The customer required precision rotary axis control in the lowest profile form factor for multiple sequential robot arm joints. Key additional requirements included minimal movement at start-up to report absolute position...

Celera Motion Case Study – Precision Rotary Axis Control

Contributed by | Celera Motion

The Challenge

The customer required precision rotary axis control in the lowest profile form factor for multiple sequential robot arm joints. Key additional requirements included minimal movement at start-up to report absolute position, and minimal signal cabling keep the overall arm size as small as possible.
 

The Solution

A series of custom-sized PCB-based encoder modules with FPGA control and BiSS communication interfaces was developed by MicroE, each featuring two standard ChipEncoder™ readheads and a Micro Motion Absolute™ (MMA) rotary grating disk. MMA grating technology employs a standard 40 micron pitch incremental counting track, coupled with a reference track where each index is uniquely spaced. With only a small movement at start-up, multiple index marks are detected, and the measured spacing is compared to a lookup table in firmware to determine the absolute position.
 

The Benefit

The output of the two encoders are averaged for improved rotational accuracy, and absolute position is determined at start-up with minimal axial rotation.  BiSS serial communication protocol allows each successive robot joint to be connected in a daisy-chain arrangement, rather than having dedicated bus cables for each axis, which along with the small size of the ChipEncoder helps reduce the overall size of the each joint. High speed serial communication (32MHz clock, 1µsec update rate) assures high robot responsiveness to the operator’s commands.
 

Specifications

PCB Diameter 20mm
Scale Diameter 15mm
Total Module Height 13.8mm
Serial Interface BiSS C
Clock Rate 32 MHz
Position Update Rate 1µsec
Resolution 85,280 CPR (15.2 arc-sec; 73.7µrad)
Cable Wire Count 5 twisted pairs, 32AWG
 
 
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