Where the magnetic field hits the road

Practically speaking, your selection of motor and amplifier will often dictate the control technique that you will use.

If you are using a brushless DC motor for positioning, then sensorless control is not an option. You will need an encoder and most likely a Hall sensor as well. If you want to gain the maximum performance from your motor you will gravitate toward field oriented control. However unless you are prepared to build your own amplifier (a daunting task for most) you will purchase a packaged drive with this feature built in.

In this configuration the drive generally includes high-level motion control functions such as profile generation, position servo loop, and PLC-style inputs and outputs. There are a number of compact, single axis drives that offer Hall-based, sinusoidal, or field oriented control. These drives are typically located on a RS/485, CANbus, Ethernet, or other serial bus. All you need to do is hook up the motor and power, and send commands.

If you are using a card-based approach, either one that you have purchased, or one that you have designed yourself, your ability to adopt field oriented control is limited. This is because most off-the-shelf amplifiers input an analog +/- 10Volt control signal and do not provide field oriented control. The ones that do are generally expensive, because they include a lot of features that you will not be using by having a separate motion card. There is good news however, which is that there are a number of vendors which offer sinusoidal commutation control. This can be done in the motion card by outputting two analog +/- 10V signals representing the A and B phase desired current. For many applications sinusoidal commutation provides a huge improvement over 6-step Hall based commutation, and is still an excellent choice for a wide range of motion applications.

OEMs steer toward intelligent drives

If you are using brushless DC motors for velocity control applications, such as in centrifuges, tape drives, or other non-positioning applications, you have a number of design options. Sensorless control is certainly a possibility, although drives that offer sensorless field oriented control are still rare. More common are sensorless drives that provide a sinusoid-like commutation function. These can be purchased at the IC level, or at the drive level.

If you are working with AC induction motors, you are probably designing for a velocity or torque control applications rather than positioning applications. Much discussed in technical journals, practical examples of AC induction motors being used as positioning are rare except for some specialized domains such as very high power drives.

In any case, you have many choices, but they generally break down into a “design it yourself ” approach, or a “buy the drive” approach. If you decide to buy a drive, you have a range of performance levels to choose from beginning at simple speed control inverters, to very sophisticated field oriented and flux vector drives.

If you decide to build your own controller card or amplifier, simple variable speed control is not that difficult to achieve if you are familiar with basic inverter design and MOSFET or IGBT switching techniques. For more advanced designs you can look to available off-the-shelf ICs to perform field oriented control for AC induction motors.