Southern African Large Telescope
Prime Focus Imaging Spectrograph
Generic Mechanism Specification
Jeffrey W Percival
||20 Aug 2002
||correct E5D part number, use AD590 can package
||13 Sep 2002
||change the home sensor
||13 Feb 2003
||add slitmask station mockup
||fix encoder & motor links
This note specifies the generic mechanism to be built up during the PFIS critical design period.
The goal of the mechanism lab is to provide a bench test environment for prototyping control software and testing various types of actuators and sensors. This will give us confidence in making decisions about the design of PFIS mechanisms.
The main areas to be addressed are:
Here is a list of items for the mechanism lab:
- Fiber-optic communication with a remote controller
- Open loop control of a stepper motor
- Closed loop control of a stepper motor
- Analog (voltage) feedback
- Digital (incremental quadrature encoder) feedback
- End-of-travel and other interlock prototyping
- Pneumatic actuation
- Analog input (temperature sensing)
- Digital I/O
- high resolution station indicator
Electronic fabrication will be limited to signal conditioning electronics, wiring, and connectors.
Mechanical fabrication will be kept to a minimum. We will need a fixture to hold the motor and encoders. The motor will drive the linear slide, as in the slitmask elevator. Scott Buckley will design and build this, and will take little or none of Mike's time.
Pneumatic air: provided at various points in the SAL lab area. We will filter it.
- PC for control (Rich will provide)
- LabView software (Done)
- PXI/MXI System (Done)
- PXI Motion Controller (Done)
- Stepper motor and driver (Done)
- Linear Bearing (Scott has one)
- Lead Screw, Power Nut & assoc. hardware (Procure from PIC Design)
- Switches & sensors (Received samples from Cherry, Done)
- Pneumatic actuator and sensors (Done)
- Quadrature Encoder (US Digital)
- Analog encoder (Cable position transducer, Done)
- Temperature sensor (AD590 & conditioning electronics, from stock)
- Miscellaneous h/w, aluminum, etc. (From stock)
Bench area: we need to be assigned some space to use for at least 2 years. Bench, network, pneumatic air. We might consider cleaning out the small shop, currently unused, or some space in the "SHS area".
- Stepper Motor and Driver. We will use the Oriental Motors PMC33A3 and the Vexta model PMD03CA driver board that we already procured. It has 500 steps per turn and produces a torque of 4.58 oz-in (0.033 N-m) at 1200 RPM.
- Linear Stage. We will make up our own linear stage. Scott has an IKO LWL 15 linear slide, with a 9.5" track rail and a slide unit with 6" of travel. Scott also has an Acme lead screw, 14" long with a lead (the amount the nut advances in one turn of the screw) of 0.1". At 500 steps per turn, this implies a linear step size of 5.08 microns, and at 1200 RPM would advance the slide at 2"/s, or 3 seconds for the full travel (ignoring acceleration). This seems acceptable.
Do we have sufficient torque to drive the screw? For torque, PIC Design offers a chart that helps. It shows the torque required to turn a screw while raising a 1 lb. load. We will be driving the load horizontally, not against gravity, so this seems like a good upper limit. Their 0.1" lead screw shows 0.64 oz-in, which gives us a margin of a factor of 7. Our cable position transducer adds 14 oz of tension, so doubling the required torque to 1.28 oz-on still leaves us with a margin of more than a factor of 3.
- End of travel (limit) switches. We will use the samples we have from Cherry. At the end farthest from the motor (the forward limit), use the VN101503 3-wire sink Hall Effect Vane Switch with the vane mounted on the moving slide. The "03" tag selects a model with leads instead of pins. We will want to adjust the trip point by moving the vane parallel to the screw. At the near end (the reverse limit), use the MP200702 2-wire Threaded Barrel proximity sensor, with the button magnet mounted on the moving slide. We will adjust the trip point using the threaded nature of the sensor body. The "02" tag selects a normally closed switch, rather than normally open.
- Home Sensor. We want a high-precision home position sensor in the middle of travel. Use the MP201801 2-wire Reed switch, with the AS201801 flange magnet mounted on the moving slide. The "01" tag selects the normally open option.
- Cable position transducer. We want to encode the moving slide's position. The cable has a 15" pull. It should be mounted so that the cable is never fully retracted or fully extended.
- Incremental quadrature encoder. We want to close the control loop by encoding the motion of the worm screw. Mount a US-Digital E6D incremental encoder to encode the position of the worm screw.
- Temperature sensor. We want to measure a temperature to test the signal conditioning circuitry, the wiring, and the sampling hardware. Mount an AD590 Flatpack sensor to the body of the stepper motor.
- Pneumatic actuator with proximity sensors. We want to control a pneumatic valve and read its end-of-travel sensors, although there is no requirement to mate the cylinder mechanically to the linear stage. It can just lay on the table, and we can use a vice to keep it from wandering around.
The control valve uses 2 2-wire +24VDC solenoids, and the actuator has 2 3-wire proximity switches, one on each end. We will use the 3-wire sensors, not the 2-wire ones; the 3-wire models have an LED.
- Wire Harness and Connectors. All the wires originating in the mechanism will be brought together into one or more male D connectors. On the benchtop, the power supply, the UMI and SCB breakout boxes, and the motor driver board wires will be brought together into the same number of female D connectors. This will allow the mechanism to be detached easily from the benchtop gear.
- High Resolution Station Indicator: the goal is to synthesize a slitmask station indicator with a very narrow "on" region. Use two vane switches reading two square-wave vanes slightly offset from one another. Combining their output using an (A & ~B) logic, providing a derived signal with a narrow "on" region. Slide the vanes with respect to each other to adjust the "on" region.
- Oriental Motors PMC33A3 Stepper Motor.
- Oriental Motors PMD03CA Driver Board
- Cherry VN1015 3-wire sink Hall Effect Vane Switch
- Cherry MP2007 2-wire Threaded Barrel Proximity Sensor
- Cherry MP1021 3-wire sink Hall Effect Switch
- Unimeasure LX-PA-15-P10K Cable Position Transducer
- US Digital E6D-2048-250-IH-PKG3 Encoder with CA-4217-6FT connector
- Analog Devices AD590 Temperature Sensor (Flatpack package)
- Festo MZH-5/3G-M3-L-LED 2-Solenoid Pneumatic Valve
- 2x Festo SME-8-K-LED-24 3-wire Proximity Switches
- National Instruments UMI-7764 Universal Motion Interface
- National Instruments SCB-68 Connector Block