Digital information is numerical information which can change only in discrete amounts. Analogue information can have any value and is not limited to variation in discrete amounts.
Both digital and analogue control systems use punched paper tape or cards as the primary source of dimensional data, i.e. both of them use digital input data. Moreover, they both use servosystems which need to have their mechanical output in analogus form, because the slides must move smoothly and not jump from point to point.
The main difference between digital and analogue control systems is that in the digital control the data are processed in their original digital form, whereas in the analogue control the digital input data are deliberately changed to the analogue form, which may be a voltage or current signal, before it is processed. The computation is carried out using the analogue data. In the digital control system, after computation, the result is converted to analogue form before being used to drive the slides as already explained.
The timing of various functions
For cutting a contour, it is essential that the ‘slides of the machine tool move simultaneously from a given datum position at given rates which are usually different, and these rates vary with time.
In many digital control systems the final data, i.e. the data obtained from the computer is fed to the local director in the form of pulse signals on a magnetic tape. This tape contains all the explicit information required, no further data processing or timing being needed. In such cases the tape must be fed through the tape reader at constant rate for the duration of the cutting process, and unless the process is divided into sections with a separate tape for each, a large cutting operation requires very long length of tape and large spools.
In other type of control systems, the data is fed to a local control cabinet in which the necessary computation is carried out and pulses are generated. In this case the data are implicit and are presented in block form. When this arrangement is used, it is necessary to provide a programmer unit (Refer Fig. 35.4) to instruct the tape reader when to read a new block of data and route the data for the various axes and the management instruction to the proper places in the equipment.
In some systems the dimensional data are fed to data stores, one for each axis and are held till they are needed in the computer. Another function of the programmer, in such cases, is to empty the stores when the data have been processed in the interpolator, so that they are ready for the tape reader to feed in a new block of data. Hence the programmer also ensures that each step in the computation process is related to one block of information and not to part of one and part of another.
Different types of analogue and digital controls.
Both these types (i.e. analogue and digital) can be further classified as positional (absolute) and incremental.
In the positional system, the command data driving the servo- system contains precise positional information with respect to a predetermined datum ; whereas in the incremental system the command data contains information in the form of increments of distance from the point at which each slide may be resting when cutting starts.
n the positional system, it is possible to stop the cutting and retract the tool and move the slide by hand ; and upon switching on again, the slide and the tool will be automatically brought back to the exact point at which cutting was stopped, and action of cutting proceeds as if there had been no break ; provided that the slides have not moved outside the range of the dimensions recorded on the tape. This facility is not possible in incremental system, but it has the advantage that it needs no predetermined datum point. Positional contouring systems can provide certain additional facilities such as cutter-diameter compensation, mirror-image production, and coarse, followed by fine, cuts without the need to provide additional control tape.
These four control systems are described below in brief :
(i) Digital incremental system. In it the command and feedback signals may consist of trains of pulses, each pulse representing a specified unit of distance, say 0’005 mm. The command and feedback signals are compared in some kind of comparator (differential), and the difference, which is not allowed to exceed a certain number of pulses, is the error signal used, after conversion to the analogue form to drive a servomotor. The accuracy with which the dimensional information can be quoted is ± half the distance represented by one pulse. The accuracy with which dimensional information can be passed on to the machine tool is ± the distance represented by one pulse, provided that adequate gain is available for the servosystem and that no pulses are lost or miscounted. In this type of control system, no zero shift device is required.
The control systems of this type are also available in which the pulses are replaced by sine waves, the control information being represented by changes in frequency and phase.
(ii) Digital Positional Control. This is shown schematically in Fig. 35.5.
(iii) Analogue Incermental Control. In this system the command signal is the differential of analogue position. In this case, error being cumulative, this method is generally not used.
(iv) Analogue Positional Control. In this system each commanded position is provided by a coded hole pattern in a punched tape and is quoted with respect to a datum position ; after a conversion process this commanded position is denoted by an analogue voltage. The slide position (feedback) signals must be analogue of the achieved position to the same scale (volts per mm) as is used for the command signal. The static accuracy of the system is the same as that with which the command and feedback analogue voltages can be generated, provided that adequate gain is available for the error signal.