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Hall sensor completes micro drive
Closed-loop control permits 3-, 5-, or 8-mm positioning systems, maximum torque output
By David J. Bak, International Editor -- Design News, October 18, 1999
Biel, Switzerland-The hall sensor mounts on a flexible circuit; the circuit fits inside a plastic housing that snaps onto the motor housing. A small magnet, glued to the motor backshaft, is the sensor "target."
By creating a sensor module at the back of the motor, instead of integrating several hall devices inside the motor's very limited space, RMB Miniature Bearings now offers modular positioning systems based on their 3-, 5-, and 8-mm smoovy® motors. The integrated sensor module also means precise control of speed, torque, and other motor parameters.
RMB's modular sensor owes its success to what Business Unit Manager Albert Birkicht refers to as a "vertical" hall device. Conventional hall sensors are usually triggered by a magnetic field that is, in principle, vertical to its surface. "Our hall sensor," he explains, "reacts on a magnetic field that is parallel to its surface.
Birkicht claims this change is achieved by using a thicker chip and by incorporating a special manufacturing process for the wafer. The trick, he says, is to limit the current laterally by a guard ring made by deep p-diffusion.
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| Three-branch sensor simplifies manufacturing and reduces the number of external connections needed for operation. The polarization current is injected in the common central electrode; sense contacts are placed between the current electrodes in each branch. |
Depending on application, the sensor's analog output signal can be treated in different ways. Sinusoidal signals, for example, can be amplified and fed back into the motor for closed-loop control. "We nicknamed this approach the 'electronic brush,'" says Birkicht, "because it accomplishes what a mechanical commutator with brushes does in a standard dc motor without all the disadvantages of the mechanical solution like wear and tear. Additionally, the electronic solution provides proper sinusoidal signals to the motor coils and does not just switch them on and off, which results in better performance and fewer EMV problems." Laboratory tests show that when used for positioning purposes, the analog signal allows an accuracy of up to 0.1 degree at the motor output shaft.
Closed-loop control of motor parameters means dramatically improved system efficiency. Again, Birkicht explains: "In an open-loop system, our motors are dominated by their resistive behavior. Current draw, therefore, is more dependent on coil resistance than load."
The effect, he states, is that energy input is more or less the same, regardless of how much torque the motor delivers. All unneeded energy is transformed into heat, i.e., loss. Additionally, there is the problem of switching the stator field at the right time--when using a square wave signal, for example. Ideally, stator and rotor field should be at 90 degrees to produce maximum torque output.
"If you don't know where the rotor is, you are never optimal. With the hall sensor, you know where the rotor is at any given moment and you can adjust the coil switching accordingly to produce maximum torque output."
Besides their "electronic brush" controller based on the integrated hall sensor, RMB offers a self-commutating PID system with positioning capabilities based on a DSP or 8-bit microprocessor design. Depending on user requirements, the interface can be analog or digital.
Additional details...Contact Thomas Rudziensky, RMB Miniature Bearings Inc, 29 Executive Pkwy., Ringwood, NJ 07456; Tel: (973) 962-1111; E-mail: TARudziensky@compuserve.com. .
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