Encoder and PC make complete motor-control system

This Design Idea combines a simple ISA-bus-resident interface circuit; a garden-variety PC; a high-resolution optical shaft encoder; and a PWM- controlled, 0.05-hp, brushed, permanent- magnet dc motor to make a high-precision and high-power motion-control system. The system sequences the precise rotation of an evacuated steel bell jar several feet in diameter, such as those used in molecular-beam spectroscopy (Figure 1).



Although the speed of the intermittent rotation of the jar isn't fast, the system needs large torques to overcome the friction of a large O-ring seal that is subjected to tens of tons of atmospheric pressure. Moving against this drag requires a heavy-duty drive that energizes a 48W (24V, 2A) motor. A different choice of MOSFET in Figure 1 would allow the system to handle even heavier loads. The quadrature-output, incremental optical shaft encoder that this Design Idea uses is popular in high-performance, bi-directional, rotation-sensing applications. Incremental encoders provide an inexpensive and reliable means for digital readout of bidirectional mechanical motion. They're usable to 10,000 rpm and higher. However, the interface logic they need can sometimes be problematic. Such logic typically includes at least one 16-bit or longer bidirectional counter. Several handy peripheral chips, such as the 8253, 8254, and 9511, are available that implement unidirectional counting, but bidirectional counter chips are comparatively scarce. ASICs can provide the needed functions, and hard-wired or programmable logic is a viable, though component-count-intensive, option. Unfortunately, none of these options is ideal from either a cost or a pc-board-area standpoint. This alternative combines the industry-standard 82C54 unidirectional counter-timer peripheral chip with simple software to create a convenient interface between quadrature encoders and the ISA PC I/O bus. Thus, you can easily digitize bidirectional motion. The trick is to use two of the three unidirectional counters in the 82C54 - one for each encoder-rotation direction.

Reference 1 describes the logic involved in detail. This Design Idea expands upon the material in Reference 1 by closing the motion-control loop with an 8-bit-resolution, digital PWM motor-speed-control circuit. The PWM modulator consists of counter-timer 0 (CT0) of the 82C54, operating in retriggerable-one-shot mode and controlling the Q1-Q2 drivers for the MOSFET, Q3. The HC4040 provides a 7.2-MHz clock to CT0 and triggers CT0 at 28 kHz (7.2 MHz/256). The result is a variable-duty-factor PWM waveform at CT0's output (O0). Q1 and Q2 buffer and boost the waveform to 12V p-p and apply the boosted waveform to the gate of MOSFET Q3. The MOSFET then modulates the 24V-dc motor-supply rail and thus generates a more-than-95%-efficient, programmable motor-speed control. The motor's armature inductance combines with flyback diode D3 to filter the high-frequency components of the 28-kHz PWM waveform and to extract the dc component. Minimizing the time the MOSFET spends between full-off and -on states is critical to the efficiency of this PWM circuit, as in all power-handling topologies. The Q1-Q2 driver circuit achieves fast MOSFET on/off transition times of approximately 10 nsec, as well as minimal 12V supply-power consumption. The driver circuit achieves these two goals by avoiding saturation-induced cross-conduction of Q1 and Q2. The circuit avoids the cross- conduction by taking advantage of the fact that Q3's gate presents an almost entirely capacitive load to the Q1-Q2 pair. Therefore, although currents of approximately 100 mA into Q3's gate are necessary during on/off transitions to ensure efficiency-promoting speed, the drive requirement drops to zero between transitions. The circuit takes advantage of the situation by using capacitive drive to Q1 and Q2. Coupling capacitors C1 and C2 deliver robust base drive during rise and fall edges but virtually no drive after the transitions. Hence, the circuit avoids cross-conduction of the bipolar transistors. Meanwhile, R1 provides just a trickle of dc bias to Q1, so that Q3 and, therefore, the motor stays off before the programming of the 82C54, regardless of whatever arbitrary state the IC may assume after power-up. Q4 is a pnp transistor that controls the direction-reversing relay, K1. Listing 1 is an MBasic demo program for the motion-control system.

You can download the software from the Web version of this Design Idea at www.ednmag.com.

Reference

1. Woodward, Steve, "Unidirectional counters accumulate bidirectional pulses," EDN, April 11, 2002, pg 72.