Benefits of Sensorless FOC

One of the two main benefits of FOC is lower cost. This is due to replacing the Hall-effect rotor position sensors, connectors, and wiring harnesses with shunt-resistor op amp circuits on the control board, which instead measure motor phase currents. The other main benefit is improved energy efficiency, since the stator field coils are excited in a precise manner that optimizes torque generation. An ancillary benefit is quieter control, since the rough on-and-off cycles of six-step trapezoidal control are replaced with the smooth PWM ramp ups and downs of sinusoidal excitation.

Executing a FOC algorithm does increase the processing requirements. A high-speed, efficient 16-bit processor with DSP capability, known as a digital signal controller (DSC), is ideal for this application. While 32-bit MCUs can also be used, they are really unnecessary, as all the sensor inputs are coming from a 10- or 12-bit A/D converter.

For tight, accurate control with FOC, a few critical A/D and PWM module features are required. The A/D converter must have multiple sample-and-hold channels. By taking a simultaneous snapshot of the currents in two or three phases, time-delay measurement errors are eliminated. Another important feature is the ability to synchronize the A/D sampling with the PWM. If the A/D takes the sample too late, then bad data are given to the control loop. Having the ability to determine when to sample each A/D on an individual PWM basis provides the optimal control-loop results.

Methods for Obtaining High Drum-Spin Speeds

Mechanical transmissions can be designed with two different gearing ratios, one for normal agitation speed and another for high spin speed. The switching system is usually mechanical or electromechanical, but hydro-mechanical systems also exist. However, the disadvantages of these systems are the extra cost and their complexity.

Another approach is to use oversized motors, such that the full rated speed is used for spinning, while slower speeds are used for normal agitation. The obvious disadvantage here is that the larger motor costs and weighs more, and provides a lot of torque capability at high speed that is not needed or used, which can make the total system cost less competitive.

Field weakening is a method for achieving high spin speeds without the cost of additional transmissions or oversized motors. Instead, the existing control circuitry is used. However, software calculations, including fractional multiplication and single-cycle multiply and accumulate (MAC) instructions, require the selection of a DSC that features a higher-speed 16-bit processor with DSP capability.

Field Weakening

Field weakening is a motor control technique that allows a motor to run faster than its rated speed. Using this control method should provide at least 1.5 times the rated speed, achieving even faster speeds with buried magnet rotor designs. Spinning the motor up to its rated speed is referred to as operating in the “constant-torque region,” where the motor’s available torque is constant as the speed is varied (see Figure 2). Faster operation moves into the “constant-power region,” where torque and speed are inversely proportional. In a washing-machine spin operation, due to momentum, little torque is needed to maintain or increase drum speed; therefore, diminished torque is an acceptable tradeoff.

To understand how field weakening works, it is beneficial to first understand the basic operation of both sensor-based six-step and sensorless FOC on a brushless permanent-magnet motor. In order to entice a rotor to turn, the motor stator coil that is slightly ahead of the rotor magnetic field is energized, drawing the rotor forward toward it. In six-step trapezoidal (also called 120-degree control), this is accomplished in a sequential manner—stepping forward in response to the Hall-effect sensor inputs identifying the rotor position. Speed is controlled by increasing or decreasing the PWM high-time (or duty cycle), thereby adjusting the power input to the motor. Even higher speeds can be obtained by commutating up to 60 degrees (in time) prior to receiving the Hall-effect sensor signal. This process is called phase advance.

In FOC operation (also called vector control), all motor stator coils are energized in a smooth, quiet sinusoidal fashion, with each of the three phases energized 120 degrees out of phase from each other. Three PI control loops regulate speed, torque (Iq), and flux (Id) (see Figure 3). Rotor position is determined by sensing one or more motor phase currents and matching the measurements to an electrical model of the motor. Speed is controlled by adjusting the voltage vector going to the motor coils. To produce the maximum torque, the stator field is energized 90 degrees ahead (by timing) of the rotor field, by setting the Iq reference to zero.

Field weakening is an optimized version of phase advance. Instead of simply energizing the stator fields to keep 90 degrees ahead of the rotor, field weakening also produces a stator field component that opposes the rotor’s magnetic field. This reduces the stator back EMF, which acts as a braking force. To accomplish this in FOC, the Id (flux) reference is set to a negative number, while the Iq (torque) reference is set to the desired torque value.