Rotary controller positions stepping motor

The circuit in Fig 1 uses a servo potentiometer, as opposed to a rotary switch or encoder, to provide the necessary drive pulses for a stepping motor. This motor positions a pointer by the rotation of a manual control. The circuit must be precise, allow a rapid response, and be simple to use. (This circuit was part of a complex apparatus used for a series of physiological tests.) Another requirement is that the control had to be a small, handheld, battery-operated device.

The angle of the pointer needs to be set at greater than 10 minutes of arc, resulting in the choice of a motor and drive system with 4000 steps per complete revolution in a system with logic pulses for step and direction control. The direction of rotation of the motor must mimic that of the potentiometer. Also, the variable stepping rate (1, 10, and 1000 times/sec) must be related to the number of degrees the potentiometer turns. That is, the maximum step rate should result when the potentiometer turns fully either clockwise or counterclockwise.

R₁ provides 180° of electrical and mechanical rotation. The device is spring-biased, so it returns to a central position when you don't actively rotate it. The circuit applies the voltage at R₁'s wiper to a series of comparators configured as double-ended limit detectors. A chain of tapped resistors provides reference voltages W_A, W_B, and W_C for the three limit detectors. The outputs of these detectors control a programmable crystal oscillator (PXO) that generates the stepping pulses. One additional single-ended comparator provides direction control. Four AA batteries provide a supply voltage of 5V.

The outputs of the detectors depend on the position of R₁. When R₁'s position is between 54 and 126°, the output of NAND gate 8 is low, which disables the oscillator through its reset pin. On either side of these positions, the output of NAND gate 8 is high, which enables the oscillator. Then, according to R₁'s position, the outputs of gates 9, 11, and 13 go low. The result is respective oscillator output frequencies of 1, 10, and 1000 Hz as the user turns the potentiometer toward either end stop. Note that the output of NAND gate 14, which comparator 7 drives, controls the direction of motor rotation. Comparator 7 changes logic level at the 90° position.

You can add or remove comparators for a different sequence comparison. If you increase the number of comparators, you should buffer the reference and potentiometer wiper voltages. Also, you can program the PXO 8640BN using controls P₁ to P₆ to oscillate at 57 frequencies or use the oscillator merely as a divider driven from an external frequency source.