Abstract:
An HVAC (heating, ventilating, and air conditioning) control and method for interpreting a broad range of input voltages generates a series of voltage pulses whose quantity increases with the amplitude of the input voltages. Upon counting the pulses or accumulating them across a capacitor, the control applies an algorithm to determine whether an input voltage should be interpreted as a logic-1 or a logic-0. The control can accept AC input voltages having nominal amplitudes of either 110 or 220-volts. In some embodiments, software-based hysteresis helps filter out electrical noise and distinguish input signals that are marginally between a logic-1 and a logic-0.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to controls that may need to receive a broad range of input voltages (e.g., 110 to 220 VAC), as is often the case with systems pertaining to HVAC (heating, ventilating, and air conditioning), and the invention more specifically pertains to converting the broad range of input voltages to control signals of much lower voltage (e.g., 0 to 5 volts). 
     2. Description of Related Art 
     Electronic controls of commercial and industrial HVAC systems often need to receive and interpret input voltage signals that can range from a nominal 110 to 220 V AC . Typically, 110 volts is used in the United States, while 220 volts is more common in other countries. However, many controls need to handle both 110 and 220 volts. 
     The relatively high input voltages typically come from sensing the condition of various HVAC devices such as motor starters, contactors, relays, limit switches, flow switches for evaporator and condenser pumps, pressure switches for condenser shells, motor winding thermostats, electric heaters, etc. The input voltage signals provide the control with feedback on the operating condition or status of the HVAC system. In response to the feedback, the control may simply monitor the HVAC system or provide various output signals that adjust or vary the system&#39;s operation. 
     Such controls often include a microprocessor for analyzing the input and providing logical output responses to the HVAC system. Since many microprocessors operate on a binary system using voltage signals of no more than about five volts, the 110/220-volt inputs need to be reduced before they reach the microprocessor. 
     After reducing the input voltages to about 5 volts, the lower voltages are preferably electrically isolated from the higher voltages. The electrical isolation helps protect the microprocessor and its associated low-voltage components from being damaged by the higher voltages. Also, when troubleshooting a low-voltage portion of the control, electrical isolation helps protect a service technician from being accidentally shocked by higher voltages. Lastly, for UL approval, Underwriters Laboratories, Inc. requires electrical isolation between electrical lines of significantly different voltages. 
     Today, step-down transformers are often used for electrical isolation. However, transformers have several disadvantages. If a transformer reduces an input signal from 220 to five volts, that same transformer could reduce a 110-volt signal to an unacceptably low 2.5 volts. Thus, separate transformers are usually needed to handle both 110 and 220-volt inputs. A transformer&#39;s bulk also makes them generally incompatible with compact circuit boards using surface-mount technology. Moreover, micro-transformers have rather delicate wire for its windings, which tends to reduce the reliability and durability of such transformers. 
     Optical isolators are also often used for electrical isolation. An optical isolator typically turns on when an input voltage reaches a certain threshold, and otherwise turns off (with some hysteresis between its on and off states). The threshold is generally a fixed value that is dependent on other electrical components associated with the optical isolator. If the electrical characteristics of the optical isolator or its other related components vary due to their manufacturing tolerances, the threshold may vary accordingly. This can become a critical problem when an input voltage is at or very near the threshold. 
     For example, if an input voltage is just barely below the threshold, the input may be interpreted as a logic-0, i.e., turned off, when actually the input might be just a weak signal that should be interpreted as a logic-1. 
     Moreover, electrical isolation circuits employing optical isolators are typically designed to handle a generally narrow range of input voltages. Otherwise, such circuits may generate a significant amount of heat when receiving higher voltage signals. 
     Voltage spikes, electrical noise, and other electrical transients may falsely trip an optical isolator. Although high-frequency filters and other circuitry can be used to block most false signals, it can be difficult to provide a circuit that can anticipate and reject every imaginable form of electrical noise. 
     SUMMARY OF THE INVENTION 
     In order to receive and interpret a broad range of input voltages, a control translates the input voltage to a pulsating voltage whose number of pulses varies with the voltage amplitude of the input. The control includes an analog, digital, and/or software component that interprets the pulsating voltage to determine the value of the input voltage. The input&#39;s value may be the actual amplitude of the voltage or may simply be a binary value, such as a logic-0 or logic-1, which respectively represents the absence or presence of the input voltage. 
     In some embodiments, it is an object of the invention to determine the value of the input voltage by counting the pulses of the pulsating voltage. 
     In some embodiments, it is an object of the invention to determine the value of the input voltage by accumulating the pulsating voltage across a capacitor and then measuring the voltage across the capacitor. 
     In some embodiments, it is an object of the invention to determine the value of the input voltage by applying software logic in interpreting a count or an analog accumulation of the pulsating voltage. 
     In some embodiments, the software provides certain time-delays and/or hysteresis that filter out electrical noise or erroneous electrical spikes, thus avoiding misinterpreting an input. 
     Another object of the invention is to provide software-based hysteresis from logic-1 to logic-0 values and vice versa. 
     In some embodiments, it is an object of the invention to electrically isolate a lower voltage portion of the control from the higher voltage input, without having to rely on an isolation transformer. 
     Another object of the invention is to employ an optical isolator that electrically isolates one pulsating signal from another pulsating signal. 
     Another object is to take multiple count readings of pulses that indicate a voltage amplitude to avoid false readings based on a single count. 
     A further object of the invention is to provide a control that can receive and interpret both 110 and 220-volt inputs. 
     A still further object of the invention is to provide a high-resolution method of sensing a voltage by converting the voltage to a series of pulses whose number of pulses increases with the amplitude of the voltage, whereby increasing the number of pulses for a given voltage increases the resolution accordingly. 
     Another object is to provide a method of reliably interpreting an input using electrical components of standard tolerance. 
     Another object is to take full advantage of surface-mount technology by not using a transformer. 
     The present invention provides a control adapted to monitor an operating status of a system in response to receiving an input voltage having an input voltage amplitude and a nominal frequency. The control includes an input terminal adapted to receive the input voltage, and a first pulse circuit coupled to the input terminal and adapted to generate a first pulsating voltage having a first frequency that varies as a function of the input voltage amplitude the first frequency is at least as great as the nominal frequency when the input voltage amplitude is above an upper limit. The control also includes a second pulse circuit adapted to generate a plurality of pulses in response to the first pulsating voltage, an electrical isolator that helps isolate the plurality of pulses from the input voltage; and a logic circuit coupled to the second pulse circuit. The logic circuit selectively creates a first binary value in response to the plurality of pulses indicating the input voltage amplitude is above the upper limit and creates an opposite binary value in response to the plurality of pulses indicating the input voltage amplitude is below a lower limit. The first binary value and the opposite binary value at least partially provide an indication of the operating status of the system. 
     The present invention also provides a method of measuring an input voltage amplitude of an input voltage having a nominal frequency. The method comprises the steps of: sensing the input voltage; generating a pulsating voltage having a generated frequency that varies as a function of the input voltage amplitude; generating a plurality of pulses that vary as a function of the generated frequency; and counting the plurality of pulses to determine the input voltage amplitude. 
     The present invention additionally provides a method of interpreting an input voltage having a input voltage amplitude and a nominal frequency. The method includes: sensing the input voltage; generating a pulsating voltage having a generated frequency that varies as a function of the input voltage amplitude; generating a plurality of pulses that varies as a function of the generated frequency; and creating a first digital value based on the plurality of pulses, whereby the first digital value indicates that the input voltage amplitude has reached a certain value. 
     The present invention further provides a control suitable for an HVAC system that conditions the air of a comfort zone. The control is adapted to monitor an operating status of the HVAC system in response to receiving an input voltage having an input voltage amplitude and a nominal frequency. The control comprises an input terminal adapted to receive the input voltage, and a first pulse circuit coupled to the input terminal and adapted to generate a first pulsating voltage having a first frequency that varies as a function of the input voltage amplitude with the first frequency being at least as great as the nominal frequency when the input voltage amplitude is above an upper limit. The control also comprises a second pulse circuit adapted to generate a plurality of pulses in response to the first pulsating voltage, an electrical isolator that helps isolate the plurality of pulses from said input voltage; and a logic circuit coupled to the second pulse circuit. The logic circuit selectively creates a first binary value in response to the plurality of pulses indicating the input voltage amplitude is above the upper limit and creates an opposite binary value in response to the plurality of pulses indicating the input voltage amplitude is below a lower limit. The first binary value and the opposite binary value at least partially provide an indication of the operating status of the HVAC system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a system incorporating the subject invention. 
     FIG. 2 is an electrical schematic of one embodiment of an input circuit. 
     FIG. 3 is a graph showing voltage (ordinate) plotted versus time (abscissa) for an input voltage and smaller voltage pulses generated from the input voltage. 
     FIG. 4 is the same as FIG. 3, but plotted over a longer period. 
     FIG. 5 is similar to FIG. 3, but showing how a lower input voltage generates fewer pulses. 
     FIG. 6 is similar to FIG. 3, but showing how a higher input voltage generates more pulses. 
     FIG. 7 is a control algorithm according to one embodiment of the invention. 
     FIG. 8 is an electrical schematic of another embodiment of an input circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An HVAC system  10 , of FIG. 1, includes some of the basic components of typical HVAC systems. However, system  10  is not meant to represent any particular system, but is rather meant to illustrate some common system components and their functional relationships to each other. 
     System  10  includes a refrigerant compressor  12 , a condenser  14 , an expansion valve  16 , and an evaporator  18 , all of which are interconnected in a closed loop to comprise a conventional refrigerant circuit  20 . In this generic example, evaporator  18  cools water that is circulated through a chilled water circuit  22 . A pump  24  pumps the chilled water through one or more heat exchangers  26  that cools an area  28  within a building  30 . Alternatively, to heat area  28 , refrigerant flow through refrigerant circuit  20  can be reversed, and/or an electric heater  32  can be used. 
     To control or monitor the operation of system  10 , a control  34  receives input or feedback from several sources. Control  34  is schematically illustrated to represent the myriad of controls that are suitable for controlling or monitoring a system in response to feedback. Control  34  can be based on digital circuitry, analog circuitry, software logic, and various combinations of the three. Examples of control  34  include, but are not limited to, computers, microcomputers, microprocessors, PLC&#39;s (programmable logic controller), voltage meters, IC&#39;s (integrated circuits), and other electrical circuits comprising discrete electrical components (analog and/or digital). Also, the system associated with control  34  does not necessarily have to be HVAC related, but rather can be almost any system or process that can be controlled or monitored in response to feedback. 
     The feedback sources or devices may include, but are not limited to, a pressure switch  36  that senses the discharge pressure of compressor  12 , a temperature sensor  38  that senses the temperature of refrigerant being discharged from compressor  12 , a flow sensor  40  that senses water flowing through circuit  22 , a limit switch  42  that senses the position of an actuator acting upon expansion valve  16 , a room thermostat  44 , a motor starter  46  having main electrical contacts for starting and stopping pump  24  and having auxiliary contacts for feedback, and an electrical terminal  48  of heater  32 . It should be appreciated by those skilled in the art that the devices just mentioned are for illustrative purposes only, and a wide variety of other feedback sources or devices are well within the scope of the invention. 
     Pressure switch  36  includes a set of normally open contacts  50  that close upon the refrigerant discharge pressure exceeding a certain limit. An electrical power source  52  delivers, for example, 110 V AC  at a nominal 60 Hz frequency to one contact  50 , while the other contact  50  connects to an input terminal  54  of control  34 . Sufficient discharge pressure of compressor  12  closes contacts  50 , which thus applies a 110 V AC  feedback signal  56  to input terminal  54 . Similarly, power source  52 , supplies voltage to the other feedback devices; however, the electrical lines to do so have been omitted for clarity. Nonetheless, feedback devices  36 ,  38 ,  40 ,  42  and  46  use the 110 V AC  that they receive to provide control  34  with feedback or input voltage on lines  56 ,  58 ,  60 ,  62  and  63  respectively. A terminal  64  on control  34  serves as a common or shared neutral node of power supply  52 . Feedback from a thermostat is typically 24 V AC  or less, but for illustration purposes thermostat  44 , in this example, conveys a feedback signal  66  of 110 V AC  to control  34 . A 220 V AC  feedback signal  68  from terminal  48  is created upon heater  32  being energized by a 220 V AC  power source  70 . 
     In response to inputs  56 ,  58 ,  60 ,  62 ,  63 ,  66  and  68 , control  34  generates outputs  72 ,  74  and  76  using analog, digital and/or software control logic that follows well-known or otherwise preferred control schemes. In the example of FIG. 1, output  72  drives an actuator that determines the opening of valve  16 , output  76  energizes a motor starter for turning compressor  12  on, and output  74  turns heater  32  on and off by way of a contactor  78 . 
     In order for control  34  to readily apply low voltage control logic to create outputs in response to relatively high voltage inputs, each input  56 ,  58 ,  60 ,  62 ,  63 ,  66  and  68  is first delivered to an input circuit  80  of control  34 , as shown in FIG.  2 . Thus, in this example, control  34  includes seven input circuits  80  to receive the seven inputs of system  10 . Circuit  80  will be explained with reference to input  56  from pressure switch  36 ; however, the same general idea applies to the other inputs as well. 
     Circuit  80  includes a 0.022 uF capacitor  82  and two 140 k-ohm resistors  84  and  86  that provide a 60-hertz pulsating voltage of 55 V AC  (i.e., 55 volts rms, 156 volts peak-to-peak) across points  88  and  90  upon receiving 60-hertz 110 V AC  (i.e., 110 volts rms, 311 volts peak-to-peak) across terminals  54  and  64 . 
     The 55 V AC  across points  88  and  90  is fed into a first pulse circuit  92  that includes a 309-ohm resistor  94 ; a diode  96 ; and a diac  98 , such as a D-30 provided by Semiconductors, Inc. of Riviera Beach, Fla. Diac  98  conducts current upon applying sufficient voltage (i.e., trigger voltage) across diac  98  and continues to conduct until the voltage drops to a minimum voltage required to sustain conduction. In some embodiments, the voltage that triggers diac  98  to conduct is about 26 to 38 volts; however, diacs of other trigger voltage levels can also be used. 
     Each positive voltage pulse (i.e., the positive half of the voltage waveform) at point  88  charges capacitor  82 . When the capacitor voltage exceeds the trigger voltage of diac  98 , capacitor  82  begins discharging through resistor  94 , diac  98 , and an LED  100  (light emitting diode) of an optical isolator  102 , such as a 4N35 provided by Siemens of Germany. Diac  98  in its conducting state drains the voltage at a point  99  across LED  100  until the voltage across diac  98  is sufficiently low to cause diac  98  to stop conducting. If capacitor  82  is still sufficiently charged, the voltage across diac  98  again increases to trigger diac  98  another time. Repeatedly triggering diac  98  creates a series of current pulses that pass through LED  100 . This continues until the voltage across capacitor  82  is insufficient to trigger diac  98 . 
     Each pulse of current through LED  100  causes a transistor  104  of optical isolator  102  to conduct a 5 V DC  source  105  to an output  106  of a second pulse circuit  108 . Thus, output  106  carries a series of 5-volt pulses that correspond in frequency and number to the pulsating voltage at point  99 . The actual number of pulses depends on the magnitude of positive charge across capacitor  82 , and thus depends on the peak voltage of each positive half of the waveform of input voltage  56 : the greater the peak voltage, the greater the number of pulses. 
     To ensure clear distinct pulses at output  106 , a 130 k-ohm resistor  110  drains any residual charge that may otherwise remain at output  106  when transistor  104  is not conducting. A 1M-ohm resistor  103  connects the base of transistor  104  to ground. A 100 k-ohm resistor  112  conveys the 5-volt pulses to a logic circuit  114  that counts the pulses to determine whether input voltage  56  should be considered as a logic-0 or logic-1. In this case, logic-0 generally means that input voltage  56  is below a lower threshold (e.g., 40 volts-rms), and logic-1 generally means that input voltage  56  is above an upper threshold (e.g., 70 volts-rms). Within a deadband or hysteresis between 40 and 70 volts, the input voltage&#39;s assigned state, logic-0 or logic-1, remains unchanged. 
     During the negative half of the waveform of input voltage  56 , diode  96  conducts to discharge capacitor  82  across resistor  94 . Also, the rather small voltage across points  99  and  101  (approximately 0.6 volts created by current passing through diode  96 ) is of a polarity that is opposite of that which is needed to operate LED  100 . Thus, pulses at output  106  generally only occur during the positive half of the waveform of input voltage  56 . 
     This can be more clearly understood by referring to FIGS. 3 and 4. When input voltage  56  is above an upper threshold (e.g., 70 volts), each positive half of the waveform generates a certain number of voltage pulses  116  at point  99  (e.g., 110 volts might produce three pulses). The actual number can be much more or less than three; however, the number increases with the amplitude of input voltage  56 . Likewise, if input voltage  56  drops from 110 volts to 70 volts, the number of pulses at point  99 , and thus at  106 , will decrease (e.g., may drop from three to one, as shown in FIG.  5 ). It is possible that no pulses would be generated if input voltage  56  drops below a lower threshold (e.g., 40 volts). When input  56  is between the upper and lower thresholds (e.g., between 40 and 70 volts), some pulses may be generated, but they may occur less frequently than at every positive half of the input waveform. 
     If the input voltage is 220-volts at 60 hertz, as is the case with feedback  68  of heater  32 , each positive half of the waveform generates many ore voltage pulses  116  at point  99  than what is produced by a 110-volt input. Thus, feedback  68  being t 220 volts also produces a corresponding higher number of 5-volt pulses at output  106 , as shown in FIG.  6 . The average generated frequency of the pulses or the average rate at which they occur is several times greater than the nominal 60-hertz frequency of input  68 . And the generated frequency of the pulses or rate at which they occur increases with the amplitude of the input voltage. 
     In some embodiments of the invention, to determine whether an input voltage should be interpreted as a logic-0 or a logic-1, control  34  counts the number of pulses  116  over a given period by counting the number of 5-volt pulses at output  106  and then applying the logic algorithm of FIG.  7 . 
     Block  118  starts the algorithm, control block  120  initializes the state of input  56  to be a logic-0, block  122  sets the count of pulses  116  to be zero, and block  124  resets a counter CTR to zero. Block  126  increments CTR to one, block  128  commands control  34  to count the number of pulses  116  over a predetermined period “T.” The period T can be any reasonable predetermined value, such as 50 ms, 500 ms, etc., as shown in FIG.  4 . Decision block  130  determines whether the number of pulses is at least as many as a predetermined high threshold pulse count (HTPC) or the number that would occur if input voltage  56  were at a minimum level indicative of a logic-1. Thus, if P is equal to or greater than HTPC), block  130  directs control to decision bock  132 . In block  132 , counter CTR is compared to a predetermined number of repetitions (REPS) that is a constant value selected to provide more or less sensitivity. Here, REPS has been assigned a value of three as an example. Counter CTR tallies the number of times that a count has been taken of pulses  116 . Since only one count has been taken so far, block  132  directs control onto block  134 , which resets the previous count of pulses  116  back to zero. Block  126  increments CTR to two, and a second count of pulses  116  is carried out at block  128 . If the second count is also greater than or equal to HTPC, then decision block  130  again directs control to decision block  132 . Since CTR is now two and still less than three, block  132  directs control to block  134 , which again resets P back to zero. Block  126  increments CTR to three and a third count of pulses  116  is performed at block  130 . Since P is still greater than or equal to HTPC and CTR is now equal to three, blocks  130  and  132  pass control onto block  136 , which assigns a binary value of logic-1 to input  56 . From block  136 , control returns to block  122 , and the process repeats until there are significant reductions in the number of pulses  116  counted over REPS periods of T (e.g., three periods of T). 
     If at decision block  130  input voltage  56  is sufficiently low to provide a count P that is less than HTPC, block  130  transfers control to block  138 , which resets CTR to zero. Block  140  then increments CTR to one. If count P is more than a lower threshold pulse count (LTPC), the assigned binary value of input  56  (logic-1 or logic-0) remains unchanged, and decision block  142  returns control to block  122  via block  143 . However, if count P is less than or equal to LTPC, block  142  transfers control to decision block  144 . Since CTR equals one and is thus unequal to three, block  144  transfers control to block  146 , which resets P to zero. Block  148  initiates a second count of P for another period of T, block  140  increments CTR to two, and decision block  142  compares the P count to LTPC. Block  140 ,  142 ,  144 ,  146  and  148  operate in a manner similar to blocks  126 ,  130 ,  132 ,  134  and  128 ; however decision block  130  sets one threshold (e.g., P≧HTPC) for changing from a logic-0 to a logic-1, while decision block  142  sets another limit (e.g., P≦LTPC) for changing from a logic-1 to a logic-0. A logic-0 is set by block  145 . 
     The difference between the HTPC of block  130  and the LTPC of block  142  provides a deadband or software hysteresis between the opposite binary values of logic-0 and logic-1 to prevent erratic oscillation between the two states. 
     Having a broad range of pulses between HTPC and LTPC to represent a relatively narrow input voltage range provides a resolution that may be appropriate for applications other than just distinguishing between a logic-1 and a logic-0. For example, such a resolution could be adequate for a voltage meter. Moreover, the value of the components (capacitor  82 , diac  98 , etc.) of input circuit  80  could readily be selected to provide an even broader pulse range between HTPC and LTPC, thereby making it possible to create a voltage meter with exceptionally high resolution. 
     FIG. 8 shows another embodiment of an input circuit  80 ′, which is similar to circuit  80  of FIG. 2, but of an analog version. Instead of counting output pulses  114 , the output pulses of circuit  80 ′ are accumulated across a capacitor  150 . The amplitude of the voltage across capacitor  150  is then measured at a point  152 . If the voltage at point  152  rises to a predetermined high threshold (e.g., 2.5 volts), a logic-1 is assigned to the input voltage being applied across terminals  54  and  64 . If the voltage at point  152  decreases below a predetermined low threshold (e.g., 0.5 volts), a logic-0 is assigned to the input voltage. A deadband or hysteresis is created by the difference between the high and low thresholds. In some embodiments, the value of the components are as follows: resistor  84 ′ is 130 k-ohms, capacitor  82 ′ is 0.027 uf, resistor  86 ′ is 150 k-ohms, resistor  94 ′ is 301-ohms, resistor  112 ′ is 33-ohms, capacitor  150  is 0.33 uf, and resistor  110 ′ is 200 k-ohms. 
     Although the invention has been described with reference to a currently preferred embodiment, it should be appreciated by those skilled in the art that other variations are well within the scope of the invention. For example, nominal frequencies other than 60-hertz can be used, such as 50-hertz and other frequencies. The terms, “isolating” and “isolator” refer to electrically insulated components that help inhibit electrical current from passing from one point to another and/or from passing from one electrical signal to another. Furthermore, the present invention is described in terms of an HVAC system, but is generally applicable to systems converting input signals to control signals of much lower voltage. Therefore, the scope of the invention is to be determined by reference to the claims, which follow.