The field of electrical motor drives is huge. They are used in machinery such as for milling, turning lathe or robotics. They are used for transportation in production such as in printing machines or assembly lines. Escalators, elevators or ergometers are for personal use. A new boom started with hybrid cars and will continue with e-cars. In industrial applications, AC drives are often used, as they are more reliable than DC motors as they are brushless. Now these motors are normally running synchronous to the power supply. Frequency inverters are used to generate supplies with required frequencies. They can further be regulated for positioning or speed and are then called servo drives. A block diagram is shown in Fig.1.38.
The three phase AC power supply is converted to a DC link voltage. Using the method of pulse-width modulation with the help of IGBTs, the DC link voltage is converted back into a three phase AC supply. The speed of the new supply is variable in frequency, so that the robust AC motors can be used over a huge frequency span and for very accurate positioning systems (for example tooling machines).
At least two currents to the motor have to be measured. The third current can be calculated, but needs to be monitored for failure. The currents are transferred into
AC supply DC link sense AC motor
Angular motor position sense Position sense of load
Digital control circuit I-sense I-sense I-sense
the rotating coordinates of the motor, so that the angular motor position needs to be sensed. This is required for the control of the torque.
More accurate systems also measure the position of a load. Often machines also include a reference position where the coordinates of the load are measured. The motor control system is storing the coordinates of this reference mark and refers all other positions relative to these coordinates. Some additional control variables of interest might be the DC link voltage and the temperature of the motor. Finally, the application might support an analog input port. This analog input is normally used to set the anticipated speed or position.
1.3.1.1 Sensing the Phase Currents
Most important are the currents to the motor. The IGBTs need to switch as soon as the currents are changing their sign. The magnitude is further required for the motor control loop, which is typically running at a switching frequency of the IGBTs of 16 kHz. The currents need to be turned off within a few microseconds, if an over- current is detected. Otherwise, additional damage can be generated inside the power stage or the motor.
Two ways for current measurement are most commonly in use. Motors with high currents use magnetic sensor, which are mostly based on the Hall Effect. These sensors output a voltage between 0 and 5 V that directly connects to the input of a SAR converter. A conversion is triggered, once the motor control loop requires the magnitude of the current. The SAR converter can trigger conversions at any point of time and the conversion rate is sufficiently fast for the system. Sign comparators are used in parallel to monitor the direction of the current. Window comparators monitor failure currents continuously. The required resolution is between 10 and 12 bit.
Shunt resistors are used for current sensing for motors with currents less than 20 A. The voltage across the shunt must remain as small as possible to keep the power dissipation in the shunt reasonable. A full-scale range of200 mV or less is normal. The small signal amplitude requires a low noise ADC architecture, so that delta-sigma modulators are suited best. Also note that the currents are measured in the phase, which will float between a positive and negative DC link voltage. Galvanic isolation to the digital control circuit is required. The bitstream of the delta-sigma modulator only requires one digital signal to be isolated (see Sect.1.2.3), which adds a cost advantage [31].
1.3.1.2 Monitoring the Position with Optical Sensors (Encoders)
The encoder consists a carrier of glass with up to 8192 stripes that is rotating with the load or the motor respectively. Light is shining through the carrier onto a set of photo diodes that are generating a current. If a diode is between two stripes the current is high and if a stripe is on top of a diode, less or no current is generated.
A sine and a cosine wave are generated out of the signals of the diodes with one period per strip.
The sine and cosine are digitized with a set of comparators. Their outputs connect to up-down counters. The angle or angular speed of the motor/load at a certain point of time can be calculated by the number of counted pulses in relation to the total number of stripes on the carrier. The resolution can be increased with two time counters that are used to compare the amount of clock cycles between two stripes and the delay from the last stripe to the time of interest. The additional resolution is dependent on the speed of the motor. The faster the motor, the less clock cycles are between two adjacent stripes. The resolution of the angle can also be increased by converting the sine/cosine wave with an analog to digital converter. Here, usually a 10 Bit ADC is accurate enough.
If a motor rotates with 6000 rpm and the encoder consists of 4096 stripes, then the input frequency will reach 410 kHz. This application is typically expecting input frequencies up to 500 kHz. The Nyquist theorem must not be fulfilled, if the signal is not continuously sampled and is not reconstructed in the digital domain. However, the bandwidth of the sample and hold stage must be high enough. The signal will then be sampled at a particular point of time in the motor control loop. Again, SAR converters have the capability to trigger a conversion at a particular point of time and are most suited here. This is also important, when the machine reaches a reference position.
Some systems try to continuously sample the sine and cosine signals. They reconstruct and filter the signal in the digital domain to improve the signal integrity, which suffers from distortion of the IGBTs and the use of long cables. In this case, the Nyquist theorem must be fulfilled. SAR ADCs with conversion rates higher than 1 MSPS are required.
1.3.1.3 Monitoring the Position with Inductive Sensors (Resolver)
A resolver is based on inductive coupling. Three coils are positioned inside the motor; two of them are positioned in a 90 angle and are in a fixed position. The third coil, which rotates with the motor, is usually stimulated with a sine wave by an external source. This sine wave is induced as carrier signal to the other two stationary coils, so that the amplitude of the induced signal is dependent on the angle between a fixed and the rotating coil. A sine and a cosine wave are generated as the fixed coils are positioned in a 90 angle, which are illustrated in Fig.1.39. The magnitude of the sine and the cosine are converted with the ADCs and the angle is calculated inside the digital control loop with an arctan operation. The resolver is cheaper and more robust compared to the encoder, but less accurate.
The sine and cosine waves of the resolver have the same frequency as the motor, while the frequency of optical sensors is a multiple of the motor frequency. Using the encoder, a high resolution can already be achieved by counting the pulses and only for the additional resolution the analog to digital converter is used. Using the resolver, the accuracy of the position is totally dependent on the resolution of the
ADC. As the system noise is usually high, a 12 bit converter is typically used together with oversampling to reach a 13 or 14 bit resolution for the angle. Some high-end systems are using 14–16 bit ADCs.
Sine and cosine need to be demodulated. This is typically achieved by sampling the signals at the peak of the carrier signal. Again, it is important to convert the signal at a particular point of time, so that SAR converters are most commonly used. The carrier signal typically operates at 8 kHz, so that delta-sigma converters would also work in this application. The bitstream from the modulator would be filtered in two steps as illustrated in Fig.1.40. A first filter generates a digital word with a medium resolution around 12 bit at a medium frequency such as 128 kSPS. Now, the signal needs to be demodulated. This means the 12 bit digital signal needs
Ymod CLK Trigger e.g. 14bit, 128kHz at OSR=128 e.g. 16.384MHz bitstream PWM Carrier signal to resolver
32bit integrator OSR=4..128 Demodulation Sinc filter OSR=4..256
Fig. 1.40 Digitizing a resolver output using a delta-sigma ADC and a dual digital filter [32] Resolver
Carrier signal Sine output
Rotating carrier coil Stationary signal coils
Cosine output
to be multiplied with +1, if the carrier is positive, and with1, if it is negative. The time delay of the carrier signal through the resolver, the wiring, the modulator and the first filter needs to be taken into account.
Frequencies, which are an integer multiple of the modulator clock divided by the used over-sampling ratio, are suppressed. These points are called notches. In this concept, the decimation ratio of the integrator is chosen in a way that the carrier frequency is falling into a notch and is canceled by the digital filter. The carrier signal can also be generated digitally. As an example, at a fixed carrier frequency is programmed in bitstream format and is applied to the carrier coil of the resolver. The resolver shows a low-pass behavior and generates a low distortion carrier signal.
1.3.1.4 Auxiliary Signals
Some applications set the anticipated speed with an analog input voltage. This voltage would remain constant for a long period of time, so that a delta-sigma approach would be ideal. A motor control environment is typically rough for analog signals as distances are large and a significant amount of current is switched with the IGBTs generating distortion. A signal range of10 V is therefore preferred in this application. As an interesting fact, there are basically no delta-sigma converters with such an input voltage range on the market, so that also here the SAR converter is frequently used.
The SAR converter can easily multiplex between channels. A SAR converter, which is used to monitor an analog input voltage, could additionally be used for other tasks such as measuring the temperature or the DC link voltage.