MMT Encoder Upgrade. D. Clark February 2010. Motivation. Recent elevation tape encoder failure with scratches and marks Failure of elevation absolute encoder resolver (coarse part of 25-bit value) Costs to upgrade encoder very nearly equal to replacement of existing tapes on elevation
Guaranteed ±1” accuracy
32768-line incremental channel
EnDat 2.2 interface
Major mounting tolerances:
Radial runout – 0.02mm
Axial runout – 0.02mm?
Mounting shoulder – 0.1mm
w.r.t shaft centerline
50% overhead on catalog starting torque (2.25Nm) implies 3.84e9N/rad stiffness on coupling shaft to stay below 1 encoder count of error – using steel (77GPa), a 100mm x 50mm shaft has torsion ≈ 61mas…windup must be taken into consideration in control design
Avoid flexible coupling if possible!
Permanently blocks Nasmyth feed. Do we care?
Mounting dimensions for 100mm dia. shaft RCN829 unit
Input shaft angle has error terms added to it, then is quantized and a backlash applied for resolution and hysteresis simulation
512 cycles/rev sin/cos gain error
1024 cycles/rev sin/cos offset, crosstalk
2048 cycles/rev sin/cos distortion
4 cycles/rev resolver null error
8 cycles/rev resolver sin/cos gain error
AD2S80 converter offset and tracking error
Aging, temperature, and drift effects add up as well
Historically ~0.25 to 0.5” RMS
Difficult to correct with pointing coefficients due to large number of cycles per revolution
Long wave error due to encoder tolerances – guaranteed ± 1”
Short-wave error due to signal period error < 1% 32768 lines 0.396” (incremental channel)
Reversal error from shaft hysteresis, guaranteed < 0.4”
RMS of 0.7” is achievable before correction
Long-wave error is repeatable and measured data is provided, and so should be able to minimize with strategic clocking of encoder shaft and many fewer pointing-correction data points compared to InductosynPointing Error Terms from Encoders
Remark: Overall Inductosyn error is smaller(?), but more scattered than RCN829. However, RCN829 error is repeatable and more amenable to LUT or polynomial correction.
Finally, generally speaking, the higher the resolution and less noisy velocity estimation, the higher the velocity loop gain can be increased for tracking stiffness; simulation studies can determine this quantitatively
12-week lead time is much longer than 2-4week ARO time for RCN829, and cost is very close to that with a new encoder
Mounting hardware cost appears to have been underestimated – recommend increase in this value by ~200% to cover FEA and stiff well-aligned mounting fixture to capture full encoder performance.
Pro Error at 1”/sec
Large increase in encoder resolution
Increased velocity-loop stiffness
Much simpler system electrical design
No vulnerability to damage as on drive arcs
Cost very competitive with repairing existing encoder hardware
Replaces ancient analog encoder hardware with modern digital system
More tractable pointing-correction model
Supports more advanced velocity-estimation techniques
Large effort required from small staff to implement
New, expensive mount(s) must be designed and installed
Unknown vulnerability to lightning or other environmental aspects
Possible velocity-loop jitter issue
Is a complete spare encoder affordable? Not having a spare is a risk…Can encoders for both main axes be afforded?
Overall pointing error may be higher, at least until proper coefficients can be fittedIs it Worth it?