Optoelectronic position control simplifies motor movements

The optoelectronic technique for achieving position control provides an inexpensive, easy- to-design method of achieving simple, repeatable movement using fixed index points with linear- or rotary-motion components. The simple, basic design in Figure 1 for sequential position control exploits the quick response time of a power op amp, working in tandem with a pair of photodiodes. The result is a low-component-count system that provides high reliability, accuracy, and repeatability when you use it in well-defined operating conditions. The circuit in Figure 1 achieves sequential position control by using a power op amp to integrate the differential output of a pair of photodiodes to drive the motor in the proper direction until the photodiode currents are equal. Movement between index points occurs when you momentartily switch a fixed input current to the amplifier's input, causing the amplifier to drive the motor in the desired direction. The charge on CF maintains motor drive as the input current switches off before reaching the index point.



Figure 1: An optoelectronic circuit uses a power op amp to achieve sequential position control.

To ensure continued motion in the desired direction, the motor drive receives reinforcement by the output from the first photodiode as it illuminates. As the second photodiode illuminates, its current reverses the motor drive, causing the system to lock to the index point. The use of a differential configuration eliminates errors from temperature and time instability in the optoelectronic devices. The entire system uses a simple switch, as Figure 1 illustrates, to generate both forward and backward motion. Because motor response time and system inertia vary greatly in different applications, you achieve proper damping by selecting CF and RF based on the application.





CF needs to be small enough to allow drive reversal before the index point passes the second photodiode; otherwise, the system continues on to the next index. If the value of CF is too small, severe overshoot or oscillation can occur, resulting in drive-train failure or motor burnout.



Figure 2: With optimum beam-sensor alignment, the light beam illuminates half the photosensitive area of each diode

To help minimize overshoot, RF1 and RF2 in Figure 1 stabilize the control loop at the unity-gain point. You can also improve response time by applying a braking force, which you create by using RL and CL to form a lead network, which enables the amplifier to modify the motor drive based on a change in the sensor output. The motor in Figure 1 has EMF (electromotive force) of 14V and can apply a 46V stress across the conducting output transistor when you reverse it. This power dissipation is a worst-case scenario; you need to check it against the SOA (safe operating area) of the amplifier. Figure 2 shows optimum alignment for the beam sensor. You achieve optimum alignment by centering the light beam in relationship to the active areas of each photodetector. The light beam needs to illuminate half the photosensitive area of each diode. When sizing the "hole," consider the distance between the location of the light beam and the photodiodes. If the beam is too large, the sensors do not produce any change for a range of positions. Too small a beam produces a nonlinear transfer function along the center line between the photosensitive areas. This non-linearity can create difficulty in selecting the value of CF for dampening the circuit and requires a light source with higher intensity.



Figure 3: This circuit imparts digital-interface control to the circuit in Figure 1.

Figure 3 illustrates how you can use a nonbipolar signal without digital-to-analog conversion for systems that integrate digital control. When logic lines are low, the signal diodes do not conduct. This condition allows the photodiodes to control the circuit. A high level on Line 2 causes current to flow to the summing junction and swing the amplifier negative. A high level on Line 1 raises the summing junction voltage above ground and swings the amplifier positive. By selecting a resistance value that allows a logic-level supply high enough to provide at least twice the maximum current from each photodiode, the circuit maintains system control regardless of the photodiode signals.