Abstract:
A contactless sensor utilizes analog and digital circuitry to provide direct interchangeability with a simple potentiometric sensor, matching all of the electrical properties of a potentiometer, including supply voltage range, power supply current, output voltage range, and having three connection terminals. The contactless sensor operates with voltages from 2 to 30 volts direct current, which includes all of the common industrial sensor power supply voltages: 5V, 10V, 24V, and +/− 15V. The contactless sensor utilizes a total current of less than 0.005 amperes, and its output voltage range includes the power supply rails. These improvements combine to enable the contactless sensor to be a direct replacement when a potentiometric sensor is removed from service.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical potentiometric devices, and to electronic ratiometric devices, more specifically, to electronic devices developed for the purpose of replacing or simulating the electrical characteristics of a potentiometric device. Further, the present invention relates to sensors having a potentiometric electrical connection, or ratiometric output signal. 
     2. Description of the Prior Art 
     A Prior Art potentiometric device, also called a potentiometer, is a three-terminal electrically resistive device. See  FIG. 1 , PRIOR ART. Such a potentiometer typically comprises at least a resistive element  1 , a wiper  2 , that makes electrical contact to the resistive element, and three electrical terminals  6 ,  7 ,  8 , for connection into an electrical circuit. An optional fourth electrical terminal may also be included to allow a ground or case connection (optional fourth terminal not shown in figures). In a rotary potentiometer, such as the one shown in  FIG. 1 , the wiper moves in an arc that pivots from a wiper pivot  3 , near one end. Linear potentiometers are also in common use, with the resistive element being made approximately in a straight line (not shown) rather than in an arc as shown in  FIG. 1 . 
     A power source is connected to the potentiometer such that a power supply voltage appears across first and third terminals  6 ,  8 , of  FIG. 1 , for example, zero and ten volts DC (direct current). There exists a constant electrical resistance between the first and third terminals, for example, five thousand ohms. A current flows through the resistance, which is two mA (milliamperes) in the example (i.e. ten volts divided by five thousand ohms). In the example, the input power would be two mA multiplied by ten volts, or twenty mW (milliwatts). The wiper  2 , of a potentiometer provides an output voltage at second terminal  7 . The wiper makes physical contact with the resistive element  1 , in at least one point. The mechanical configuration of the wiper is such that its point of contact with the resistive element is movable along at least a portion of the length of the resistive element. As the point of contact between the wiper and the resistive element moves along the arc or the length of the resistive element, an output voltage appearing on the second terminal  7 , varies as a percentage of the voltage across the resistive element  1 , and in proportion to the relative position of the wiper  2 , along the resistive element. 
     The resistive element  1 , typically comprises a substrate of ceramic or other mechanically suitable electrically insulative material, having at least one surface that is coated with a thin layer of electrically resistive material. Typical power supply voltages for a potentiometer are 5, 10 or 24 volts DC, but other voltages may be used. It is uncommon for the power supply voltage to be above 30 volts DC. Typical resistances of the resistive element are one or two thousand ohms when used with a five volt power supply, five thousand ohms when used with a ten volt power supply, or ten thousand ohms when used with a twenty four volt power supply. It is not desirable to use a lower resistance element, such as one thousand ohms, with a higher power supply voltage, such as 10 or 24 volts, due to the higher current that would be drawn from the power source, and the resulting increase in power dissipation of the potentiometer. 
     As shown in  FIG. 1 , first terminal  6 , is connected to resistive element  1 , at a location approximately along one end of the resistive element, forming a first resistive element connection  4 . Likewise, a third terminal  8 , is connected to resistive element  1 , at a location approximately along another end of the resistive element, forming a second resistive element connection  5 . It is desirable that wiper  2 , remain in constant contact with resistive element  1 , and to be prevented from riding up onto the areas of connections  4 , and  5 . This will prolong the life of the wiper  2 , and also will help to reduce intermittent loss of contact between the wiper  2 , and the resistive element  1 . Therefore, motion of the wiper is commonly restricted to a range slightly less than that required to obtain output voltages equal to the power supply voltage. For example, with terminals  6 , and  8 , connected to 10 and 0 volts DC (Direct Current), respectively, full wiper motion over the mentioned restricted range will result in output voltages up to approximately 9.900 volts DC, and down to approximately 0.100 volts DC (or, when connected to 5 and 0 volts DC, output voltages can be obtained of up to approximately 4.950 volts DC and down to 0.050 volts DC, respectively). 
     The main advantage of using a potentiometer as a means for providing a variable voltage is its simplicity. The major disadvantage of a potentiometer is that mechanical contact between the wiper and the resistive element constitutes a mechanism for wear. Wear resulting from repeated mechanical motion of the wiper normally limits the lifetime of a potentiometer. End of service life of a potentiometer typically occurs when wearing of the surface of the resistive element causes erratic voltages to appear on its output (represented here as second terminal  7 ), due to several factors, including buildup of particles that have been scraped from the resistive element by the wiper movement, partially bare spots where the coating of the resistive element has been removed from the underlying substrate, as well as changing the contact properties of the surface of the resistive element. 
     It is common in the prior art for other types of electronic devices to be developed in attempts to simulate the simplicity of wiring that is inherent with a potentiometer, but having other undesirable attributes which limit such an electronic device from being directly interchangeable with a potentiometer. Some of the undesirable attributes of such prior art electronic devices include a higher current draw from the power source, a more narrow range of allowable power supply voltage, and a more narrow range of output voltage available. Many such prior art electronic devices require a power supply voltage in the narrow range of 5.0 volts +/−0.5 volts, and provide an output voltage range of 10% to 90% of the power supply voltage for indications of zero and full scale, respectively. Such prior art electronic devices typically draw from 10 to 150 milliamperes of current from the power source. 
     A potentiometer is commonly employed to provide a variable output voltage in response to a physical parameter being measured (that is, in response to a parameter). Such a potentiometer is often configured as a position-measuring sensor, but potentiometric devices can be used to sense other parameters such as pressure, flow, etc. when coupled to a mechanical system that provides a mechanical motion proportional to the parameter. 
     The physical parameter can be mechanically coupled to the potentiometer directly, or transduced from one form of mechanical energy or motion into another as appropriate for the given parameter. For example, a diaphragm or bellows can be used to transduce a pressure measurand into a linear motion. The linear motion can be coupled to a linear potentiometer. Such a potentiometric device or combination of potentiometer and transducer can be called a potentiometric sensor. 
     A potentiometric sensor with a wiper that contacts and rubs along a surface of the resistive element is called a “contact-type” sensor, that is, the wiper makes mechanical contact with the resistive element. Because the typical potentiometer has only three wires, it is relatively simple to connect into an electrical system, and is also easily understood. 
     Various electronic devices, and especially sensors, have been developed which simulate the function of a potentiometric device to some extent. The output of such a device or sensor is typically called ratiometric. In a device having a ratiometric output, an output voltage is developed that is similar to an output voltage developed in a potentiometric sensor, in that the output voltage is a percentage of an applied power supply voltage. Many ratiometric electronic sensors have an advantage over an actual potentiometric contact-type sensor, because they can utilize capacitive, inductive, or magnetic sensing, for example, and thereby make their measurement without physical contact among moving and non-moving members comprising the device or sensor. This type of sensor arrangement is called a non-contact ratiometric sensor. This eliminates mechanical wear, and can provide an increase in the service lifetime of the sensor. 
     By virtue of having a three-wire electrical connection, a non-contact ratiometric sensor as described above can sometimes be used as a replacement for a potentiometric contact-type sensor. A typical sensor of this type uses an electronic circuit that requires an input voltage of 4.5 to 5.5 volts DC at a current level of between 10 mA and 150 mA, and produces an output voltage in the range of 10% to 90% of the power supply voltage in response to a 0% to 100% range of a measurand. For example, with a 0 to 1 inch linear position sensor having a power supply voltage of 5.0 volts DC, an output voltage range would typically be 0.5 to 4.5 volts DC for positions from 0 inches to 1 inch. Although this can be accommodated by some types of receiving electronics with appropriate adjustments, it is not serviceable as a direct replacement of a potentiometric contact-type sensor in many applications. 
     To the contrary, the present invention teaches an apparatus which can directly replace a potentiometric contact-type sensor in virtually all applications, while preserving its desirable performance characteristics and simplicity of wiring. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention teaches a method and apparatus for providing a direct replacement of a potentiometer by an electronic circuit, and in some cases, a sensing element, while retaining the simple connection scheme and electrical performance of a potentiometer. This is accomplished by using a novel mix of digital and analog circuit techniques, providing output voltages very close to the voltages of the power supply, for example, 0 and 10 volts DC, while also accommodating a power supply voltage over a wide range of voltages (including for example, 5, 10, and 24 volts DC), and with a very low power supply current (for example, less than 5.0 milliamperes). The analog circuits provide compatibility with a potentiometer application, while the digital circuits maintain accuracy that could otherwise be lost with a primarily analog circuit. The digital circuit techniques include using standard logic levels while the input signal is in the form of a frequency or a duty cycle, and using the voltage potential across the simulated potentiometer as special logic levels during translation to the analog circuitry. An analog circuit technique converts the duty cycle representation of the signal into an output voltage that is centered on the power supply voltage, and so that the output voltage has a range extending to the power supply rails. In some cases, extended power supply voltages are developed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       For further understanding of the nature and objects of the present invention, reference is made to the following figures in which like parts are given like reference numerals and wherein: 
         FIG. 1  shows the internal components of a Prior Art potentiometer, illustrating the wiper and resistive element. The resistive element is curved, as it would be in a rotary potentiometer 
         FIG. 2  is a pictorial representation of an electronic circuit for implementation of a preferred embodiment of the present invention, in which the generation of extended power supply voltages is included. 
         FIG. 3  is a chart listing various parameters of a circuit according to  FIG. 2 , and is included in the description of the invention as an aid to understanding the function of a preferred embodiment of the invention. 
         FIG. 4  is a pictorial representation of an electronic circuit for implementation of a preferred embodiment of the present invention, in which the generation of extended power supply voltages is not included. 
         FIG. 5  is a pictorial representation of an electronic oscillator circuit, such as can be used to interface with a sensing element in a preferred embodiment of the invention. 
         FIG. 6  is a pictorial representation of conductor patterns and a target of a rotational sensing element, such as can be implemented in a preferred embodiment of the invention, and which shows the target in two positions (views A and C) and also shows the bottom conductor pattern (view B). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Unless otherwise stated, a power source to the various circuits described here will be assumed to be from a power supply having a voltage in the range of 5 to 30 volts DC. Except for actual contact-type potentiometers, Prior Art has not disclosed an electronic potentiometric or ratiometric device having such a wide range of power supply voltage. If using mainly analog techniques, it has been difficult to maintain accuracy over such a range. If using mainly digital techniques, transition from logic levels to a wide voltage range has been difficult. But to the contrary, the present invention uses a novel mix of analog and digital techniques to circumvent such difficulties. 
     In this description of some preferred embodiments of the present invention, the positive terminal of a power source will be called power supply, and the negative terminal of a power source will be called common. This is a configuration that is often used with industrial applications in the field of the invention. 
       FIG. 2  shows a first preferred embodiment of the present invention, in which resistors  27 ,  28  form a voltage divider. They divide a difference in voltages between first terminal  10  and third terminal  12 , thus providing a percentage of that voltage difference to a non-inverting input of amplifier  29 . Connections to third terminal  12  will be referred to as common. Amplifier  29  operates as a unity gain voltage follower, thus presenting the divided voltage, in buffered form, at its output to resistor  31 . The buffered output of amplifier  29  will be called the reference voltage (Vref). Amplifier  32  generates an output that is connected to second terminal  11 . The voltage of the output (Vout) of amplifier  32  is equal to a signal voltage (Vsig) at its non-inverting input, minus the voltage at the output of amplifier  29 , with that difference multiplied by one plus the ratio of resistances of resistor  33  to resistor  31 . The resistances of resistors  33 ,  31  will be called R 33 , R 31 , respectively, thus:
 
 V out =( V ref− V sig)*(1+ R 33/ R 31)  (1)
 
Frequency input  13  is an alternating current (AC) voltage having a frequency indicative of a parameter which is desired to be represented on output voltage terminal  11  as a DC voltage, in the form of a percentage of the voltage difference between first terminal  10  and third terminal  12 . This frequency operates a monostable multivibrator, also called a one-shot. The output of one-shot  15 , is normally at a logic level zero when it receives no input transitions, but changes to a logic level one for a fixed period of time with each low-going transition of the frequency input. There is one low-going transition for each cycle of the frequency input. After that fixed period, the one-shot output returns approximately to the voltage of common, which is logic level zero. Logic level one is a regulated positive voltage, with respect to third terminal  12 . While at logic level one, the one-shot period operates transistor  26 , turning it on, through current limiting resistor  20 , so that the collector of transistor  26  goes approximately to the same voltage as common while the transistor is turned on. First terminal  10 , second terminal  11 , and third terminal  12  of the present invention according to  FIGS. 2 ,  4 ,  5 , are analogous, respectively, to first terminal  6 , second terminal  7 , and third terminal  8  of the Prior Art, according to  FIG. 1 .
 
     Using digital circuitry as described, that is, logic levels rather than analog voltages, allows the signal to be represented accurately, without any degradation as would be evident with analog voltages. 
     When the output of one-shot  15  goes back to logic level zero, transistor  26  turns off, and its collector voltage becomes approximately equal to the voltage of first terminal  10 . 
     So, when considering the waveform of the collector voltage of transistor  26  over several cycles of the input frequency, the collector voltage goes to a positive voltage and to common with a duty cycle proportionate with the input frequency. Low pass filter  30  filters the waveform of transistor  26  collector, thereby presenting a variable DC voltage to the non-inverting input of amplifier  32 . As stated above, the voltage appearing at the non-inverting input of amplifier  32  is called Vsig. 
     Inverter  14  drives a positive charge pump circuit comprising capacitors  16 ,  21 , and diodes  22 ,  23  to provide an extended positive supply voltage, V++, to amplifier  32 , which is more positive than the voltage at first terminal  10 . Inverter  14  also drives a negative charge pump circuit comprising capacitors  17 ,  25 , and diodes  18 ,  19  to provide an extended negative supply voltage, V−−, to amplifier  32 , which is more negative than the voltage at third terminal  12 . Powering amplifier  32  in this way allows the output of amplifier  32  to range up to the voltage of first terminal  10  and down to the voltage of third terminal  12 , even though amplifier  32  may not be able to produce outputs equal to the extents of its power supply voltage. Even so-called rail-to-rail output operational amplifiers are not able to produce outputs equal to their power supply rails, and even less-so when having a load resistance connected. 
     Assuming some typical values, in which resistors  27 , and  28  each have a resistance of 49.9 k ohms (k representing a factor of 1,000), the resistance of resistor  31  being 100 k ohms, the voltage at first terminal  10  equal to 10 volts DC, the voltage at third terminal  12  at zero volts DC, and the one-shot period being listed as P 1 s, the table of  FIG. 3  describes the voltage on second terminal  11 , listed as Vout in the table because it is connected to the output of amplifier  32 , for respective frequencies supplied by frequency input  13 . 
     In  FIG. 3 , Fsens is the sensitivity of a signal being provided by frequency input  13 , representing a parameter. The frequency of frequency input  13  has a maximum frequency of Fmax, and can vary by a factor called sensitivity, which is represented in  FIG. 3  as Fsens. For example, with an Fmax of 100 kHz and an Fsens of ½, then the frequency of frequency input  13  can vary from a maximum of 100 kHz to a minimum of 50 kHz. The table includes calculations for conditions of Fmax being 100 kHz, and Fsens being ½, ⅓ and ¼. These sensitivities are representative of sensing elements that have high sensitivity (½), medium sensitivity (⅓), and low sensitivity (¼). In  FIG. 3 , Fcalc is a percentage of Fmax that will be used for that row of calculations. For this table, Fcalc is shown for three frequencies: the minimum frequency, the frequency in the middle between the minimum and maximum frequencies, and at the maximum frequency Fin is calculated at each value of Fcalc. 
     Period is the reciprocal value of Fin, and is in microseconds. P 1 s is the on-time of one-shot  15  after it is triggered. Power supply voltage, Vps, is a voltage applied across first terminal  10  and third terminal  12 . Vsig is derived as:
 
 V sig= V ps* P 1 s/ Period  (2)
 
Gain is derived as:
 
 G =(2/ F sens)−1  (3)
 
The value of resistor  33 , which is R 33  in the table, is derived as:
 
 R 33=( G− 1)* R 31  (4)
 
and R 31  had a value of 100 k ohms for generation of the table.
 
     Vref is one half of the power supply voltage. Vout is derived according to formula (1), with R 31  being 100 k. 
       FIG. 4  shows a preferred embodiment of the invention which may be suitable for applications in which it is not required that voltage of second terminal  11  be able to go as far positive as first terminal  10 , or as far negative as third terminal  12 . In such a case, amplifier  32  can be of a type with rail-to-rail output, thus enabling the voltage of second terminal  11  to come relatively close to the voltages of first terminal  10  and third terminal  12 . The circuit operates in the same way as the circuit of  FIG. 2 , with the exception that the circuit of  FIG. 4  does not include the positive or negative charge pump circuits. 
     The frequency input  13 , shown in  FIGS. 2 and 4  represents a parameter that is desired to be indicated by the voltage of second terminal  11 .  FIG. 5  shows a circuit configuration that can be used to provide such a frequency input. In  FIG. 5 , terminals  10 ,  11 ,  12  are connected to like numbered terminals in either  FIG. 2  or  FIG. 4 . 
     A first sensing terminal  45 , and a second sensing terminal  46 , are to be connected to a resonant circuit, such that the resonant frequency is representative of a parameter. If the parameter is that of a rotational angle or arc, then sensing apparatus such as shown pictorially in  FIG. 6  can be used. Otherwise, a linear sensing element or other resonant circuit can be applied. 
     Voltage regulator  40 , in  FIG. 5 , connects across first terminal  10  and third terminal  12  to receive power. Voltage regulator  40  provides a regulated voltage for inverter  42 . The regulated voltage, such as +3.3 volts DC, then determines the voltage of logic level one. The voltage of logic level zero can be approximately equal to the voltage of third terminal  12 . Resistor  43 , and capacitors  41  and  44 , ensure that inverter  42  will oscillate according to the resonant frequency of the resonant circuit that is connected across first sensing terminal  45  and second sensing terminal  46 . 
       FIG. 6  shows the basic parts of a rotational sensing element. Substrate  50 , is made of an electrically insulative material, and carries top conductor pattern  50  on one plane, and may carry bottom conductor pattern,  55 , on another plane. In  FIG. 6 , bottom conductor pattern  55  is shown separately in view B as it would appear if substrate  50  was transparent, and without top conductor pattern  53 . This enables one to observe the direction of winding of bottom conductor pattern  55 , and compare it to that of top conductor pattern  53 . Top conductor pattern  53  would typically be disposed directly above bottom conductor pattern  55 . 
     First sensing terminal  45  in  FIG. 6  matches up to the same numbered item as shown in  FIG. 5 . Likewise for second sensing terminal  46 . Starting at first sensing terminal  45  as shown in view A of  FIG. 6 , it can be seen that conductor pattern  53  winds around in a clockwise fashion until arriving at its proximate center at feedthrough  54 . Looking next at view B, feedthrough  54  connects to bottom conductor pattern  55  and continues in clockwise fashion until coming to second sensing terminal  46 . 
     Target  51  is made of an electrically conductive material, and is shown in view A such that it does not cover any part of top or bottom conductor patterns  53 ,  55 . In this position, the resonant circuit formed by a sensing element according to  FIG. 6  will have its lowest resonant frequency. Target  51  is made rotatable around target pivot  52 . As target  51  rotates around target pivot  52 , there will come a position in which target  51  starts to cover over a portion of top conductor pattern  53 , and this likewise aligns above bottom conductor pattern  55 . As target  51  rotates to align more and more directly above top conductor pattern  53 , the resonant frequency of the sensing element will increase. View C shows target  51  partially positioned above top conductor pattern  53 . The maximum resonant frequency of the sensing element shown in  FIG. 6  is reached when target  51  is fully aligned directly above top conductor pattern  53 . Thus, the resonant frequency of the sensing element of  FIG. 6  is indicative of the rotational position of target  51 . A second target, similar to target  51 , may also be disposed below bottom conductor pattern  55 . 
     In like manner, a linear position sensor can be fashioned to use in place of the rotational sensing element of  FIG. 6 . Mechanical transduction elements can be added to form sensors of various types, such as making a pressure sensor by adding a diaphragm to a linear sensing element, or making an inclinometer by adding a seismic mass to a rotational sensing element, or making a humidity sensor by using a humidity sensitive capacitive sensing element for the resonant circuit, or making a flowmeter by non-uniformly winding a resonant coil circuit around a rotameter with an electrically conductive or ferromagnetic (depending on the oscillation frequency range) float, etc. 
     The present invention may also be useful in any application where it is desired to represent a variable frequency input (to insert as frequency input  13 ), as a potentiometric output voltage. This may include many types of applications where a sensing element is not used, and in which it is not desired to sense any physical parameter, other than a parameter represented by the frequency input.