Abstract:
A closed-loop, temperature-controlled cooling system includes a Stirling cycle refrigerator driven by a dc motor, wherein the motor excitation derives from pulse width modulation. Temperature control of the cooling system is provided by a temperature-sensitive bridge circuit which responds to variations in temperature by generating an error voltage which effects a change in the modulator duty cycle. The amount of cooling is directly related to the average voltage applied to the motor, which is a function of the modulator duty cycle and the amplitude of the supply voltage. A compensation circuit is disclosed which responds to variations in the voltage amplitude of an unregulated supply by coupling an error signal to the bridge circuit to thereby affect the modulator duty cycle.

Description:
The present invention relates generally to control systems and, more particularly, to a control system providing a pulse width modulated drive signal, wherein a compensation circuit adjusts the pulse widths in response to variations in the power supply voltage. 
     BACKGROUND OF THE INVENTION 
     The purpose of cooling electronic equipment is to keep the temperature of the electronic components at some desired temperature. In some cases, in order to achieve the desired electronic performance, the electronic component must be cooled to a temperature below the temperature of its surroundings. In these cases, refrigeration equipment is used to pump the heat from the component into the surroundings. A typical electronic device requiring refrigeration is an infrared detector. 
     The Stirling cycle is a power-producing thermodynamic cycle using four basic thermodynamic processes. A Stirling cycle engine is basically a closed cycle system using regenerators to aid in the transfer of heat into and out of a working fluid. Although the Stirling cycle engine did not compete successfully with the later-developed steam and internal combustion engines, the Stirling cycle is still widely used in refrigeration. 
     When the motive power for a Stirling cycle refrigerator is a dc electric motor, it may be approximated that the amount of heat removed by the refrigerator is proportional to the voltage applied across the stator windings of the motor. Thus, in a closed-loop, temperature-controlled cooling system, the amplitude of the motor drive signal determines the amount of heat removed by the refrigeration apparatus and, therefore, the temperature of the cooled device. 
     In some cooling systems of this type, the motor is driven from a linear amplifier. Amplifiers are easily and efficiently compensated for supply voltage variations; however, in this type of system, a linear amplifier is highly dissipative, thereby contributing to inefficiency in the cooling system. Another form of motor drive uses pulse width modulation, which, for a sufficiently high pulse frequency, is very efficient in this type of cooling system. 
     In a typical pulse width modulation scheme, the duty cycle is controlled by an error signal generated from a temperature-sensitive transducer. However, because a dc motor is responsive to the average applied voltage for its rotational speed, it is easily seen that the pulse width modulation method depends upon a constant supply voltage. An increase in supply voltage will increase the average motor voltage, increasing the motor velocity, causing the temperature of the cooled device to drop. Although the inherent nature of the closed-loop system will eventually bring about self-correction, such temperature swings resulting from power supply variations are highly undesirable. 
     The effects of power supply variations can be reduced either by regulating the power supply or by compensating for the effects. The cost and complexity of regulating a power supply, particularly where it is employed as a power source for a large number of systems having current drains which vary over time, makes this an impractical option. It is therefore more practical to include in the cooling system an apparatus which compensates for the effect of power supply variations. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention, an apparatus is disclosed for use in a control system. The control system includes a sensing bridge circuit for producing an error voltage proportional to deviations from a predetermined sensed parameter. The control system also includes a driver circuit coupled between a power supply and a load circuit. The driver circuit provides pulses of the voltage of the power supply to the load circuit. The driver circuit is responsive to the bridge circuit error voltage for determining the width of the pulses. The load circuit is responsive to an average voltage level applied thereto, the average voltage level being the product of the power supply voltage and the pulse duty cycle. The disclosed apparatus compensates for variations in the average voltage level due to variations in the power supply voltage. The apparatus comprises means coupled to the sensing bridge circuit and to the power supply for providing a signal proportional to the power supply voltage, the signal affecting the balance of the sensing bridge circuit such that a change in the power supply voltage effects a change in the width of the driver circuit pulses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representation of a closed loop Stirling cycle cooling system including the present invention; and 
     FIG. 2 is a pulse width modulated waveform useful in illustrating the operation of the system of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a temperature-controlled cooling system according to the present invention. Device 10, which may typically comprise a charge-coupled device (CCD) infrared (IR) detector, is to be maintained at a constant temperature well below the ambient temperature, typically at 73° K. Device 10 is mounted for cooling directly on cold head 12 of Stirling cycle refrigerator 14, which is driven through its cooling cycles by dc motor 16 via connecting shaft 18. 
     Device 10 is heated via its I 2  R drop, by the heat of its ambient surroundings (shown as wavy arrow 20) and, in the case of an IR detector, by the radiant energy of the scene being detected. In the present example, the power dissipation across device 20 is typically 180 milliwatts, the heat absorbed from its surroundings is typically 300 milliwatts, and the scene radiance is relatively negligible. The heat of device 19 (shown as wavy arrow 22) is drawn into cold head 12 which, in turn, transfers its heat (shown as wavy arrows 24) into refrigerator 14 during its cooling cycles. 
     The temperature of cold head 12, and therefore the temperature of device 10, is measured using sensing bridge circuit 26 which comprises temperature sensitive resistor 28, mounted directly on cold head 12 adjacent device 10, and resistors 30, 32 and 34. The series combination of resistors 28 and 30 comprises the variable resistance leg of bridge circuit 26, and the series combination of resistors 32 and 39 comprises the fixed resistance leg. 
     Resistor 28 exhibits values of resistance which vary with temperature. In the present example, resistor 28 is a platinum resistance sensor which may be a type 118MF-2000 sensor sold by Rosemount, Inc., Eden Prarie, Minn. At 73° K, the resistance of resistor 28 is 340 ohms and, in the temperature range of interest, the resistance of resistor 28 varies by approximately eight ohms per °K. Fixed-value resistors 30, 32 and 34 are precision resistors and may be a type similar to Model S102C resistors, sold by Vishay Intertechnology, Inc., Malvern, Pa. In the present example, resistors 30, 32 and 34 have resistance values of 340 ohms, and are extremely stable over wide temperature ranges, illustratively having a spread of ±1.5 parts per million per °C. over the range of interest. 
     Current is provided through the two legs of bridge circuit 26 from a regulated power supply (not shown), which is coupled between input terminal 36 and ground. The current which passes through bridge circuit 26 also passes through resistor 38, typically having resistance of 82.5 ohms. Resistor 38 is used to set the desired power dissipation through sensor resistor 28 which, for a regulated power supply voltage of 5.0 volts, is 12 milliwatts. 
     As is well known in the art, the balance of bridge circuit 26 is determined by detecting the differential voltage between the node joining resistors 28 and 30 and the node joining resistors 32 and 34. The differential voltage is applied at the two input terminals or error amplifier 40, which may typically be an operational amplifier. 
     The amplified error voltage which is the output signal from amplifier 40 is coupled to the input terminal of pulse width modulator 42. Modulator 42 generates a signal comprising a sequence of pulses of fixed amplitude and frequency, but which vary in width according to the amplitude of the error signal provided by amplifier 40. In the present example, modulator 42 may be similar to type SG1524B Regulating Pulse Width Modulator, sold by Silicon General, Inc., Garden Grove, Calif. The SG1524B modulator includes an error amplifier of the type shown as amplifier 40 in this description. In the preferred embodiment, modulator 42 generates pulses having amplitude switching between 0 and 5.0 volts, at a frequency of 82 KHz. 
     The pulsed output signal from pulse width modulator 42 is coupled to the input terminal of driver 44. Driver 44 is additionally coupled to a source (not shown) of high-current potential which is applied at input terminal 46. In the present example, the potential at terminal 46 is unregulated; it assumes a nominal value of 28 volts, although it may range between 24 and 32 volts. The switching element of driver 44 may typically comprise a power MOSFET transistor, such as type IRF130, sold by International Rectifier Corporation, Los Angeles, Calif. 
     FIG. 2 illustrates the waveform of the output signal from driver 44 which is coupled to the input terminal of dc motor 16, thereby controlling the amount of heat removed by refrigerator 14. The waveform of FIG. 2 shows the ON and OFF pulses of the driver 44 output signal. It may be seen from earlier paragraphs that modulator 42 responds to the error signal from amplifier 40 for establishing the time duration t of the ON pulse relative to the period T of the pulse cycle, the quantity (t/T) being referred to as the modulator duty cycle. It is also easily seen that the average voltage V AVE  =V SUP  (t/T), where V SUP  is the amplitude of the unregulated supply voltage coupled at terminal 46. Thus, it is clear that variation V SUP  will cause in changes in V AVE , ultimately resulting in changes in cooling, which changes are unrelated to the temperature sensed by temperature sensing resistor 28. 
     In accordance with the present invention, a compensation circuit 54, comprising resistors 48 and 52 and capacitor 50, is coupled between supply voltage terminal 46 and bridge circuit 26 for the purpose of correcting the bridge error voltage for the effects of variation in the unregulated supply voltage. Resistors 48 and 52 are coupled in series between terminal 46 and the juncture of resistors 32 and 34 in the fixed reistance leg of bridge circuit 26. Capacitor 50 is coupled between the juncture of resistors 48 and 52 and ground, and, together with resistors 48 and 52, comprises a low pass filter, such that high frequency pulses of the voltage at terminal 46 do not effect changes in the error voltage generated by bridge circuit 26. 
     While the purpose of the low pass filter network is to prevent instability, as mentioned above, the main function of the series combination of resistors 48 and 52 is to unbalance bridge circuit 26 is response to variation in the unregulated supply voltage V SUP  in such a manner that the resulting pulse width change at the output of driver 44 produces essentially no change in the average voltage V AVE  coupled to motor 16. That is, compensation circuit 54 responds to a change in V SUP  by causing a pseudo-temperature change in the balance of bridge circuit 26 which induces modulator 42 to adjust the width t of the ON pulse, thereby maintaining a constant value of V AVE  and keeping V AVE  independent of V SUP . In particular, if the amplitude of V SUP  increases, bridge circuit 26 will unbalance in a direction so as to decrease the duty cycle output of pulse width modulator 42. Similarly, if the amplitude of V SUP  decreases, bridge circuit 26 will unbalance in a direction so as to increase the duty cycle output of modulator 42. 
     In the present example, using the voltages and component values indicated earlier, resistors 48 and 52 may each have resistance values of 121 kilohms and capacitor 50 may have capacitance value of 0.1 microfarad. One effect of the inclusion of compensation circuit 54 to bridge circuit 26 is the need to slightly adjust the resistance value of resistor 34 in order to provide a zero voltage error output from bridge circuit 26 at the desired temperature for the nominal value of V SUP . 
     A closed-loop, temperature-controlled cooling system having the component values and embodying the principles described above was built and found to perform with a temperature regulation having a standard deviation equal to 0.0015° K. 
     While the principles of the present invention have been demonstrated with particular regard to the illustrated structure of the figures, it will be recognized that various departures from such illustrative structure may be undertaken in practice of the invention. As an example, while the variable resistance of the bridge circuit has been shown as a temperature sensitive resistor, it may be any form of transducer that produces a resistance change in response to, for example, changes in force, light or displacement. Thus, the system of the present invention may be useful in controlling operating parameters other than temperature. The scope of the this invention is therefore not intended to be limited to the structure disclosed herein but should instead be gauged by the breadth of the claims which follow.