Southern African Large Telescope

Prime Focus Imaging Spectrograph

Control System Design Philosophy


Jeffrey W Percival

Modification Record
Version Date Comment
1.1 13-Aug-2002
1.2 02-Mar-2003 Change +/-12 to +/-15V
1.3 22-Dec-2004 Update links to data sheets

This page establishes a vocabulary and lays out some guiding principles for designing the PFIS control system.


Actuator: something that causes an action. Examples are motors, pneumatic valves, and solenoids.

Sensor: A device that detects some physical state or event. Examples include switches, pressure sensors, temperature sensors, strain gauges, Hall sensors, and photodiodes.

Transducers are a subset of sensors; transducers have the additional nuance of changing one physical quantity into another. For example, a temperature sensor might convert temperature into electrical current. A strain gauge might convert mechanical strain into electrical resistance. The nuance is useful, in that transducers often require signal conditioning electronics, whereas switches, Hall sensors, and magnetic proximity sensors may just be configured as simple digital inputs.

An excellent source of information on all kinds of sensors and signal conditioning can be found on the National Instruments company's Developer Zone. Click on the "Development Library" link or the "Measurement Encyclopedia" link.

Indicators and Limits: It is useful to distinguish between indicators and limits. An indicator senses a normal operational state, and is used by the software to manage its actions. A limit has the connotation of being bad; limits should not normally be activated during normal operations. Examples of indicators are when the slitmask elevator is at a magazine station appropriate for fetching a slitmask, or when it is at the position at which it is appropriate to insert the slitmask into the beam. Limits usually imply some end-of-travel condition, where further movement would encounter a hard stop or other illegal mechanical state.

Interlocks prevent the instrument control software from putting the instrument into an illegal state. Interlocks guarantee the safety of the hardware. There are two levels of interlocks, software and hardware.

Software interlocks are implemented in the LabVIEW instrument control software running on the Windows PC. Software logic will use indicators (not limits) to sense and manage the state of the instrument. This logic should prevent the instrument control software from sending the instrument into a limit condition.

Hardware interlocks take over if the software interlocks fail, due to a programming error, computer crash, network outage, or other PC-related anomaly. Hardware interlocks are implemented with discrete or programmable logic hardware on a custom circuit board in the instrument electronics box. Hardware interlocks can also rely on the limit detection capabilities of the motion control cards used in the PXI chassis.

It is of great importance to be able to understand and diagnose a hardware interlock situation.

Limit Switches

Limit switches detect harmful situations like end of travel. A broken wire to the limit switch should indicate the limit, so the interlock system will protect the system in the presences of a failed switch.


PFIS will receive AC power from the facility, and will convert it to various DC voltages to run the actuators and sensors. We are already planning to supply

Additional voltages (such as +40V) are possible, but represent possibly significant changes to the power and distribution system (weight, heat, wires, connectors, and so on). Such additions should be considered carefully.

Stepper Motors

Stepper motors need motor driver electronics. Some vendors offer only motors, requiring a 3rd party solution for the driver. Some vendors, like Oriental Motors, offer motor/driver combos. We prefer single-vendor combos, to minimize the chance of incompatibilities. The Oriental Motors PMC series offers small motor/driver combos, and the CSK 2-Phase series and CSK 5-Phase series offer larger motors from the PK series.


The standard National Instruments motion control scenario is to close the loop around a stepper motor with an incremental quadrature encoder, or an analog signal. They offer hardware support for both kinds of encoder inputs, together with end-of-travel limits and a home index mark. This is a good model to stick to, as they offer LabVIEW modules that can manage the whole home-seek/initialize process.

With quadrature encoders, we distinguish between a "home" indicator and an "index" indicator. A home indicator is used to initialize an incremental encoder to a known value. It is sought out during initialization. We use "index" to mean the digital signal that some incremental encoders put out, usually indicating a full turn of an encoder disk.


We like pneumatics for their low power dissapation and power-free holding. We prefer proximity sensors at the home and extended positions, but don't need encoding of intermediate positions.


Solenoids can get hot with extended operation. Our strategy is to use them with short duty cycles. Energize them for some brief operation, like moving a wheel, and then de-energizing them for, say, a prolonged exposure. This implies working against a spring for the short part of the cycle, and letting the spring act during the long part.