A simple arrangement of a two-dimensional control system in the block schematic form is shown in Fig. 35.2. It may be noted
that the numerical input data (dimensional control information), which consists of numbers (punched in coded form into a paper tape or a series of cards), relating to the dimensions of the finished parts, is fed to the data handling equipment which processes (i.e. calculates the further data from original data) usually by means of a computer or an interpolator, stores (i.e. records data in any form of long or short-term storage medium, e.g. punched tapes, magnetic tapes or cores, and relays or switches), converts the analogue signals to digital form and vice versa. It may be appreciated that the data preparation which is the abstraction of data from a source or sources and the arrangement of them in a suitable form for their presentation in the form of a primary control tape, is an important feature. The output from the data handling equipment passes all along separate channels, (the number of channels is equal to the number of dimensions being controlled) one relating to each axis of machine motion, to the servosystems which, in turn, drive the machine slides.
Usually closed control loop is employed because it is not sufficient for the numerical input information system to tell slides what positions they are to take up, but it is equally important to ensure that the desired position has been moved. This is done by measuring actual position of the slides by some transducer and comparing it with the commanded position and making any difference cancel itself. It is important that the output of the displacement (position) measuring devices be of a form directly comparable with that of the command signal. The block schematic of the servosystem, including the measuring device, for a single axis of control is shown in Fig. 35.3.
Thus in the closed loop system, the tape transmits a command to the control system, which tells where the tool slide or table should be positioned. A feedback device transmits a signal back to the control system indicating the true position, which creates an un-
balanced condition. After the difference is accurately measured by the electronic control system, the system transmits electrical signals to the drive mechanism, causing the table tool slide to move in a direction tending to reduce this difference. When the variation between the feedback signal and the input command is zero, the moving element has reached the prescribed position and stops.
Some machines utilise only semi-closed loop system, in which the exact position of the tool slide or table is not measured as the linear distance directly, but by the rotary position of a lead screw or pinion gear. The linear displacement of the tool slide or table is inferred from the rotary displacement. The accuracy of position is then subject to the accuracy of the lead screw or a gear train. High precision lead screws utilising ball-bearing nuts guarantee the accuracy required for most numerical control machine tools.
Some machines utilise open-loop systems also, which do not have a feedback device and thus the actual position of the tool slide or work table is not measured and verified. This system depends upon the quality of the drive unit. The machine tool does not become part of the control system and the effect of vibrations and the other characteristics of the machine are less critical than in close loop control. Modern open loop control systems use pulse motors (also called digital motors or stepping motors) to actuate the mechanical movement. Each pulse that is fed into the motor causes it to rotate a given amount.