Switching regulator drives robot motor

Donald Comiskey, Power Trends Inc, Warrenville, IL -- EDN, 5/13/1999

The circuit in Figure 1 shows an example of how you can use an integrated switching regulator (ISR) to efficiently vary the speed of a permanent-magnet dc motor. In this application, the rotating head of a robot senses the presence of an oncoming object (perhaps a human). If the robot senses an object, it slows the rotation of its head to closely survey the surroundings. If an object is indeed present and comes within a certain distance of the robot, the robot's head stops rotating and "looks" in the direction of the object. The robot uses a typical proximity-detection device that senses reflected light (Figure 2). Its nose contains a photodetection device that senses the light that reflects from an oncoming object. The primary source of the light is the robot's eyes, which contain two lamps. The photodetector in the nose resides in a black tube to reduce the effects of ambient light.

The sensed light causes a photodetection current to flow through the photodetector and R₄, developing a voltage across R₄ (Figure 1). The LM324 op amp, which you configure as a noninverting amplifier with an adjustable gain of 1+R₂/R₃, amplifies this voltage. The amplified voltage is a control voltage, V_C, which routes to the ISR's Adjust pin via R₁. A decrease in V_C, relating to a decrease in light level, causes the ISR's output voltage and the corresponding speed of the dc motor to increase. An increase in V_C, relating to an increase in light level, causes V_OUT and the corresponding motor speed to decrease. A further increase in light level causes V_C to increase to a point at which V_OUT becomes low enough to stop the rotation of the motor. A linear relationship exists between V_C and V_OUT: V_OUT=-V_C+6.25V.

In the absence of light, the output voltage of the op amp saturates near 0V. The equation shows that a control voltage of 0V produces an ISR output voltage of 6.25V, causing the motor to rotate at its maximum speed. A light level that produces a 6.25V control voltage results in an ISR output voltage of 0V, causing the motor to stop rotating. Intermediate motor speeds result from control voltages between the extremes of 0 and 6.25V. You can adjust feedback resistor R₂, which sets the op-amp gain, to obtain any desired sensitivity. The PT6101 ISR in Figure 1 is 85 to 90% efficient and supplies motor currents as high as 1A. (DI #2353).