Motor Control Circuits

The PIC18F2331/2431/4331/4431 family of microcontrollers have peripherals that are suitable for motor control applications. In this application note, we will see how to use these features to control a Brushless DC (BLDC) motor in open loop and in closed loop. Refer to the Microchip application note, “AN885, Brushless DC (BLDC) Motor Fundamentals” (DS00885), for working principles of Brushless DC motors and basics of control. Also, to obtain more information on motor control peripherals and their functions, refer to the PIC18F2331/2431/4331/4431

If the transmitter stick-potentiometer delivers a voltage about 2 - 3 V, this circuit will be suitable. If you want to avoid using the battery cable (supplying Vcc for IC1and -2), you can use a separate 5V supply for IC1 and -2. In that case you should test the mixer function when the battery voltage is low.

This is a reasonably nice design.

The design in Figure 1 allows full direction and step control of a 12V, four-phase stepper motor from a 5V, TTL/CMOS-compatible logic controller. The circuit uses a Philips SAA 1027 control IC to generate the correct step sequences from the count input. The IC also has a reset function that allows you to temporarily stop the motor and reset the count and a mode control that specifies direction. The TIL199 optoisolator isolates the control signals.

The circuit in Figure 1 provides slewing control for stepper motors that you use in sophisticated applications, such as monochromator movements in optical experiments. The LM331 VFC plays a vital role in this circuit. The constant 10V dc from the IC9596 voltage reference routes to the VFC through the DG303 CMOS switch. The DG303's configuration is such that the VFC initially receives 0V through the grounded 33-kilohms resistor.

Controlling a small dc motor without speed control sounds like a trivial task; a switch or a relay should suffice. However, several problems accompany this approach. For one, the switch, because of the inductive load and the low starting resistance of the motor, tends to wear out prematurely (with all the related sparks and EMI problems). Second, when you cut the power, the motor continues to rotate for a certain time, depending on its initial speed and inertia. The circuit in Figure 1 can be useful for designs that don't need precise control of speed and stopping position but can benefit from enhanced deceleration.

Fan-control circuits range from simple switches that boost the fan speed at a certain temperature, to digitally controlled fans with continuously-variable speed. High-speed/low-speed switches are inexpensive, but the sound of sudden speed changes can be annoying. Digitally controlled fans perform well, but those circuits are more costly, and the system must include a serial bus.

Fan noise is becoming a significant issue as electronic equipment increasingly enters the office and the home. Noise is proportional to fan speed, and the airflow—hence, fan speed—necessary for cooling is less at low ambient temperatures. Because ambient temperature is lower than the upper design point most of the time, a fan can run slower, making it easier on the ears. Fan-control circuits range from simple switches that boost the speed from low to high, to digital, proportional speed-control designs.

The simple circuit design in Figure 1 lets you measure all components of a current flowing in a dc servo motor. The rectified output of the circuit uses ground as a reference, so you can measure the output by using a single-ended A/D converter. The current-sense resistor, R1, has a value of 0.1Ω. The Zetex (www.zetex.com) ZXCT1010 IC converts the differential signal across R1 to a single-ended signal. Two of these ICs form a signal rectifier.

The two-chip circuit in Figure 1 provides fan control and overtemperature warning and shutdown signals to protect systems from excessive heat. The circuit monitors the temperature of the pc board and the die temperature of a CPU, an FPGA, or another IC with an on-chip temperature-sensing transistor. IC1 is a temperature detector and fan driver for cooling fans with nominal operation of 250 mA.