Cryogenic and Superconductive Techniques Laboratory

SCPS control board (Fig. 5) is based on the Atmel AT89S8252 microcontroller running at 22.1184 MHz clock frequency. It has 8Kb static RAM to keep all data. CPU has external watch-dog, which attends to controller program faults and resets CPU as fast as in 0.5 s in this case.

Fig. 4. Functional layout of the SCPS control board.

Separate AT89C2051 micro-controller handles 3 phase PWM regulation. It takes setpoint from the main CPU via serial connection and uses internal timer and phase synchronization signals for regulation. Every phase is regulated independently. In case of missing synchronization signal of any phase all three phases are blocked. This mode of regulation is rough but it saves power and, consequently, allows one to work without water cooling.

Control signals for superconductive switch heaters and 3 phase switch go through buffer [ULN2803] to solid state relays on a separate board. Current regulation signal and setpoints for coil signals come from 16-bit DAC [DAC7634]. All analog signals from sensors come to 16-channel multiplexer and then go through AD711 amplifier to 16-bit ADC [ADS7807]. These signals are:

1. Amplified differential signal from coil 1.
2. Amplified differential signal from coil 2.
3. Amplified signal from current shunt.
4. Direct signal from coil 1.
5. Temperature sensor 1.
6. Temperature sensor 2.
7. Temperature sensor 3.
8. Temperature sensor 4.
9. Temperature sensor 5.
10-16. Additional analog inputs (±10V).

Control board is equipped by RS-485 and RS-232 serial interfaces and could be connected to the slow control system. SCPS itself does not have any indicator or control panel because it is planned to be installed at the bottom of electronic rack. But in addition to computer serial interface control board has special RS-485 serial link to connect external indicator and control panel.

The device is equipped by an external indicator and control panel and has digital RS-232 (RS-485) interface, which allows it to be included into common slow control system.