JKT dome


The dome is an aluminium structure on a fabricated steel backbone. The steel fabrication consists of the base ring girder and arch girders together with the roller springs and supports. The dome segments are bolted together and the joints sealed against the weather. The dome rotates on rollers mounting in the base ring girder. There are twenty rollers, eighteen of which are idlers and two of which are driven. There are also a series of rollers mounted on the wall below the dome which act as guides to limit lateral movement.

Each roller/spring combination is suspended by a single leaf spring which is supported and anchored to the concrete drum of the building. The drive rollers are each connected via an intermediate chain drive through a helical/wormdrive gearbox to the TASC unit which provides an infinitely variable speed and torque control.

The dome shutters are of the bi-parting type and are constructed in the same way as the dome. They open and close on roller guideways on their top and lower faces. These guide roller assemblies are supplemented by sets of lift-off rollers which prevent the shutters from being displaced by wind or other effects. The shutters are opened and closed by a synchronising drive system which open the shutters until limit switches are activated. The DOME push buttons on the control desk indicate whether they are in an OPEN or CLOSE position. A windshield is also available to reduce wind in the dome due to gusts. If the wind speed exceeds 50 km/hour, the dome shutters MUST be closed.

The dome, shutters and windshield can be operated in LOCAL mode from the observing floor, but for operation from the control room the switches on the local control box MUST be switched to REMOTE.

Position encoding for the dome is achieved using a 10 bit absolute T + R encoder mounted on a bracket fitted to the wood paneling above the control room. This is spring loaded and the encoder is driven through a gearbox and sprocket which runs in an endless chain attached to the dome. A line driver box mounted on the encoder bracket sends position information to CAMAC.

Three phase power for the shutter motors, windshield and dome crane is supplied to the dome through slip rings.

Dome Control Equipment

The Dome and Shutter control unit consists of a series of relays which interface between the 110v AC and 240v AC contactors contained in the control boxes on the dome wall. These relays are controlled from either the control desk push buttons or from CAMAC (dome rotation only) when in Computer Mode. The CAMAC modules associated with dome control are:

PR2402  Input register
Address: B4  C2  N10  A0   Reads position data from the dome encoder

OD2407 Output driver
Address: B4  C2  N11  A0   Dome drive (CW and CCW directions)
Address: B4  C2  N11  A1   Dome speed

See the JKT CAMAC manual for more details

Full information on the JKT dome can be found in:

JKT Technical manual: 13    JKT Dome
JKT Technical manual: 14    JKT Dome control circuits

Dome encoder re-alignment

n.b.  It should be noted that the encoder sprocket can slip on occasions. A reference mark at 235o (easily seen through the control room window) should line up with the MIDDLE mark of the dome aperture.

If in doubt (and with the TCS running), drive the dome to 235o and check that the marks line up. If this is NOT the case, using the manual +/- dome direction buttons, rotate the dome until the marks line up then read the ACTUAL dome position from the TCS Info Display window. To re-align, pull back the encoder to disengage the spocket and rotate it by hand until the position reads 235o . Once achieved, re-mesh the sprocket (without moving it) back into the chain.

Dome TASC unit

The Dome rotation motors are connected via a helical/wormdrive gearbox to the TASC unit. This is basically a magnetic clutch fitted between the motor and dome drive.

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

Velocity control is achieved by comparing a reference voltage supplied by the speed control potentiometer (always set to maximum)with a feedback signal from the tachogenerator. Differences between these voltages cause the output drive circuit to supply a varying current to the excitation coil thereby maintaining the set speed constant. Additional facilities are provided to control the acceleration and torque. Refer to the TASC unit technical manuals for further information.

Last updated: Sept 2002  ejm