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 RemarksDo Nothing Potentially compromised servo performance (25- vs. 27-

bit 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, high-resolution 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 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

Replace Encoder with RCN829

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

Absolute Encoder System

512-cycle Inductosyn

Resolver

Preamplifier

Preamplifier

Excitation Signals 10kHz Oscillator

Rot

atio

n

Angle Conversion

SIN/COS

SIN/COS

RE

F

Power Supplies

Optocoupler Interface16-bit Raw Values

IP-Digital48 I/O

Mount Computer IP-Carrier

Absolute Encoder Block Diagram

2/19/2010

MMT Observatory

Absolute Encoder

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.

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 RCN829Guaranteed ±1” accuracy

29-bit resolution

414 counts/arcsecond

32768-line incremental channel

EnDat 2.2 interface

Upgraded Absolute Encoder Block Diagram

Absolute Encoder Block Diagram

2/19/2010

MMT Observatory

Rot

atio

n

Absolute Encoder

Heidenhain RCN829 EnDat 2.2 Interface

InterpolationElectronics

SinusoidalScanning Signals

IK220

FPGA PCI Counter

A/B Quadrature Signals

Mount PC

(Future) Period Counting Interface for high-resolution velocity estimation

29-bit Absolute Position Values

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

Avoid flexible coupling if possible!

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

Permanently blocks Nasmyth feed. Do we care?

Mounting dimensions for 100mm dia. shaft RCN829 unit

Rear-mounting arrangement

Front-mounting arrangement

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

Inductosyn512 cycles/rev sin/cos gain error1024 cycles/rev sin/cos offset, crosstalk2048 cycles/rev sin/cos distortion4 cycles/rev resolver null error8 cycles/rev resolver sin/cos gain errorAD2S80 converter offset and tracking errorAging, 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

RCN829Long 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

Remark: Overall Inductosyn error is smaller(?), but more scattered than RCN829. However, RCN829 error is repeatable and more amenable to LUT or polynomial correction.

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?

ProLarge increase in encoder resolutionIncreased velocity-loop stiffnessMuch simpler system electrical designNo vulnerability to damage as on drive arcsCost very competitive with repairing existing encoder hardwareReplaces ancient analog encoder hardware with modern digital systemMore tractable pointing-correction modelSupports more advanced velocity-estimation techniques

ConLarge effort required from small staff to implementNew, expensive mount(s) must be designed and installedUnknown vulnerability to lightning or other environmental aspectsPossible velocity-loop jitter issueIs 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—– 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