Sensors – Information for efficient motor control

For efficient and precise motor control, the processor unit needs, in particular, position information. Data collected by the control unit is also actively used to control and monitor the current flowing in each individual motor phase.

While in analog systems the speed of a motor is measured with a tachogenerator, in digital motor control various sensors such as encoders or resolvers are used for position detection - in less precise applications also Hall sensors or Back Electromotive Force, i.e. the electromotive resistance.

Optical and magnetic encoders

The encoders can be optical or magnetic. For optical encoders, there are optical disks with reflective or transmissive tracks for the reflective or transmissive type of optical encoder. The granularity of the tracks on the disk determines the resolution of the encoder. Magnetic encoders use external magnets mounted on a rotating part and utilize various magneto resistive sensing solutions such as AMR, GMR, and TMR.

Both types of encoders give a relative position (pulses), but use an index signal to determine the starting position within one revolution. With two phase-shifted signals, the resolution is usually increased by a factor of four and information about the direction can also be generated.

Absolute encoders

The absolute position of the motor or motor shaft can be determined with absolute encoders. They have a similar design to an optical encoder, but instead of a simple slotted disc, they have a disc which generates a visual bit code (usual gray code where only one bit changes at once). The measurement accuracy is dependent on the length of the bit code. The sensor reads the bit code and determines the rotor position based on this. It is also possible to measure rotation speed as the code changes with the rotor position and the rate of change is directly proportional to the rotation speed of the rotor. Absolute encoders normally have a digital output, with the resolution being determined by the number of bits. 

Figure 1: Design of an optical absolute encoder

Resolver

A resolver is another type of absolute position sensor, consisting of three coils. Two of these are stationary, placed at a 90° angle to one another, and one is mounted on the rotating shaft. The rotor coil is supplied with high frequency alternating voltage. The power flow generated from this coil is dependent on the rotor position and induces voltage in the two stator coils, both of which are also position dependent. The measured voltages of the stator coils can then be used to measure the absolute position of the rotor. Resolvers are very resistant to external influences such as dust, heat and vibrations and are used mostly in the automotive industry and industrial sector.

Back EMF

Brushless DC motors provide simple, sensorless control in applications that require only constant speed, such as vacuum cleaners, fans, or remote-controlled toys like model airplanes. The back EMF is the electromotive force induced when the motor rotates. The biggest challenge with this control solution is low speeds and starting the motor.

Hall sensors

Hall sensors give the motor control system approximate information on angle position, used to improve Back EMF approach at low speeds and at start They are used most commonly in brushless direct current motors: they are fitted with three Hall sensors which are attached to the stator at intervals of 120 electrical degrees. When they are exposed to the rotor’s magnetic field, they produce a corresponding digital pulse which are used for improved control.

Hall sensors have a very simple design, improve accuracy of control over sensorless solutions and are therefore cost-effective. They are easy to integrate in a motor control system and are largely impervious to environmental factors such as humidity, temperature, dust and vibrations.

Inductive position sensors

Recently, there has been a trend for magnetic position sensors (mainly angle sensors) to be replaced by inductive sensors: They do not require external magnets, are much less sensitive to stray fields and can be mounted on or next to the shaft. An eddy current method is used, requiring only an appropriately shaped metal part on the rotor and appropriately shaped excitation and receiving coils on a printed circuit board. Dual inductive sensors are available as an extension, allowing multiple coils, which on the one hand provides better resolution and on the other hand allows the absolute position to be maintained over a full revolution.

Current sensing

Most motor control applications use a resistor shunt for current measurement. It is quite accurate and has good dynamic range, but is less efficient and not isolated. Therefore, Hall-effect-based current measurement solutions that can measure currents of up to 1500A with a bandwidth of up to 1.5MHz are in vogue. In addition, the Hall-effect-based current measurement is isolated from the high-voltage stage.

Capturing environmental conditions

In addition to motor control, sensors that can be used to record environmental conditions, such as temperature, vibrations and noise especially in ultrasonic range, are also increasingly being used, which is particularly useful for predictive maintenance. Drive manufacturers are working increasingly on integrating sensors and control electronics into one compact unit together with the motor itself.

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