Thermal transient test system

Abstract

A pyrotechnic device (UUT) is automatically tested for thermal response cacteristics using an automatically balanced Wheatstone bridge. One branch of the bridge is a series of relay connected resistances which are selectively connected to coursely balance the bridge using a voltage developed across the UUT. An FET in series with the series of resistances, in response to the bridge null currents, finely balances the bridge. A low constant current pulse is used to generate a voltage across the UUT and to balance the bridge. A second high constant current pulse is used to measure the transient thermal response of the UUT. The transient test current pulses are generated automatically or manually and whose amplitude and repetition rate are manually set. The pulses are generated by a D/A converter whose output is fed to a constant current generator coupled to a manually settable resistance range for setting the value of the high current transient test pulse amplitude. The lower value initial balance current pulse is separately settable by internal calibration switches and straps. The entire test is less than 40 ms.

What is claimed is: 
     
       1. A test apparatus for providing a resistance of a certain value comprising: control means for generating at least one control signal;   first circuit means including a settable resistance responsive to said at least one control signal for setting said settable resistance to a first value approximating said certain value; and   second means responsive to said at least one control signal for setting said settable resistance to a second value which is substantially the same as said certain value.   
     
     
       2. The apparatus of claim 1 wherein said control means includes means for automatically causing initially the setting of said settable resistance to said first resistance value and then automatically causing the setting of said settable resistance to said second resistance value. 
     
     
       3. The apparatus of claim 1 wherein said first circuit means includes a resistor ladder network of series connected resistances of different first values and said second means includes a second variable analog resistance means in series with said first circuit means. 
     
     
       4. The apparatus of claim 3 including a bridge network having first and second pairs of opposing nodes wherein a resistance is between each said nodes, the first pair of nodes comprising an input to said bridge network and said second pair of nodes comprising an output of said bridge network, the current value at said second pair of nodes being the same for providing a null current value therebetween when the resistances between each input node and a first output node have the same relative value as the resistances between each input node and the other output node, said settable resistance being between one of said output and one of said input nodes. 
     
     
       5. The apparatus of claim 4 wherein a second resistance is between the other output and said one input nodes and having a second value, said control means including means for sensing said second value and for producing said control signal in response to said sensing. 
     
     
       6. The apparatus of claim 5 including means for providing a first constant current pulse of a first magnitude to said bridge network for providing a current through said second resistance manifesting said second value and for nulling said bridge network with said settable resistance, said first magnitude being sufficiently low such that the resistance of said second value exhibits negligible temperature change during said providing and said nulling and then providing a second constant current pulse of a magnitude sufficiently greater than the first magnitude for increasing the temperature of said second resistance. 
     
     
       7. The apparatus of claim 6 including means for automatically successively applying said first and second pulses to said bridge network and including means for recording the thermal response of said second resistance caused by said temperature increasing. 
     
     
       8. The apparatus of claim 6 including means for setting the duration of each said pulses. 
     
     
       9. The apparatus of claim 6 including means for setting the amplitude value of said first and second current pulses. 
     
     
       10. The apparatus of claim 6 include current range setting means for setting the current values of said pulses to different ranges of values. 
     
     
       11. The apparatus of claim 3 wherein said second resistance is non linear, said first circuit means including means for generating a first signal manifesting the value of said first resistance, means for digitizing said first signal, said control means including means for causing said digitized signal to select certain of said series connected resistances to provide a digitized linear approximate value of said series connected resistances, and said second means including means for coupling said first and second means in series to form a third resistance, said control means for coupling said first and second means so the third resistance forms one branch of a wheatstone bridge and for applying a current to said bridge such that the value of said third resistance assumes a nulling value for the bridge. 
     
     
       12. The apparatus of claim 11 wherein said bridge has first, second, third and fourth branches, said third resistance comprising the first branch, a fourth resistance comprising the second branch, a fifth resistance comprising the third branch and a sixth resistance comprising the fourth branch, the first and second branches having resistance values substantially different than the resistance values of the third and fourth branches, said control means including means for generating said control signal to response to a sense signal generated in response to sensing the value of the fifth resistance. 
     
     
       13. The apparatus of claim 12 wherein said control means includes amplifier means for amplifying said sense signal, analog to digital converter means for digitizing said amplified sense signal and address means responsive to said digitized signal for selecting said certain series connected resistance and wherein the value of said fifth resistance is substantially lower than the value of the third resistance. 
     
     
       14. The apparatus of claim 13 wherein said fifth resistance has an unknown value approximately of the same magnitude as the sixth resistance, said control means including means for determining the value of said fifth resistance. 
     
     
       15. The apparatus of claim 14 wherein said control means includes means for opening said bridge to provide a voltage across said fifth resistance manifesting the value of said fifth resistance. 
     
     
       16. The apparatus of claim 13 including means for providing a current pulse through said fifth resistance to elevate the temperature of said fifth resistance after said nulling and means for recording the thermal response of said fifth resistance to said elevated temperature. 
     
     
       17. Apparatus for determining the thermal transient response of a resistor comprising: wheatstone bridge means including a plurality of resistor branches, a first resistor forming one of said branches;   a settable resistor forming a second of said branches; and   means for automatically setting the settable resistor value so that the relative values of said first and settable resistors are the same as the relative resistor values of the remaining branches.   
     
     
       18. The apparatus of claim 17 including means for generating a signal manifesting the value of said first resistor, means responsive to said signal for automatically setting the value of said settable resistor to approximate the resistance value of the resistance in a third branch, means for automatically nulling said bridge means with said settable resistor to substantially the same value as said resistance value in said third branch, means for injecting a current through said first resistor to elevate its temperature after said nulling and means for recording the thermal response of said first resistor in response to said elevated temperature. 
     
     
       19. The apparatus of claim 17 wherein said means for automatically setting including a resistor ladder network of series connected resistances of a plurality of first linear values and a second variable analog non linear resistance means in series with said first circuit means, and first means responsive to the sensed value of the first resistor for selecting certain of said series connected resistances and and second means responsive to the null current value of the bridge for setting the analog resistance means so that the settable resistor value is such that said bridge means is substantially nulled. 
     
     
       20. The apparatus of claim 17 including means for providing a first current pulse of a first magnitude to said bridge means for nulling said bridge means with said settable resistance, said first magnitude being sufficiently low such that said first resistor exhibits negligible temperature change during the nulling and then providing a second current pulse of a magnitude sufficiently greater than the first magnitude for raising the temperature of said first resistor. 
     
     
       21. The apparatus of claim 20 including means for automatically applying said first and second pulses to said bridge means and means for recording the response of said first resistor to said temperature raising. 
     
     
       22. An apparatus for automatically nulling a wheatstone bridge comprising: first, second, third and fourth resistances each forming a different branch of said bridge, the first and second resistances having different values, the third and fourth resistances having values corresponding to the respective values of the first and second resistances, said bridge having input nodes between the first and second resistances and between the third and fourth resistances and output nodes between the first and third and between the second and fourth resistances;   means for supplying said second resistance of unknown value and approximately the value of the fourth resistance; and   means for automatically setting the value of the first resistance to null said bridge at said output nodes.   
     
     
       23. The apparatus of claim 22 wherein said first resistance comprises a linear resistance in series with a non linear resistance, said means for automatically setting including means for approximating the value of said third resistance with said linear resistance and means for changing said approximate value of said third resistance with said non linear resistance to substantially null the bridge by setting the value of said non linear resistance. 
     
     
       24. The apparatus of claim 23 wherein said linear resistance comprises a ladder network of serially connected fixed value resistances and said non linear resistance comprises a field effect transistor (FET) connected in series with said ladder network and servo means responsive to the null signal value of said bridge at said output nodes for setting the resistance of said said bridge with the resistance of said FET. 
     
     
       25. The apparatus of claim 23 wherein said means for approximating the value of said first resistance includes means for injecting a current through the second resistance to produce a voltage manifesting the value of said second resistance and means responsive to said voltage for generating a control signal for selecting the value of said linear resistance to approximate the value of said third resistance and means responsive to said control signal for sensing the current at said output nodes caused by said injected current and for setting the value of said non linear resistance so that the values of the currents at said output nodes match. 
     
     
       26. The apparatus of claim 22 further including means for automatically injecting a constant current through said second resistance after said bridge is nulled for determining the thermal response of said second resistance. 
     
     
       27. The apparatus of claim 26 including a constant current generating means for generating a first current pulse of a first value for said nulling and a second constant current pulse for determining said thermal response, said first and second pulses being generated successively. 
     
     
       28. The apparatus of claim 27 including a current generator for generating a constant current of a given amplitude corresponding to the first current and dummy load means for receiving said current of a given amplitude corresponding to said first current pulse prior to applying said first pulse to said bridge. 
     
     
       29. The apparatus of claim 27 wherein said current generator comprises means for providing a digital signal manifesting the amplitude of the first and second currents, digital to analog converter means for creating an analog signal corresponding to said digital signal, amplifier feed back means for generating a constant current from said analog signal and terminal means for supplying said constant current to said bridge input nodes. 
     
     
       30. The apparatus of claim 29 including pulse duration set means for setting the duration of each said first and second pulses. 
     
     
       31. The apparatus of claim 30 including pulse amplitude setting means for setting the amplitude of said first and second pulses. 
     
     
       32. The apparatus of claim 29 including means for automatically generating said first and second pulses. 
     
     
       33. The apparatus of claim 29 include means for selecting current amplitude values of different ranges for said first and second pulses.