February 2, 1998

Optocoupled gate detects motor operation

Jim Knipfer, Liebel-Flarsheim Co, Cincinnati, OH

Most applications require redundant system checks to ensure that devices are operating as expected. The circuit in Figure 1 detects when a PWM-controlled servo motor is running. You can monitor the motor-running signal output with a CPU or tie the output to hardware that indicates a fault if the motor is running when it should be off. The motor's output connects to input resistors R_IN1 and R_IN2. The use of two resistors protects the motor or servo amplifier in the case of a short circuit. R_IN1 and R_IN2 also serve as current-limiting resistors for the LED in the opto gate, IC₂.

The value of R_IN1 and R_IN2 in Figure 1 is suitable for 150V nominal servo output voltage. Because the input voltage can reverse, the circuit uses a full-wave bridge (D₁ to D₄) before the input of the opto-gate IC. The circuit needs a high-speed opto gate, because when the motor runs at low speeds, the servo-output duty cycle can be only a fractional percentage, thereby providing only 1- µsec or narrower pulses to sample. When no signal is present at the input resistors, the output of the opto gate is high, causing the output of inverter IC₃ to be low. The low output allows the MC14541 counter, IC₁, to count at the frequency determined by R₁, R₂, and C₁.

You can determine the time-out by multiplying the count range (selected by inputs A and B) by the period of the frequency set in IC₁. Table 1 shows approximate time-outs. Upon the initial application of power, the motor-running signal remains active until the time-out occurs. Upon detection of a servo pulse of 500 nsec or longer, the master reset pin (Pin 6 of IC₁) goes high, resets the internal counter, and asserts the motor-running signal. The motor-running signal remains low for the amount of time shown in Table 1. The time-out for the circuit in Figure 1 is approximately 10 msec.

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