1. absolute encoders and motors for -80°C

2. motors Choosing a drive system for our application.
To be taken into account:
speed
mechanical load characteristics
smoothness of motion
resolution
temperature
other environmental conditions
our choose for IRAIT telescope: stepper motors
a stepper motor is an electromagnetic driving system that moves around a particular reproduceable step angle due to defined wiring and triggering of windings (phases). With a motor control that triggers the phases, a quasi-continuous rotation is achieved.
The smallest possible resolution of positioning depends on the motor as well as on the type of drive. An usual value for the resolution of commercial stepper motors is 200 steps per revolution; however, resolution can be considerably increased by electronic micro-stepping.
With the right design of bearings and windings, a stepper motor is able to work in extreme environmental conditions.

3. motors PHYTRON stepper motors, model ZSH87/3.200.5 PLE80/1 3:1
stepper motors advantages for our application:
high torque at low speeds, e.g. during tracking
smoothness of motion, at least using good driving electronics
low temperatures tolerant, choosing the right commercial product
no need for an encoder to get reproduceable positioning (*)
(*) but a loss of steps must be avoided! Typically a bad design and/or a bad tuning of the driving system, in presence of irregular loads, can produce a loss of steps.
Anyway, as you will see, we have ALSO absolute encoders on telescope axes...
our choose:
PHYTRON stepper motors, model ZSH87/ PLE80/1 3:1

4. motors PHYTRON ZSH87/3.200.5 PLE80/1 3:1 2 Phase Stepper Motor
integral gearbox (3:1 ratio)
for -30°C/+8 0°C
5A per phase
5.4 Nm holding torque (before the gearbox)

5. frequency behaviour (control pulses) for two supply voltage values
motors
PHYTRON ZSH87/ PLE80/1 3:1
2 Phase Stepper Motors
integral planetary gearbox (3:1 ratio)
for -30°C/+8 0°C
bipolar current per phase: 5A
holding torque: 5.4 Nm
max. permissible output torque: 40 Nm
torsional backlash: <15‘
number of magnets: 3
permissible bearing axial load: 180N
permissible bearing radial load: 280N
200 steps, 1.8° step angle
insulation class: F
Phytron is well known for its catalogue of in-vacuum and cryo stepper motors
frequency behaviour (control pulses) for two supply voltage values

6. motors our choose for driver electronics: PHYTRON SLS-4xPAB93-70
4 axis Rack Mount Power Drive
including 4 power stages PAB93-70
230VAC power supply
supplier in Italy: MICOS Italia Gmbh
power stage PAB micro 256

7. motors Climatic chamber tests (from 0°C to -80°C) in Germany (Phytron)
Tested system behaviour in working conditions at various temperature values.
Tested speed constance and motion smoothness and resolution at low temperatures, especially for low speeds (tracking).
The system has been calibrated at -30°C. It works without problem at this temperature; with lower temperatures, there will be no damage. Giving power supply to the motors leads their temperature over -30°C; at that point we can move the telescope.
A derating has been applied, to take into account the effect of low pressure values.
We are using commercial off-the-shelf products; the cost-benefits ratio is very good.

8. motors IRAIT drive groups mechanical configuration
2x motors (primary and bk)
2x 90° 1:1 gearboxes
2x 1:20 gearboxes
2x pinions
redundancy and backlash reduction

9. motors IRAIT azimut drive group 2x motors 2x 90° 1:1 gearboxes
2x pinions
1x slewing bearing

10. motors
IRAIT azimut drive group

11. motors IRAIT elevation drive group 2x motors 2x 90° 1:1 gearboxes
2x pinions
1x gear sector

12. motors
IRAIT elevation drive group

13. motors
mounting/dismounting operations...

14. motors special grease... www.solvaysolexis.com
High degree of STABILITY; unmatched chemical and solvent resistance; thermal and oxidative stability; excellent radiation stability.
COMPATIBILITY with a broad selection of materials inert to most plastics, metals, and elastomers, liquid and gaseous oxygen.
SAFE; non-flammable, non-toxic, no volatile organic compounds, NSF certified, BAM approved.
Broadest operating TEMPERATURE range of all classes of lubricants (- 90°C to 290°C).

15. absolute encoders Heidenhain RCN 700 series absolute encoders
Choosing a position feedback system for our application.
We don’t have encoders on motor axes; we use an angular position readout directly
on telescope axes, at the end of kinematic chain (1:1440 total ratio).
To be taken into account:
resolution and repeatability of angular measure
problems with heating (below primary mirror and around optical path -Nasmyth-)
operating temperature
other environmental conditions
our choose for IRAIT telescope:
Heidenhain RCN 700 series absolute encoders

16. absolute encoders
Heidenhain RCN 700 series absolute encoders

17. absolute encoders 29 bits = 536.870.912 positions per revolution
1 arcsec on sky → 414 counts
hollow through shaft Ø100mm

18. absolute encoders
HEIDENHAIN encoders with optical scanning incorporate measuring standards of periodic structures known as graduations. These graduations are applied to a carrier substrate of glass or steel. These precision graduations are manufactured in various photolithographic processes. Graduations are fabricated from:
extremely hard chromium lines on glass or gold-plated steel drums,
matte-etched lines on gold-plated steel tape,
three-dimensional structures etched into quartz glass,
Manufacturing processes and typical grating periods:
40 µm with AURODUR
20 µm with METALLUR
10 µm with DIADUR
4 µm with etched quartz glass
Circular graduations of absolute angle encoders

19. absolute encoders Photoelectric scanning HEIDENHAIN uses the imaging
scanning principle for
absolute angle encoders:
Parallel light hits the grating structure (scanning reticle) and light passes through the gaps.
When the measuring standard moves in relation to the scanning reticle, the incident light is modulated (light / no light).
Photovoltaic cells convert these variations in light intensity into electrical signals.

20. absolute encoders

21. absolute encoders
Heidenhain RCN 700 series absolute encoders

22. absolute encoders Heidenhain RCN 700 series absolute encoders
interface card IK200

23. absolute encoders
Heidenhain RCN 700 series absolute encoders (over LAN)

24. absolute encoders
Climatic chamber tests (from 0°C to -45°C) in Italy (Perugia University)
Tested system behaviour in working conditions (encoder rotated by a motor at different speeds) at various temperature values.
A specialist from Heidenhain supervised the tests.
Heidenhain suggested a series of tests over the electrical signals coming from the encoder, with a particular attention to the incremental signal, to check for a signal level degradation with low temperatures.
The behaviour of the system has been perfect.
The tests doesn’t cover the full range of expected temperatures at DomeC, but the self-heating of the device will keep it over the minimum value tested.

25. absolute encoders tests setup Supervisor PC Climatic Chamber
Chamber RH% sensor
Chamber T sensor (pt100)
Encoder T sensor (pt100)
Motor T sensor (pt100)
Motor Heater
RCN 727 Encoder
PLC
Control
Box
Climatic Chamber
Supervisor PC
RS-232
Power
Supply
EnDat
IK 220 Pci Card
Main PC
tests setup

26. good luck IRAIT!