Design Ideas: November 10, 1994

The digital tachometer in Fig 1a is a component of the digital seek servo loop in Fig 1b. The loop produces an error signal that causes a motor to follow a sophisticated velocity profile (Fig 2). Although you could use an analog tachometer, the digital one costs less, performs better, is less sensitive to temperature, and implements easier in a custom IC.

My design's tachometer controls the actuator for the heads of an optical-disk drive. But the principle also applies to other motor-control applications.

Upon receiving a start command, the actuators accelerate to a desired speed and follow the specified velocity profile until they reach their destination.

As the actuators move across the tracks, they generate two pulses for each track they cross. The circuit samples these pulses at different rates depending on how close the actuators are to their final destination. Table 1 lists the specific sampling rates.

A 10-MHz clock provides the fundamental sample rate; but the actual sample rates vary. Therefore, the circuit must divide the time between samples by that same sample rate to obtain the correct time interval for each pulse. Otherwise, the output A/D converters would not produce a properly scaled error signal.

In Fig 1, two 4-bit counters divide the input-pulse signal and the 10-MHz clock by the appropriate divisor from Table 1. The overflow output of the input-pulse's 4-bit counter loads all zeros into the 12-bit counter. The 4-bit counter's overflow output clocks the 12-bit counter. Thus, the output of the 12-bit counter always represents the time required for a single input pulse, regardless of the sample rate.

A 4-kbyte PROM converts the output of the 12-bit counter to velocity. The circuit in Fig 1b compares the actual velocity with the desired velocity, producing an error signal for the actuator. (DI #1612)