Transcript MMT Encoder Upgrade talk

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
• Ongoing azimuth servo
upgrades open opportunity for
main-axis upgrade effort
Options for System Repairs/Upgrades
Option
Remarks
Do Nothing
Potentially compromised servo performance (25- vs. 27bit resolution)
Parts and repairs on existing Inductosyn encoders are
difficult and expensive to carry out
The recent resolver failure is a point of vulnerability due to
a lack of spares for internal parts critical for encoder
operation.
Add RON905 “stack” for
high-resolution counting
Uses existing absolute encoder for pointing data, highresolution incremental channel for 1/T period counting for
velocity feedback
Saves cost at the expense of mechanically complicated
mounting that may make servicing absolute encoder
difficult
Replace Tapes
Replace Encoder with
RCN829
Brings elevation tape encoder back into full operation
Exposure to a repeat of damage to tapes
Encoder alignment is critical; potential for velocity jitter
due to alignment being less than perfect
Costs almost the same as a new absolute encoder
Simplifies wiring and system cabling for encoders
Lower exposure to damage compared to tapes
Increases resolution by a factor of 16X
Allows 1/T counting implementation in parallel with 29-bit
absolute output
Costs very nearly equal to the cost of new tapes
Ro
tat
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Absolute Encoder System
Excitation Signals
512-cycle Inductosyn
Power Supplies
REF
Absolute
Encoder
10kHz Oscillator
SIN/COS
Resolver
Preamplifier
Angle Conversion
16-bit Raw Values
Optocoupler Interface
SIN/COS
Preamplifier
Encoder shaft turns resolver once per
revolution. SIN/COS angle signals from
resolver and Inductosyn are converted to
16-bit digital values via Analog Devices
AD2S80 converters. Oscillator board
provides excitation to both resolver and
Inductosyn, and reference signals to
AD2S80s. Parallel digital output data go
to mount computer via opto-isolators and
IP-Digital48 units on IP-carrier board.
Software then converts the raw values
into a single 25-bit absolute angle.
IP-Digital48 I/O
Mount Computer
IP-Carrier
MMT Observatory
Absolute Encoder
Block Diagram
2/19/2010
Problems with Absolute Encoders
• 512- and 1024-cycle/rev error terms in Inductosyn
• Null error at cardinal angles of resolver and Inductosyn
electrical cycle
• Low-frequency spatial error from resolver outputs
• Complex analog signal-processing hardware produce
error terms, some of which are subject to aging and drift
• 25-bit resolution lower than tapes OR absolute encoder
offerings
• Velocity estimation noise considerably higher compared
to using tapes or RCN829
• IP-Digital48 is end-of-life and not easily replaceable
• Internal encoder parts are extremely difficult to repair or
replace as the recent resolver failure shows
Proposed New Encoder
Heidenhain RCN829
Guaranteed ±1” accuracy
29-bit resolution
414 counts/arcsecond
32768-line incremental channel
EnDat 2.2 interface
Upgraded Absolute Encoder Block Diagram
tat
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on
Absolute
Encoder
Ro
29-bit Absolute Position
Values
Heidenhain RCN829
EnDat 2.2 Interface
IK220
Sinusoidal
Scanning
Signals
Interpolation
Electronics
A/B Quadrature Signals
(Future) Period Counting Interface for highresolution velocity estimation
FPGA PCI Counter
Mount PC
MMT Observatory
Absolute Encoder
Block Diagram
2/19/2010
Considerations for Encoder Replacement
• Mechanical – Can the RCN829 be mounted to achieve full
accuracy?
• Electrical – Can the RCN829 be cabled and powered for correct
operation?
• Environmental – Does it work at 2.5km altitude over -10 to 30C, and
is it ozone-resistant?
• Integration – Can the RCN829 and associated IK220 readout board
be used in the existing mount control system software?
• Performance – What performance in pointing and tracking can be
expected, and is it sufficiently higher than the existing system to
justify it?
• Cost – How does the cost compare to replacing the tape encoders
or improving the performance (e.g. RON905 stack) of the existing
encoder system?
• Time – What is the lead time? How long to complete the
replacement?
• Staffing – How many people and how much effort will be needed?
RCN829 Mechanical
Mounting Overview
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?
Front-mounting arrangement
Rear-mounting arrangement
Mounting dimensions for 100mm dia. shaft RCN829 unit
One Implementation
Electrical
• One M23 coupling cable is required for
connection to encoder
• Max cable length is 150m for EnDat 02 option,
MMT can easily use 25m cables
• Power required is 3.6-5.25V @ 350mA
• For implementation of period counting,
intermediate connection is required to split out
incremental signals for separate interpolation
• Initial deployment simply connects directly to the
IK220 board for both power and signals
Environmental
• Viton seals are supplied, ozone resistance is very good
• -10C will stiffen the bearing grease and seals a bit, and
testing in Perugia showed self-heating is sufficient to
keep the encoder working (in Antarctica…)
• Altitude is also within the operating tolerance
• Clean dry air can be supplied to the unit to reduce
contamination (recommended!)
• Unknown what to do for lightning protection; connection
to PC via IK220 is technically a violation of long-standing
electrical isolation policy for mount control systems
Integration Issues
• Mount computer has no available PCI slots; we are
ordering new passive-backplane PCI system to address
this problem for other reasons
• IK220 board needs to have a driver developed for use
with VxWorks and xPC Target for testing and operations
• Software using the new readout electronics and control
design needs to be validated
• Removal of existing encoder is a potentially
unrecoverable change in terms of re-acquiring the
original position should installation of RCN829 fail and
require changing back
• New pointing coefficients must be gathered, requiring a
scheduled night (or two) for this activity
Performance Modeling
Input shaft angle has error terms added to it, then is quantized and a
backlash applied for resolution and hysteresis simulation
Heidenhain Error Chart
Pointing Error for 36 Random Positions 0…360°
Pointing 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.
Inductosyn
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
RCN829
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 Inductosyn
High-speed and Sidereal Tracking Simulation using Elevation Velocity
Estimator
Comparison of 25- and 29-bit Position Error and Velocity Error at 1”/sec
Velocity Estimation Remarks
•
•
•
•
In every case, higher resolution gives finer velocity estimation outputs (5.6”
p-p vs. 0.35” p-p, 16X as expected)
Estimation jitter frequency linearly related to quantization noise and
proportional to velocity (Fjitter ≈ (1.7 * velocity) + 28) (at 25 bits)
Jitter noise gets amplified in controller’s forward path – minimize wherever
possible
But:
–Shaft windup can be an issue; a high-quality coupling is absolutely
necessary
–Velocity jitter as was experienced with tape encoders may be
encountered; fix with velocity-jitter estimator(?)
–Modeling period counting may tell us more about what to expect
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
Cost to Repair Tapes
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
New Equipment Cost
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.
Time, People and Operations
• Lead time for RCN829 is 4 weeks ARO, 12 weeks for
replacement tapes
• Already working on IK220 readout board drivers to
support other encoders
• New mechanical mount needed, design should start very
soon if this course is taken
• Encoder repairs have been effected, but we are currently
running essentially without spares for encoders
• Already working on upgrading azimuth servo, opportunity
exists to do parallel development to support
• Need 2 mechanical (1 designer, 1 drafter), 1 electrical, 2
software people to complete work by 2011
Is it Worth it?
Pro
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 velocityestimation techniques
Con
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 fitted
Recommended Action
• Short term—
– Apply whatever repairs/fixes can be to bring the tape encoders back into
operation as much as possible
– Repair/replace bad elevation resolver, buy a spare
– Begin development of software for IK220 readout boards
– Replace slot-limited mount computer hardware
• Long term—
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–
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Plan to replace encoders with RCN829 units
Due diligence w.r.t. study of alternate vendors/offerings
Study mounting requirements and FEA of shaft coupling
Can RCN829 be mounted on West side of cell for
testing/integration/tracking performance verification before installation
on azimuth?
– More detailed design simulation (e.g. period counting) to determine
performance pitfalls and explore limits on controller gains