Precise position? Absolute.
Mario Zecchin
From Motion Control Applications to Digital Sales, creating Technical Webinars, e-Learning and Trainings.
Many applications need position information: filter wheels, robotic arms, grippers, surgical tools and thousand of device benefit from the position detection during operation.
During Power-ON the position of each mechanical part in the application is unknown. The system must be set in a defined position for the correct synchronization of every section: this process is called homing, and depending on the application complexity it takes several seconds before operating the system. Then you need switches or proximity sensors, wiring, testing...
With the help of absolute encoders the position is known directly at power-ON, without losing time for the homing operation.
Going deep, FAULHABER absolute encoder delivers a numerical value between zero and a defined maximum (by default is 0 to 4095, resetting at every revolution). Switch-ON, read the value, control the position. Easy.
There are two types of absolute encoders that can be integrated in FAULHABER motors: single-turn and multi-turn.
A single-turn absolute encoder can be used for example to drive a small optical filter wheel in direct drive operation. Every filter has its defined position, so in case of a 6-optical filters disc the position of every filter is on step of 60°. Dividing 4096 counts in 6 we obtain a value of 682.666, let's make a rounded value of 683. The six position will be at 683, 1365, 2048, 2730, 3413, 4095 counts (yes... to compensate the rounding error I have also alternated the step values 683 and 682). Then the six numerical values will be used for the target position. At Power-ON the system will move the filter disc to the desired position.
A multi-turn absolute encoder can be used for example with linear actuators, or with geared systems where more than one revolution must be tracked. The multi-turn value is delivered just before the single-turn value on the data telegram sent by the encoder. The number of turns is in 16-bit format (0 to 65535), while the single-turn position value is in 12-bit format (0 to 4095).
How to read the absolute encoder values? With the BISS-C standard.
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There are two digital lines used by the motion controller to read the encoder values: the clock line and the data line. The motion controller sends a train of clock pulses to the encoder, which responds with Acknowledge (Ack), Start of frame and the Control Data Slave (CDS) bit. Then the number of turns is transmitted as 16-bit sequence (most significant bit as first). After the 12-bit sequence for the single-turn value is transmitted (again, first the msb). The nERR bit is low whan an error is detected, and the nWARN indicates a warning. The 5-bit Cyclic Redundant Check ends the telegram. The receiver must calculate the CRC value of the received telegram. When the two CRC values are equal, then the data transfer has been successfully performed.
You can read more about the absolute encoder in this application note:
For the Multi-turn absolute encoder operation you can also read this application note:
Thank you to have read until the end of this article!