
         [0033]    As can be seen from FIG. 1, the line voltage is introduced to a converter  300 . The converter  300  is a standard ac-to-dc converter that includes a step-down transformer so that it converts the nominal line voltage of 117 Vac to an unregulated dc voltage with a nominal voltage of 8 Vdc which is output on unregulated-dc line  2 . In addition to stepping down the ac voltage, the step-down transformer portion of the converter  300  isolates the line voltage from the stepped-down ac and hence from the unregulated dc voltage appearing on unregulated-dc line  2 . Also produced by the converter  300  is a zero-crossing pulse train  7  consisting of a 120-Hz train of positive-going pulses each pulse synchronized to the instant that the ac line voltage passes through zero (120 times a second for the 60-Hz signal) in such a way that half of each pulse precedes the zero-crossing and half follows it. The individual pulses have a fixed width of approximately 1 ms, a height on the order of one volt, and are output over a zero-crossing line  3 . (See FIG. 1 and FIG. 3.) It is the zero-crossing pulse train  7  that provides the synchronizing and counting means for the control system. Both the rising and the falling pulse edges are used, the rising edge serving to “wake up” the controller and the falling edge to cause the motor&#39;s turn-on signal to be issued. This permits a well-defined turn-on time, and hence minimizes jitter in the triggering point location from one turn-on phase to the next.  
         [0034]    The unregulated-dc line  2  is connected to a digital control unit  10  and to a dc regulator  400 . The dc regulator  400  converts the unregulated voltage from the unregulated-dc line  2  to a regulated (constant) 5 Vdc output on regulated-dc line  1  that is coupled directly to the control unit  10 , to which it provides operating power. The digital control unit  10  is in major part a microprocessor. Note that in addition to the regulated-dc line  1  and the unregulated-dc line  2 , the control unit  10  has as an input the zero-crossing line  3 ; the zero-crossing pulse train  7  plays a clock and synchronizer role for the control unit  10 .  
         [0035]    As further depicted schematically in FIG. 1, a keypad  12  is coupled to the control unit  10 . The keypad  12  is the means by which the oven operator interacts with the control unit  10  either to enter specific operational commands or to vary certain pre-programmed tasks. As will be discussed further below, there are buttons corresponding to the most common food types expected to be placed in the oven for toasting. In addition, there is a button to depress to slightly increase the dwell time, whatever the pre-programmed protocol calls for, and another to decrease the dwell time slightly. The keypad  12  and its configuration with the control unit  10  also provides the operator more advanced control options such as the capacity to change default settings for the power-up and shut-down procedures, respectively. Further, in the Preferred Embodiment, the keypad  12  allows the operator to select 30 min, 60 min, or 90 min as the time interval that must pass with no operator input before the oven enters stand-by. For monitoring the operation and changes in the oven, the system is equipped with a visual display  14  coupled to the control unit  10 . Visual information presented by the display  14  includes such things as (a) the time remaining (during power-up or start-up from standby) until the oven reaches operating temperature, (b) the time remaining before the oven goes into standby mode absent an input, (c) time remaining in standby mode before complete shut-down occurs absent input, and other time information useful to the operator in planning his/her production.  
         [0036]    Additional control signals generated by the control unit  10  include a top-heater-control signal, which is output on a top-heater-control line  9 , a bottom-heater-control-signal which is output on bottom-heater-control line  11 , a fan-control signal, which is output on a fan-control-signal line  13 , and a conveyor-control signal, which is output on a conveyor-motor control signal line  15 . All of these control signals are binary in nature, with a HI-to-LO difference being on the order of a few volts. These control signals all look into high-impedance inputs, details of which are set out below.  
         [0037]    As can be seen with further reference to the block diagram of FIG. 1, the Preferred Embodiment includes a top-heater control circuit  18 , which is basically a switch interposed between the first ac line L 1  and a first end of a top-heater element  16 , a second end of the top-heater element  16  being connected directly to the second ac line L 2 . In the Preferred Embodiment, the top-heater control circuit  18  incorporates a phase controller (not shown) such was described earlier. The control unit  10  determines the delay angle of the phase controller and hence the fraction of each cycle for which full power is to be applied to the top-heater element  16 . The top-heater element  16  being a simple resistance, with no reactive component, the power that top-heater element  16  dissipates is directly proportional to the mean square voltage applied to it, the mean square voltage value being determined by the delay angle. The full ac line voltage is turned on by a brief logic HI signal, but turns off by itself when next the current through the phase controller passes through zero. Because the top-heater element  16  constitutes a non-reactive load for the top-heater control circuit  18 , the current through it will pass through zero at essentially the same instant that the voltage applied to the it passes through zero.  
         [0038]    A similar arrangement determines the average power dissipated in a bottom-heater element  20 , which is also a simple resistive element. The bottom-heater-control line  11  provides control input to a bottom-heater control circuit  22  which is interposed between first ac line L 1  and a bottom heater element  20 . The current through the bottom heater element  20  is controlled in the same manner as described above for the top-heater element  16 .  
         [0039]    The fan-control line  13  is connected to a fan control circuit  26  which in turn is coupled to a muffin fan  24 . The speed of the muffin fan  24  is controlled by the fraction of the line voltage cycle that is applied to it. This fraction in turn is controlled in the same manner as described above in the description of the control of the current through the top-heater element  16  and the bottom-heater element  20 .  
         [0040]    The monitoring of the unregulated-dc signal by the control unit  10  is the key to the steps taken by the control unit  10  in compensating for variations in the line voltage amplitude. In other words, the signal on unregulated-dc line  2  is a fluctuation surrogate for the ac line voltage. The unregulated-dc signal will have an amplitude (magnitude) that is directly proportional to the ac line voltage amplitude. For example, a variation in the ac line voltage amplitude by ±10% about its nominal peak-to-peak amplitude of 117 volts will result in the unregulated-dc voltage on unregulated-dc line  2  also varying by ±10%, with a resulting range of 7.2 to 8.8 Vdc.  
         [0041]    In order to use the varying amplitude of the voltage on the unregulated-dc line  2  directly to determine line voltage drift, it is necessary to recognize and take account of variations in the unregulated-dc voltage that arise from sources unrelated to the ac line voltage variation. The most significant such source in the Preferred Embodiment is the change in the unregulated-dc voltage that occurs because of changing current demands put on it by the regulator  400 . The regulator  400  has as its sole function the maintenance of a constant 5 Vdc output on regulated-dc line  1  in the face of the current demands put on the regulated-dc line  1  by the load it powers. As the regulator  400  meets this function, its demand for current from the unregulated-dc voltage line  2  varies causing the voltage on the unregulated-dc voltage line  2  to vary also, as a function of the output impedance of the converter  300 . The visual display  14  is the major cause of the variation in current demand placed on the regulated-dc line  1 , primarily because of the varying information the visual display  14  is called on to present. (All the other outputs of the control unit  10  go to high impedance connections.) The Preferred Embodiment deals with this effect by ensuring that the measurement of the variation of voltage on the unregulated-dc line  2  is always done with the same load on regulated-dc line  1 , by returning the visual display  14  to a specific reference mode for the fraction of a second that the variation is measured. That is, the interval for which the circuit must be held at the reference mode is very short, only long enough for the voltage on unregulated-dc line  2  to arrive to a level reflective of the ac line voltage, a small fraction of a second, and hence not enough to interfere with the operator&#39;s visual observation of the visual display  14 .  
         [0042]    As stated above, the “clock” for the control unit  10  is provided by the signal on the zero-crossing line  3 , the zero-crossing pulse train  7  on that line providing 120 Hz “ticks” of the clock, with the individual pulses synchronized to the zero-crossing times of the ac voltage input to the power supply. Everything that is done by the system is done for an even number of such ticks.  
         [0043]    The key control signal from the control unit  10  is a motor-control signal  70  output on motor-control-signal line  15 . The motor-control signal  70  reflect all the information that the control unit  10  has been given or has calculated regarding the demand for the quantity of energy that the bread items are to be exposed to. This motor-control-signal line  15  can be seen in FIG. 1, where it is shown as an output from the control unit  10 , and also in FIG. 2, where it is shown as the input to a motor control circuit  100 .  
         [0044]    As depicted schematically in FIG. 2, a conveyor  28  is driven by a conveyor motor  30  coupled to the conveyor  28  through a gearbox  32  and a chain drive  34 . The motor  30  is powered by the ac line voltage, the first ac line L 1  being connected directly to the motor  30  and the second ac line L 2  being connected to the motor  30  through the motor control circuit  100 .  
         [0045]    The control unit  10  monitors the zero-crossing pulse train  7  with pulses synchronized to the zero crossings is shown in FIG. 1. A zero-crossing pulse will appear every 8.33 ms for a 60 Hz line voltage. The zero-crossing pulses identify the cycles of the ac line so that the control unit  10  can produce the pulse that switches the motor  30  on or off to within a precision of {fraction (1/120)} of a second. That is, the control unit  10  counts pulses on the zero-crossing pulse train  7  and, when the total equals a number predetermined based the desired dwell time, it outputs an appropriate signal on the motor-control-signal line  15  to the motor control circuit  100  so as to interrupt the ac line voltage to the motor  30 . Then the pulse count by the control unit  10  begins again and when the total reaches a predetermined number, the control unit  10 , again acting on the control circuit  100 , allows the full ac line voltage to be applied once again to the motor  30 . This pattern continues to repeat until the operator changes the dwell time through inputting new information to the control unit  10  through the keypad  12 . The details about the way in which the motor control circuit  100  operates are given in the next paragraphs.  
         [0046]    As with the other control signals in the Preferred Embodiment, the motor-control signal  70  is binary in nature. When the motor-control signal  70  is HI, it causes the control circuit  100  to interrupt completely the ac line voltage to the motor  30 . This is the low-voltage regime. When it is LO, it causes the control circuit  100  to permit the full ac line voltage, the high-voltage regime. FIG. 3 illustrates this sequence. The top line in FIG. 3 represents the zero-crossing pulse train  7 ; the second line is the motor-control signal  70  output by the control unit  10  on motor-control line  15 ; the line below that depicts a motor input voltage  80 ; and the bottom line roughly depicts a motor speed  90 . The four lines are synchronized and indicate the following. The motor-control signal  70  is initially HI, resulting in the motor input voltage  80  being zero, holding the motor speed  90  to zero. Coincident with the second pulse in the zero-crossing pulse train  7 , the control unit  10  causes the motor-control signal  70  to switch from HI to LO at a turn-on point  71 . As a consequence the control circuit  100  causes the full ac line voltage to be applied to the motor  30 , as depicted by the motor input voltage  80  trace between a voltage start  81  and a voltage stop  82 , all as set out in FIG. 3. With continuing reference to FIG. 3, it can be seen that coinciding with the voltage start  81 , the motor speed  90  becomes non-zero and, after going through a speed-up phase  91 , the motor speed  90  reaches full speed  92 . Similarly, when, after four zero-crossing pulses, the motor-control signal  70  switches back from LO to HI at a turn-off point  72 , all input voltage is removed from the motor at a voltage stop point  82 , and the motor  30  coasts to a stop during a coast-down phase  93 .  
         [0047]    [0047]FIG. 2 depicts the control circuit  100 , for the purpose of illustrating the means by which commands from the control unit  10  cause the motor input voltage  80  to the motor  30  to be switched between zero and the full ac line voltage, that is, for the line voltage to the motor  30  to be switched on and off. The motor-control signal  70  from the control unit  10 , carried on conveyor-motor-control signal line  15 , effects these changes in a series of steps designed to isolate the ac line voltage from the control unit  10  and its associated circuits. The conveyor-motor-control signal line  15  is connected directly to the negative side of a light-emitting diode D 1 , as can be seen in FIG. 2. The positive side of the diode D 1  is biased to +5 Vdc by regulated-dc voltage line  1  (connection not shown) through a first current-limiting resistor R 1 , 330 ohms in the Preferred Embodiment. When the motor-control signal  70  is switched from HI to LO, current flows through diode D 1 , causing it to emit light which, in turn, activates (turns on) a light-activated switch Q 2  (a triac) so that light-activated switch Q 2  becomes freely conducting in both directions. Once light-activated switch Q 2  is fully conducting, the full ac line voltage appears at node  150 , causing a coupling triac Q 1  to turn on, so as to complete the circuit between first ac line L 1  and second ac line L 2  through the motor  30 . The full ac line voltage continues to be applied to the motor  30  as long as diode D 1  is emitting light, that is as long as conveyor-motor-control signal line  15  is held at LO.  
         [0048]    The control circuit  100  also includes a snubber circuit consisting of snubber capacitor C 1 , 0.05 pF in this embodiment, and snubber resistor R 3 , 100 ohms in this embodiment, shunting the coupling triac Q 1 . The function of this snubber circuit is to reduce or eliminate oscillations in the control circuit that otherwise would tend to occur at the turn-on and turn-off times.  
         [0049]    In the Preferred Embodiment, the light-emitting diode D 1  and the light-activated switch Q 2  are included in a standard off-the-shelve device of the type used to provide optical coupling (and electrical isolation) between two electrical circuits. The part number of this unit is TLP 160J, manufactured by TOSHIBA.  
         [0050]    The control unit  10  is configured by well-known techniques to incorporate the algorithms needed to effect the various toasting protocols called for by the particular toaster specifications. In the Preferred Embodiment, it is configured to allow the operator to vary the duty cycle from essentially 100% (conveyor running continuously at full speed), down to 5% (resulting in products spending 20 times as long in the cooking zone as they do at 100% duty cycle). Also in the Preferred Embodiment, the operator is able to set up a shift-to-stand-by protocol whereby if the toaster has not had any inputs from the operator for some multiple of 30 minutes, it goes into standby mode, causing the heaters to be cut back to half power and causing the fan speed to be cut back as well. If a further predetermined period (typically 30 minutes) passes after it enters standby mode, the toaster is shut off completely.  
         [0051]    [0051]FIG. 4 shows the keypad  12  in the Preferred Embodiment. It includes an “on” button  201  and an “off” button  202 . Also on the keypad  12  is a status display  203 , which indicates various key time intervals, such as time-to-standby, time-to-shutdown, and time-to-full-turn-on (during power-up), and also provides the operator information about the current status of the toaster, such as the type of bread item it is set to toast. Below the status display  203  is a step-down button  204  for making the toast one-half step darker, on the fly. Depressing it causes a slight reduction in the duty cycle of the motor. Similarly, a step-up button  205  enables the operator to make the toast in process one-step lighter. In the Preferred Embodiment, the step-down button  204  is labeled “DARKER” and the step-up button  205  “LIGHTER. The oven is pre-programmed for several common types of bread products, permitting the operator to simply press the button corresponding to each of those types in order to obtain the-proper toasting parameters. Thus, there is a toast button  206 , a bagel button  207  (which will cause a long dwell time while limiting heat to just the top-heater), and a muffin button  208  (calling for a heater/dwell time combination appropriate for most English muffins). Also there is a particular-pre-programmed-protocol button  209  for calling up a particular protocol that has been programmed by the operator or, during original set-up, by the manufacturer. Closely related to this function is the control provided by a protocol-choice button  214 , which enables the operator to make a menu-based choice from a number of pre-programmed protocols, the difference being that the protocols accessible through the protocol-choice button  214  are more difficult to modify than is that accessible through the pre-programmed-protocol button  209 .  
         [0052]    The keypad  12  also allows the operator to easily introduce variants to the pre-programmed protocols. Perhaps the most used of these variants will be the LIGHTER and DARKER commands. For example, with the bagel button  207  depressed, and the legend BAGEL displayed on the status display  203 , the operator may push the step-down button  204  (DARKER) once. This has two effects. One is to extend the toasting time by approximately 10% and to cause the display on the status display  203  to begin to alternate between BAGEL and DARK. After a time interval sufficient for the operator to load the untoasted bagel and for the item to pass through the toasting chamber, the control reverts automatically to the default dwell time for bagels. If the operator pushes the step-down button  204  twice, the dwell time is extended for approximately 20% and while the piece is toasting the display on the status display  203  alternates between BAGEL and XDARK—until the appropriate time interval has elapsed, and the controls go back to the bagel default dwell time and the display goes by to a continuous BAGEL. A similar variant is available for making the item one or two stages lighter than the default dwell time for the species of bread item would result in. Although in the Preferred Embodiment, each step lighter or darker results in a change of about 10% in the dwell time, this increment can be modified by the supervisor to be any desired step. As a safeguard, the apparatus can only be re-programmed through password-protected access, presumably limited to managers and the like.  
         [0053]    Also, through a top-heat button  210  and a bottom-heat button  212 , the operator can choose to have one or the other of the heaters (or both or neither!) operating. Through a power-saver button  211 , the operator can determine whether the standby-mode option is activated. A manual dwell-time-adjust button  213  permits a manual adjustment of the dwell time over a continuous range (in contrast with the single discrete change in dwell time available through either the step-up button  205  or the step-down button  206 ), by adjusting the motor duty cycle over a continuous range.  
         [0054]    The details of one particular embodiment, the Preferred Embodiment, have been set out above. In so doing, there is no intention of limiting the invention claimed to this Preferred Embodiment. The full scope of the invention is defined in the Summary; those skilled in the art can readily develop alternatives to the Preferred Embodiment while staying within the invention&#39;s scope.