By Jim Woodward
Copley Controls Corp.
Edited by Miles Budimir
There is a critical relationship in brushless servomotors between current and torque. The amount of current dictates the magnetic field strength of the motor's stator which, in turn, determines torque. If the rotor and stator magnetic fields are not perpendicular, the motor doesn't produce maximum torque. But a technique called field-oriented control promises to maximize torque output of servomotors by precisely controlling magnetic-field alignment.
Field-oriented control, combined with sinusoidal commutation, lets servomotors deliver a flatter torque-speed curve without roll-off at higher speeds. It also lets amplifiers and motors run cooler, letting smaller motors do the job of larger ones. It takes less power to produce motor torque at high speeds, reducing amplifier and motor heating and making the system more reliable.
Field-oriented control depends on a critical relationship between motor and stator magnetic fields. Specifically, for maximum theoretical motor torque, the magnetic fields of rotor and stator must be at right angles to each other. A 40-Mips Motorola DSP executes an algorithm that maintains 90° angular displacement for loads and speeds to 30,000 rpm.
The motor has three stator coils, each energized by respective coil currents. These sinusoidal currents set up the motor rotating magnetic field. The rotating field interacts with the rotor's permanent magnetic field to produce drive torque. The amplifier gets shaft-angle information from the motor-driven encoder and adjusts the coil currents to maintain the torque. Field-oriented control uses a current-loop algorithm that gives the amplifier direct control over drive currents at all speeds.
In conventional drive methods, when the alignment deviates from 90°, a fraction of drive current is applied to correct the problem. This usually ends up generating more heat rather than torque. The greater the departure from 90°, the worse the torque loss becomes, and conversely, the higher the motor heating. But field-oriented control devotes the entire load current to useful torque production.
Field-oriented control delivers more torque-per-watt of power input to the motor than conventional motor-drive techniques. Better motor efficiency means less power dissipation for a given load. For a given power supply voltage, field-oriented control delivers both a higher torque range and speed.
Lower voltages also mean less heat. Reduced heat dissipation simplifies system cooling, which lets convection cooling play a bigger role. As a result, amplifier weight, bulk, and cost decline.
The control loop produces wide bandwidth with a sampling rate of 20 kHz in the current loop, 4 kHz in the velocity and position loop. With such high bandwidth, the performance looks like that of an analog amplifier.
From the mathematical end, the essence of field-oriented control is an algorithm. Two of the three phase currents are measured and run through a mathematical transformation. This gives not only the magnitude but also the angle of the current. So the current angle is being adjusted as opposed to just the magnitude of current as happens with a regular current loop. Phase-lag due to inductive time constants are compensated for by active control of the output voltage angle. This keeps the field at 90° and ensures that none of the current produces any stray magnetic field which would weaken the field or create more back-EMF.
A regular current loop compares a current command with a feedback signal. In field-oriented control, there's still only a single current command. But there are a couple of internal inputs that control the phase angle, making it essentially a dual-loop system.
Although not a new technique, sinusoidal commutation helps the servoamplifier drive brushless servomotors with constant torque. This eliminates torque ripple, making the motor run smoothly with significantly reduced hum.
Sinusoidal commutation creates a rotating magnetic field in the stator's air gap and follows the formula:
The servomotor stator coils sit at 120° increments around the motor stator. Each of the stator motor coils energize in a special sequence. The ability to develop constant torque throughout the full 360° of rotor travel is critical in many applications. Torque ripple can cause vibration in machinery and mar surface finishes in polishing and cutting equipment.
Motor-drive power is developed by a MOSFET inverter circuit that generates pulsewidth-modulated signals. The inverter develops bipolar output power for four-quadrant motor control which permits motor reversal and regenerative braking from a single-polarity dc supply. During regenerative braking, the motor converts kinetic rotational energy into electrical energy and returns this energy to the dc supply via the amplifier power bridge. The motor kinetic energy is absorbed by a power supply reservoir capacitor or by a special energy dumper connected in parallel with the dc supply.
The inverter uses a digital switching technique called space-vector modulation. This produces higher motor terminal voltages from the same power-supply voltage than conventional sinusoidal PWM techniques. Combined with field-oriented control, it produces not only a wider torque band but higher motor speeds.
Carrier-cancellation PWM switching minimizes motor-ripple current as well. The 20-kHz carrier signal at each of the inverter outputs produces ripple currents at double the carrier frequency, or 40 kHz. Excessive ripple current at high frequency can cause hysteresis heating of the stationary motor and because it lacks fan cooling, increases the potential for motor damage. Raising the ripple-current frequency reduces these currents. Because ripple currents don't produce useful torque, lowering them increases motor efficiency, which means more torque per watt.
Peak currents as high as three times the continuous current also produce rapid motor acceleration which reduces motor positioning time. Pick-and-place systems, for instance, typically spend more time accelerating and decelerating (which depends on high-peak current) than traveling at full speed.