Abstract:
This switching amplifier is for driving inductive loads, stepping motors in particular. This amplifier uses dual threshold concept with improvements to the current feedback. These improvements compensate for errors in the feedback signals of the prior art. These improvements measure currents erroneously omitted or included in the feedback signal and add or subtract said measurements from said feedback to improve signal fidelity.

Description:
BACKGROUND OF THE INVENTION 
     This invention is an improvement to the switching amplifier disclosed in my U.S. Pat. Nos. 4,087,732 and 4,140,956. This switching amplifier is used to drive stepping motors and is related generally to switching amplifiers for inductive loads. 
     Stepping motors are specialized synchronous motors designed to be driven over a wide range of speeds including being stopped. The low-frequency energization of stepping motors must limit the motor current to keep the motor from incinerating. The prior art contains many approaches to limiting the motor current. Of these, switching amplifiers provide the greatest performance with the best efficiency. The switching amplifier disclosed herein is a single supply voltage variety and not the dual voltage type which uses the higher voltage to create the rapid current rise needed for high speed operation and a lower voltage to maintain the motor current without overheating. More specifically, the amplifier disclosed herein attempts to control the motor current between two thresholds, whose values are the amplifier input offset by an internally generated hysteresis signal. It is important to compare these thresholds against accurate representations of the motor current so that the amplifier is more nearly linear. 
     OBJECTS OF THE INVENTION 
     The first object of this invention is to improve the fidelity of the feedback signal which represents the motor current. 
     The second object of this invention is to filter out of the feedback signals unwanted switching noise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the prior art. 
     FIG. 2 is the schematic of this invention. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     The prior art switching amplifier circuit excluding diodes 2 through 6 was disclosed in my U.S. Pat. No. 4,140,956 and included herein by reference. This circuit exhibited inductive spikes from the windings 8 when the transistors 10 and 12 were turned off. These spikes could burn out transistors 10 and 12. Diodes 2 and 4 and zener 6 were added in the standard fashion to protect the output transistors 10 and 12 from the leakage inductances of the motor windings. 
     The inductive windings 8 are shown as two ideally coupled windings 8B and 8C with leakage inductances 8A and 8D. If these leakage inductances were not present, the inductive spike would be entirely clamped by diodes 22 and 24 and the inductive coupling. Unfortunately, inductances 8A and 8D do exist in the motor and its associated wiring and create a flyback voltage which must be clamped by diodes 2 or 4 to protect the switching transistors 10 and 12 from excessive voltages. During the short time that these diodes are conducting, the motor currents are not accurately represented by the voltages on sensing resistors 14 and 16. Although the amplifier of FIG. 1 can ignore these error signals when the hysteresis signal of resistor 26 is large, having large hysteresis signals forces a low switching frequency which can create unwanted motor motions and which is irritatingly audible. 
     The currents in sense resistors 14 and 16 are supposed to be representative of the total motor current. Said currents cannot be representative of the motor currents if some of the motor current is diverted through diodes 2 and 4. Also, said currents cannot be representative of the motor currents if said sense resistor currents include significant base drive currents from amplifiers 18 and 20. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The improvements disclosed in FIG. 2 compensate the error currents in the sensed current for clamping diodes and base drive circuits. The FIG. 2 schematic also includes a filter capacitor to filter feedback noise and resistor network designed to facilitate amplifier gain changing. 
     The circuit of FIG. 2 operates in a feedback manner as follows: the sense voltages at nodes 30 and 32 are compared differentially against the input 34 and the hysteresis signal 36 by comparator 38 and resistors 40, 42, 44, and 46 to create output/positive feedback 36. This output drives delay 50 directly and delay 52 via inverting amplifier 48. These delay circuits delay the rising edge of the input and do not delay the falling edge. This delay may be implemented with an AND gate 54 and RC delay 56 and 58. The delay circuits control transistor driving amplifiers 60 and 62. The base drive currents are sensed by resistors 64 and 66. The voltages across resistors 64 and 66 are connected to nodes 30 and 32 via resistors 68, 70, 72, and 74 so as to cancel the effects of the base drive currents as sensed by resistors 76 and 78 which are connected to nodes 30 and 32 by resistors 80 and 82. Resistors 64 through 82 should have proportional or equal values as listed below to properly cancel the base drive signal. The base drive currents in resistors 64 and 66 operate transistors 84 and 86. These transistors control the currents in windings 88. The winding currents in transistors 84 and 86 pass through resistors 76 and 78 to provide nodes 30 and 32 an indication of the motor current. The primary inductive flyback, as created by windings 88B and 88C are conducted by diodes 90 and 92 back into the power supply connection 94. Resistors 76 and 78 also sense the flyback currents in diodes 90 and 92 in an attempt to measure the net current within winding 88. The secondary inductive flyback of windings 88A and 88D are clamped by diodes 94 and 96 and zener 98. As mentioned herein above the currents in diodes 94 and 96 need to be sensed also so that nodes 30 and 32 differentially indicate the net currents in winding 88. Consequently, differential current transformer 100 is inserted in the current path of diodes 94 and 96. Resistor 102 loads the transformed output winding 100B. Preferably, resistor 102 is the same value as resistors 76 and 78 so that the current transformer may be trifilar wound in the ratio of 1:1:1 for the tightest possible coupling. The voltage across resistor 102 is connected to nodes 30 and 32 by resistors 104 and 106. 
     The symmetric operation of the switching amplifier may now be explained. Suppose the output of the comparitor is high and the time delay 50 has expired, then transistor 84 is on and 86 is off. Depending upon the direction of the current flow in winding 88, the current in winding 88 is then either flowing in transistor 84, or in diodes 90 and 96. If the current is flowing in transistor 84 then the emitter voltage is positive and becoming greater in magnitude. If the current is flowing in diode 90, the emitter voltage is negative and decreasing in magnitude, i.e. increasing towards zero. According to the prior art, any current flowing in diode 96 should be flowing in diode 90. To compensate, transformer 100 transforms the transient flyback current for sensing circuit while isolating the sensing circuit from the power supply 94. The transformed flyback current then makes the dotted end of winding 100C negative, and compensates the voltage on the emitter of transistor 84. Further compensation of the differential voltage between nodes 30 and 32 for the base current in transistor 84 is accomplished by resistors 64, 68, and 72. In any situation, the voltage on node 30 rises with respect to the voltage on node 32. When the voltage on 30 rises so that the negative input of comparator 36 exceeds the positive input, the output 36 goes low. This immediately turns off transistor 84, drives the positive input of the comparator further below the negative input, and after delay 52 turns on transistor 86. The symmetric operation of this circuit now follows. The base current of transistor 86 is sensed by resistor 66 and compensates the voltage between nodes 30 and 32 via resistors 70 and 74. If transistor 84 was conducting prior to the comparator switching, then diodes 92 and 94 conduct the flyback. Transformer 100 and resistor 102 sense the current in diode 94 and compensate nodes 30 and 32 via resistors 104 and 106. If diodes 90 and 96 were conducting, then transistor 86 conducts. In the former case, the emitter of transistor 86 is negative and decreasing in magnitude. In the latter case, the emitter of transistor 86 is positive and rising. In either case, the voltage at node 32 rises with respect to node 30. When node 32 rises so that the positive input of comparator 38 is greater than the negative input, then the output switches to a high. This immediately turns off transistor 86, drives the positive input of the comparator further above the negative input, and after delay 50 turns on transistor 84. The switching cycle is thus completed. 
     Capacitor 108 is placed across nodes 30 and 32 to form a filter with resistors 68, 70, 72, 74, 80, 82, 104, and 106 and to reduce the switching spikes which can create spurious oscillations in the amplifier and which can cause excessive transistor dissipations. 
     Preferably the components of FIG. 2 have the following relationships: 
     R76=R78=R102 
     R80=R82=R104=R106 
     R68=R70=R72=R74 
     R64=R66 
     (R76)(R68)=(R64)(R80) 
     R40=R42 
     R44=R46 
     Typical comparator produce too large an output voltage to be used directly. In that case, resistor 46 is replaced by a network such as found in my patent 4,140,956. 
     The gain of the amplifier is varied by changing resistors 40 and 42. 
     The resistors 76 and 78 form a first or primary current sensing means as established in the prior art resistors 14 and 16. The diodes 94, 96, and 98 form a current path which diverts motor current from the primary or first current sensors. A second current sensing means consisting of transformer 100 and resistor 102 compensates the primary sensor for the current flowing in diodes 94, 96, and 98 so that the combined signals of the first and second sensors more accurately reflect the net current in the windings 88. 
     Similarly, the current in resistors 64 or 66 also flows in the primary current sensing means making said primary sensing means less indicative of the current in winding 88. A second current sensing means, resistors 64 or 66, compensate the primary sensor for the currents which control the load switching transistors 84 and 86 so that the combined signals of the first and second sensors more accurately reflect the net current in the windings 88. 
     The capacitor 108 and resistors 68, 70, 72, 74, 80, 82, 104, and 106 filter the signals from the various sensors to eliminate spurious responses of comparator 38. 
     Although this invention has been disclosed in detail its scope is the correction of the primary current sensor signal by a secondary current sensor so that the combined signal of the sensors more accurately reflect the load current. The above description is an example of this invention whose scope is limited only by the appended claims.