Southern African Large Telescope

Prime Focus Imaging Spectrograph

Power Distribution and Management Plan


Jeffrey W Percival
Don Michalski

Modification Record
Version Date Comment
1 19-Feb-2003
1.1 21-Feb-2003 Add power budget

This note presents the PFIS power and switching plan.

Table of contents:

Power and Switching Diagram

Switching Diagram

Discussion of Switches

There are 4 mechanical switches:

SALT Isolator Switch: this is the SALT-provided switch in the electrical room that applies 220V to PFIS on the payload. 220V is supplied to the PFIS PXI chassis, and to the etalon controller switches. This allows the etalons to be powered up (using the PXI chassis) long in advance of using the etalons. Ted Williams reports that the controllers like to be on for a day prior to use, to allow the electronics to settle.

This switch controls the two PFIS items that will usually be left on. The PXI chassis will always be left on (in normal usage) because cycling its power requires the PFIS computer (PCON) to be rebooted. Turning off the etalon controllers will require a day or more of settling time before use. We therefore expect that the SALT isolator switch will be left on except during maintenance periods.

PFIS Instrument Power: this switch is the PFIS "maintenance mode" switch. Whenever any invasive work is to be done on PFIS, this switch will be opened. It cuts both actuator and sensor power, but maintains power to the PXI chassis and the etalon controllers. This is also the emergency stop button.

PFIS Motor Power: this switch cuts power to the motors. Sensors and pneumatics are still enabled. This is used when motorized movement is not desired, but still allows pneumatics to hold elements in place.

PFIS Pneumatic Power: this cuts power to the pneumatic relays. Sensors and motors are still enabled.

Power Budget

PFIS dissipates power in two ways: steady state power (e.g. power supplies, electronics) and intermittant power (motors). Our steady-state power is all enclosed in glycol cooled enclosures. Only the intermittant power is used in the beam without cooling, and only in polarimetric modes is it used during an observation. In non-polarimetric modes, the intermittant power is used to configure the instrument prior to an observation.

Steady State Power (cooled)

PFIS will dissipate power when it is in use. The steady-state power dissipation occurs in glycol-cooled boxes.

PFIS Steady-state Power

Power (W) Comment
Top Hex

2 x etalon ctlr 184 Mfgr spec; measured power is about half this
PFIS Main Box

PXI Chassis 4 Data sheet
3 x Motion Ctlr 15
Power Supplies 36 15A, 24V, 90% efficiency
PFIS Box 1

4 x Drivers 9.6 estimate based on measured Oriental Motors board
Interlocks 1 estimate from EE
PFIS Box 2

4 x Drivers 9.6 estimate as above
Interlocks 1 estimate from EE

SDSU Array Ctlr 4.3
SDSU Pwr Supply 80
CCD Ion Pump Ctlr 40
Star Tracker

Base 15 Measured


PFIS will dissipate 400 W steady-state, in 6 separate glycol-cooled boxes. The allocation to PFIS is 1.1 kW.

Intermittant Power (not cooled)

Intermittant power dissipation occurs when PFIS runs its motors. PFIS motors are not cooled. The situation is mitigated in several ways:

  1. Most PFIS motors have a very low duty cycle
  2. PFIS motors are turned off when not in use; stages are held in place using brakes or back-torque.
  3. Most PFIS motors are used for configuration, not for observing

The exception to these statements involve polarimetric observations, where the waveplate rotation occurs during an observation, and in some situations may have quite a high duty cycle.

Power (W) Comment Time (s) # Ops/hour Energy (J) Average Power (W)
Mask Elevator 14.4
30 1 432 0.120
HWP Rotate 3.5 est. winding = 5V 1 900 3150 0.875
QWP Rotate 3.5 est. winding = 5V 1 900 3150 0.875
Focus 5 estimate 5 1 25 0.007
Grating Magazine 7.5 est. winding = 5V 20 1 150 0.042
Grating Rotate 9.6
5 1 48 0.013
Filter Magazine 14.4
43 1 619.2 0.172
Articulation 25.1
70 1 1756 0.488

We point out that this is a worst-case mode, doing 2-waveplate high-duty cycle polarimetry. Without the polarimetry, the time averaged dissipation drops to about 0.8 W.

This analysis assumes a 1-hour exposure for a typical observation.