[Engineering Essentials]
Mechatronics Means Motors
Movement requires motors, and motors are now including smarter controllers to deliver better efficiency and performance.
Piezoelectric motors are a special class of electric motors that do not use magnetic flux for movement. Instead, they use the deformation properties of some crystals when power is applied. They tend to be small and can be extremely small.
Piezoelectric motors are easier to construct than conventional electric motors of comparable size and performance. Their small size makes them a match for many portable applications, such as the lens zoom support in cell phone cameras.
MOTOR CONTROL An on/off switch is often the only thing sitting between a motor and its power supply, but it provides limited control. It also can be inefficient. The lack of control takes this approach out of the realm of mechatronics, where computer control is key.
The next step up is electronically controlling the on/off switch. This is accomplished by using transistors for dc control and devices such as silicon controlled rectifiers, thyristors, and triacs for ac control. For ac control, variable power and speed can be attained by turning on power for a fraction of the power cycle because it is easy to set the threshold of these devices.
The H-bridge is normally used for handling dc motors, including BLDC motors, where rotation may be reversed as in drills or servos (Fig. 2). Current flows through opposite pairs of transistors when they are turned on. Turning on all the transistors isn’t a good idea. In this case, the motor doesn’t turn and transistors get rather warm before acting like blown fuses.
Different power transistor technology, such as MOSFETs and bipolar, can be employed. The diodes are included in the circuit to protect the system during transition periods where back electromotive force (EMF) would cause undue stress.
Pulse-width modulation (PWM) signals from a microcontroller normally control the transistors. This provides the motor with speed control, and it works because motors are an inductive load. Another advantage of driving the transistors using a PWM signal is that the torque essentially remains constant. Changing speed by varying the voltage would also change the torque. This could be desirable in some instances, but an H-bridge generally will be driven via PWM signals.
The precision and accuracy of the PWM output along with the power components and power supply will affect the efficiency and operation of the motor. Also, it is good practice to allow some “dead time” between turning off a transistor and turning on the other transistor when changing polarity. This is especially true in multiphase motor control.
Motor control with feedback makes the job of designing a system a little more complex, but there are benefits depending upon the kind of feedback provided. For example, quadrature encoders can provide position information in addition to velocity information. Hall effect and optical sensors are two common external feedback systems. Hall effect sensors are mounted to detect the field from the electromagnets that are part of the rotor. LEDs tend to be aimed at a target on the rotor that reflects light to sensors near the LED.
Sensorless feedback refers to the use of a sensor that isn’t an external device like Hall effect and optical sensors. Instead, the sensor is electronic and examines the electrical subsystem used to drive the motor. The sensors use the back EMF that is inherent in an electric motor’s operation.
Sensorless feedback is normally used on multiphase motors. A similar approach can be used on a single-phase, H-bridge implementation, but it is done in a polling mode. In this case, the controlling microcontroller would stop the PWM signal train so all the transistors are off and the motor essentially acts like a generator. It then uses an analog-to-digital converter (ADC) input connected to the motor to check the output. This takes a few milliseconds, so the effect is minimal and allows a sample rate of about 50 Hz. This approach can be used with Acroname’s Brainstem Moto 1.0 power control module, which is used in robotics.
A typical three-phase BLDC controller includes three ADC sensor inputs, one for each phase of the drive system ( Fig. 3). One way to implement this is by including a series resistor in the drive circuit and using a differential ADC to measure the voltage across it. A small resistance minimizes overhead but increases the range and precision requirements of the ADC.
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