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Blanking Circuit for Switched Motor Control System

IP.com Disclosure Number: IPCOM000060025D
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Commander, RD: AUTHOR [+3]

Abstract

In a switched motor control system a transistor bridge switches current through a motor in either direction. Transitions of an analog input waveform are detected by a comparator to switch the bridge. Sensitivity to input noise is removed by a blanking circuit which blanks the response of the comparator after a good transition in either direction. The blanking circuit is not in series with the comparator which avoids additional phase delays, and equal mark/space operation is provided. Fig. 1 shows the position of the blanking circuit within a motor control loop. FETs T1 to T4 form a bridge around the motor. At any time either T1 and T4 or T2 and T3 can conduct to switch current in either direction through the motor. Amplifier A2 has a differential input from resistors RSENSE.

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Blanking Circuit for Switched Motor Control System

In a switched motor control system a transistor bridge switches current through a motor in either direction. Transitions of an analog input waveform are detected by a comparator to switch the bridge. Sensitivity to input noise is removed by a blanking circuit which blanks the response of the comparator after a good transition in either direction. The blanking circuit is not in series with the comparator which avoids additional phase delays, and equal mark/space operation is provided. Fig. 1 shows the position of the blanking circuit within a motor control loop. FETs T1 to T4 form a bridge around the motor. At any time either T1 and T4 or T2 and T3 can conduct to switch current in either direction through the motor. Amplifier A2 has a differential input from resistors RSENSE. When the analog input drive starts to increase, the comparator output goes low, T1 and T4 conduct and current starts to flow from left to right through the motor. As current builds up in the motor, amplifier A2's output increases until the comparator negative input exceeds the 'positive' hysteresis voltage at the positive input. The comparator output then goes high, and the hysteresis circuit switches the hysteresis voltage to the other input. The 'Break Before Make' circuit ensures that T1 and T4 shut off before T2 and T3 turn on. The motor current starts to decrease. This state holds until the comparator once again switches negative and the cycle is repeated. In this way the average motor current follows the analog input drive. The blanking circuit monitors the output of the comparator for a change of state, then operates on the comparator to ensure that this is held for a predetermined period. This is illustrated in Fig. 2. Since the blanking circuit is not in series with the feed-forward section of the motor control loop, no additional phase lag is introduced. Fig. 3 shows the blanking circuit and comparator circuit in detail. The lower portion of Fig. 3 shows the comparator with a low current, high gain input stage followed by a lower gain stage. Transistors 1, 2 and diodes D form an output level shifting network. The upper portion of Fig. 3 shows the blanking circuit. Assume that the comparator output is low and has been low for some time. Then, via long tailed pair 3, 4 the voltage across capacitor C is equal to the comparator output voltage. Thus current I flows through transistor 3, I flows through transistor 4, and 2I through...