On-Line Data-Acquisition Systems in Nuclear Physics, 1969: Chapter 2 - A PROCESS-CONTROL SYSTEM

In 1965, a system based on an SDS 920 computer was put into operation at the Brookhaven National Laboratory to control data-acquisition processes involving eight neutron spectrometers and one x-ray spectrometer. The neutron spectrometers are located on the floor surrounding the High Flux Beam Reactor (HFBR); the x-ray spectrometer was placed in the same building in order to facilitate linking it to the computer. The system can control the execution of experiments on all nine sets of apparatus simultaneously, yet each experimenter feels that he is working essentially independently of all other users. The system controls all angular rotations of crystals and counters, all detector counting, the data displays, the input and output operations, and automatic error responses. It can also perform most of the calculations necessary for real-time guidance of the course of the experiments.

For example, the experimenter can mount a crystal on a goniometer at approximately the correct angular orientation, then he can specify to the computer where several peaks should be found, whereupon the computer will direct the execution of a trial experiment to find where the peaks do, in fact, occur, executing least-squares calculations in the process, after which the error in crystal orientation is known and the angular scales are automatically corrected. In another example, the computer is given as input information the crystal constants (unit cell) and the zone orientation of the crystal on the goniometer and is asked to produce a scanning of a given part of reciprocal space. The computer then calculates where to look, turns to a correct angle to check the intensity of a central peak, and performs the other necessary steps, making many decisions as it controls the execution of the entire experiment.

When it was first assembled, the system included only two teletypes, both located near the computer. Early in 1969, a communications network was added to permit the installation of a local, assigned typewriter at each of the nine spectrometer stations, as well as three assignable remote teletypes located in the Chemistry and Physics buildings. This network incorporates a Varian 620i computer. It permits any ordinary operation to be carried out from any of the 12 remote stations, except program loading, which still must be done via the high-speed paper-tape reader at the computer.

FIGURE 10 The multiple-spectrometer control system at Brookhaven National Laboratory.

FIGURE 11  Block diagram of a single-spectrometer control station of the MSCS shown in Figure 10. [From D. R. Beaucage, M. A. Kelley, D. Ophir, S. Rankowitz, R. J. Spinrad, and R. Van Norton, Nucl. Instrum. Methods 40, 26 (1966).]

The major parts of the system (Figure 10) are the SDS 920 computer with a 16k, 24-bit memory, a bulk storage memory section comprising two magnetic tapes units and one 32,010-word drum, the communication network, and the nine local control stations (SCS) at the spectrometers. Each SCS (Figure 11) contains the stepping motors required for computer control of angular rotations of crystals and counters, together with shaft rotation encoders (optional, incremental type) to feed information back to the computer. Each SCS also includes manual controls, the electronic counters associated with the radiation detectors, counter displays, a decoding and control section, and other related equipment.

    a. The system now does "all things imagined to be necessary."

    b. The computer has proved to be remarkably reliable, with a record of about 40,000 hours of use without a breakdown.

    c. A reasonable amount of preventive maintenance is done, mostly during the one week of four that the reactor is shut down.

    d. One person serves as operator and programmer (for simple jobs). He also transports magnetic tapes to the computing center for off-line data processing and performs smaller tasks. The average user does not need to do any programming.

    e. Fortunately, the people who have written most of the programs have remained in attendance and have updated the programs frequently. Machine-language programming has not proved to be a bad chore because the system is a fixed-hardware setup.

    f. Modes of data collection can easily be changed.

    g. The overall performance is excellent. The only problem is an occasional wiping out of a program due to the fact that there is no hardware memory-protection feature in the computer. These accidents are estimated to cost at most a loss of a few percent of the running time.

The costs in manpower and dollars of the MSCS are given in Table 7.