The field of motion control is heavily dependent on the feedback device. There are a world of issues encompassed in this statement. But I will skip the majority of them and jump to the conclusion that magnetic encoder technology has long been a favorite of mine as a potentially ideal solution for a number of reasons. Magnetic position technology tends to be more resistant to environmental problems such as temperature, dust, dirt and humidity. Since all motors are heat producing systems, temperature restrictions for feedback technology can be a problem. And since electric motors are frequently found in environments where there is dust, dirt and humidity, magnetic feedback would be ideal.
On the other hand, magnetic feedback has been complex and expensive in the past. Resolvers require high precision windings in the sensor and precision power supplies to excite them. Can you spell “expensive”?
Enter the Hall sensor. In spite of the fact that the Hall Effect has been understood since 1879, the use of the technology has only recently become widespread with the fabrication of semiconductor level Hall devices. The Hall sensor as a transistor found very popular application in sensing the three phases of brushless dc motors’ permanent magnet rotor. The bldc motor technology was essentially impossible without this crucial piece of technology because in the early versions of the control, it was impossible to start the motor without knowing which phase to energize. This has been less of a problem with the advent of low cost, high performance microprocessor controls that are able to run brushless dc motors with or without Hall sensors.
New arrangements of the Hall devices into arrays with greater capabilities is where the Hall effect technology intersects the position feedback technology. The Hall arrays are capable of sensing small permanent magnet domains on rings that permit rotorary position to be sensed in either analog or digital form.
While there are a number of suppliers of Hall sensing arrays for motion control, a couple of new twists have been added. The Timken company has added some new features to the Hall array that have additional benefits. Among them are the ability to program the numerical value of the digital output, which can be a very helpful feature that eliminates fractional remainders and rollover error in control systems.
In addition, Timken is introducing a new linear version of the technology which is a real first for the motion community. Most linear motion is the result of converting the rotary motion through a linear mechanical device, either a belt or leadscrew. But the control system is measuring the position from the feedback on the motor that’s driving the system and not on the load. So the mechanical error of the leadscrew or belt is the limiting factor in high performance linear.
And to make the new magnetic feedback really interesting, it is comparable in cost to conventional optical encoders. Which is really going to create some new opportunities for everyone in the motion control field.
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