Field-oriented control for motors

Sophisticated motor control is no longer the sole province of custom hardware and proprietary control techniques. Advances in powerful, low-cost digital-signal controllers (DSCs) let designers use advanced motor- control techniques. For example, power-factor correction (PFC) and field-oriented control (FOC) save energy and quiets rotating machinery.

Prior to the arrival of the digital- signal controllers, power factor correction and field-oriented control techniques required costly, custom-made applicationspecific ICs. Proprietary motorcontrol hardware was inflexible, lacking provisions for modifying it to better fit an application.

Older motor-control schemes relied on scalar motor control with power semiconductors. Scalar control varies a generated sinusoidal wave’s frequency along with its voltage. Feedback, when used, tracked only the coarsest and most rudimentary functions, such as motor speed. The control elements could not process data fast enough to measure direct parameters like instantaneous current in the motor windings.

Prior motor-control drives with advanced algorithms, such as those for field-oriented control, contained costly digital-signal processors (DSPs). The high-current MOSFETs and other powersemiconductor devices made the drives physically bulky.

Though the latest motor drives still need MOSFETs and IGBTs, they are much more compact and efficient. More significantly, new motor drives use the DSC’s built-in logic and control capacity, eliminating the expense and installation of additional components. This resulted in smaller, lessexpensive drives that consumed little electrical power, yet had the speed and processing power to perform more advanced control functions.

Permanent-magnet synchronous motors (PMSMs) with dynamic response react quickly to speed changes, such as switching between agitation and spin in front-loading washing machines. Sensorless FOC algorithms controlling PMSMs appear to provide such responses with greater efficiency than scalar control on standard induction motors. Controlling stator current by FOC also reduces torque ripple for quieter motor operation.

Digital-signal controllers contain interfaces tailored for motor control, such as pulse-width modulators (PWMs), analog-to-digital converters (ADCs), and quadrature- encoder inputs. Most of their instructions execute in a single clock cycle for fast response times. The combination permits software-based digital filters and improves event response times without the need for additional hardware.

Some analog-to-digital converters in DSCs convert input samples at rates up to 1 million samples/sec while handling four simultaneous inputs. The high sample rate is crucial for current sensing in motor controllers. The only additional component needs are inexpensive sensing resistors to measure motor-phase-winding currents.

Admittedly, DSCs are overkill in some low-end applications that do not require high performance. When compared to motor control with application-specific ICs and 8-bit MCUs with custom motor controls, DSCs have complex peripherals that would appear to make design cycles longer and raise costs. Engineers may feel forced to invest in high-level-language development tools, such as C compilers and other mathematical software libraries, adding to initial development costs. They may find it hard to justify these additional costs when simpler control techniques will suffice.

On the other hand, there are many areas where DSCs have major advantages over the “blackbox” ASIC approach. These include both sensored and sensorless motor control in appliances, fuel pumps and power steering in cars and trucks, and high-speed servos in automation and robotics. In all of these, motors need high-speed response under varying loads.