Benefits of Sensorless FOC
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.
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
Methods for Obtaining High Drum-Spin Speeds
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.
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.
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.
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
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.
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.
Figure 3. Sensorless FOC of a permanent-magnet synchronous motor with field weakening provides very high spin speeds and optimal torque output without expensive Hall sensors and cables.
high deceleration rate is also important. Due to momentum, the drum and
motor can take several minutes to spin down and stop naturally.
Spinning down quickly after the spin cycle is completed saves time.
More critically, for safety reasons, when the lid or door is opened,
the drum must be stopped quickly to avoid potential injuries.
meet these deceleration goals, braking is often employed. This can be
accomplished by a supplemental mechanical braking system, which adds
materials cost and system complexity. Another approach is to slow the
motor electronically using a large external brake resistor. In the
latter case, a resistor is switched into the circuit and abruptly slows
the motor by limiting the current flow from the motor to ground.
Although this method works, the braking resistor and switch still add
significant cost to the system. A lower-cost method is to implement
braking works by controlling the magnetic flux of the stator coils. To
change from high-speed spinning to braking requires that the FOC Id
reference (flux) be stepped down from a negative number back to zero
(i.e., the constant-torque operation range). At the same time, the
reference for the speed PI control loop must be set to zero. The Iq
torque reference can also be changed to a negative number (implying
negative torque). These software variable changes direct the FOC
control algorithm to effectively slow both motor and drum spin, and
should be implemented at a rate that optimizes deceleration without
blowing up the power electronics. Experimenting to find this optimal
rate can be an exciting part of the motor-control development process.
further increase energy efficiency and improve torque control, the
trend in washing machine motor control is moving from split-phase AC
induction motors to three-phase permanent-magnet synchronous motors. At
the same time, to reduce cost, the trend is going from sensor-based to
sensorless FOC. The field weakening software control technique helps by
providing much higher spin speeds to extract water more quickly,
without the additional component costs.
About the Author: