WHT Dome

The Dome and Shutter heavy current control equipment is contained in cabinets located on the dome rotating platform. These comprise of the Brittain Steel/Grubb Parsons cabinets and the KONI shutter control cabinet. It should be noted that the shutter cannot be driven from the control room.

The dome can be rotated locally from the dome control cabinet (keyswitch to LOCAL and DOME POWER button pushed) or in either Engineering or Computer mode (keyswitch in REMOTE) from the control room. In 2008, the dome control system was upgraded using PLC's and the old TEM-L system taken out of service.

Dome position is measured using a T + R absolute encoder mounted on a bracket which is located on the mid balcony level walkway. A continous chain near the slip rings drives the encoder. Signals from the encoder pass to an ENCODER LINE DRIVER BOX containing differential line driver chips and a 5V PSU which supplies both the encoder and the logic. A multicore cable carries the signals back to the control room. The dome encoder is read by the TCS via a HYTEC 450-4 module in the CLIP CENTRE crate (Address : B6 C3 N5 A0).  Refer to the CAMAC manual for more details.

n.b. Although it was originally planned to encode the dome shutters, this was never implemented and the Baldwin absolute encoders were removed from the dome shutter drives many years ago.

Associated drawings: ED183  ED184  ED200  ED202  ED211  ED214

Dome Slip Rings

To enable continuous dome rotation, two slip rings are used. These are located at the mid balcony level:

Dome control system

To improve reliability and ease of maintanance, the TEM-L system was replaced in 2008 with Rockwell Micrologix ML1100 Programmable Logic Controllers (PLC) working over a WiFi radio link. Using this technique, the slip rings previously used for TEM-L signalling are no longer required. At the moment, there is no Engineering Mode available from the control desk, but this will be added soon. The control room PLC is located above the in the old TEM-L rack in Bay 3. A network connection from the PLC being made to the hub in Bay 1

The radio link is made using Netgear G54 Wireless Access Points. One is located in GRACE and is plugged into the network hub there. The G54 WAP at the other end of the radio link is fitted into an old TEM-L cabinet on the moving dome platform. This connects to another ML1100 PLC, the analog output going to the dome control TASC servo unit. The analog output direct from the PLC made the old Analog to Digital converter in the dome control cabinet obsolete and was therefore removed. A small UPS in case of power failure has also been provided.

See: PLC dome controller document for more details.

On the rare ocasion of a major fault on the PLC controllers, follow this procedure to clear the fault.

Dome TASC unit

The Dome rotation motors are connected to three drive rollers via a combination helical/wormdrive gearbox to a TASC unit which provides an infinitely variable speed or torque drive. In effect, there is no direct mechanical coupling between the motors and the dome wheels.

A poled rotor which is fixed to an output shaft is surrounded by a tube. This is driven by a constant speed AC motor. A stationary brushless coil held on the stator is fixed to an outer casting. Magnetic flux is set up when the field coil is energised generating eddy currents in the tube and causing the rotor to turn. A tachogenerator mounted to the output shaft of one of the drives generates a voltage proportional to speed. Cooling air is provided by a constant speed fan, driven by the motor.

Speed control is achieved by comparing a reference voltage (supplied by the speed control potentiometer or via the DAC in the Dome Control System) along with the feedback signal from the tachogenerator. Differences between these voltages generate more or less current to the excitation coil thereby maintaining the set speed constant. Additional facilities are provided to vary the acceleration and the torque. Refer to the TASC unit technical manuals for further information.

First issued: 28th June 2002  ejm
Updated: 18nd January 2011  rjp