DIGITAL CONTROL SYSTEM FOR A MILLING MACHINE

B. A. Lapin

(Presented by the scientific seminar of the Electromechanical Faculty)

Automation of production processes is the principal direction of technical progress in all branches of industry. At present, the development of technology, in particular metalworking technology, has reached such a level that in many cases further increases in productivity, and increases in the speed and accuracy of machining, are limited by the physical capabilities of operators who monitor the course of the technological process and control the equipment. The complete removal of people from direct participation in the technological process requires the use of special automatic control systems.

As is known, the evolution of control systems for metal-cutting machine tools has led to the creation of productive copying automatic machines operating from templates. However, this method of control has drawbacks associated with the limited accuracy of the templates and the laboriousness of their manufacture.

Progress in the field of automation, information theory, and computer technology has led to the creation of digital program-control systems built on electronic computing elements of discrete action. The program for such control systems is specified analytically, in the form of a digital model of the product, consisting of the coordinates of successive machining points. The digital method of control makes it possible to specify the program, in principle, with any degree of accuracy. The labor required to produce the digital model of the product is also much less than that required for a geometric one.

The process of manufacturing a product on a machine tool with digital program control is divided into several characteristic stages, independent of the particular structure of the control system (Fig. 1). The source of information about the product is the technological assignment. It must contain data on the dimensions and shape of the part, on machining allowances, on the material of the workpiece, on the size and type of tool, and on cutting conditions. As a result of the mathematical processing of this technological information, a mathematical model of the product is obtained, recorded in the form of increments that the coordinates of the center of the milling cutter must receive during the machining process. The final stages of programming are the coding of the mathema-

technical information and its recording on the program tape. The coding system is determined by the logical structure of the control system and does not change when different products are machined. The program tape contains, in coded form, the complete algorithm of the technology for machining the product and is the specifying document for the control system. The final stage of the product-manufacturing process is the running-in of the program by the control system and the machine tool.

The advantages of numerical control are realized most fully in the individual and small-batch production of parts of complex configuration. In this case the cost-effectiveness of machine tools with numerical

Fig. 1.

program control proves to be tens of times higher than that of copying machines.

Equipping existing metal-cutting machine tools with numerical program-control systems significantly expands their technological capabilities. Described below is a two-coordinate system for controlling a model 6441A copy-milling machine. During modernization the machine underwent minimal alteration, which consisted in the installation of displacement sensors and a plug-in switch for the operating mode “copying—program control.” The control part of the system is assembled on electronic tubes and is housed in an attached cabinet. For recording and reading

Fig. 2.

the program, standard telegraph equipment is used—a perforator and a transmitter. A standard five-track telegraph tape is also used as the program carrier. A section of the program tape is shown in Fig. 2. Commands for movement along co-

the “\(X\)” coordinate, for the positive direction, are recorded on track No. 1; for the negative direction—on track No. 2. Tracks No. 3 and No. 4 are used for the “\(Y\)” coordinate, and track No. 5 for the “Stop” command. When recording the program on the program tape, a unit code is used, in which the magnitude of the displacement along a coordinate is proportional to the number of punched holes, and the speed of displacement to their frequency.

Structural Diagram of the System

The structural diagram for one coordinate is shown in Fig. 3. The digital tracking system is based on a scheme in which

Fig. 3.

the mismatch signal is generated in a reversible counter that compares the number of pulses arriving from the program with the pulses arriving through the feedback circuit. The program pulses arrive at the counter through the “OR” circuit and the sync-

synchronization. These same pulses, depending on the programmed direction of motion, set the “\(T_{\text{sign prog.}}\)” flip-flop either to the “\(+\)” position or to the “\(-\)” position. Switching the counter to addition or subtraction is performed by the synchronization circuit in such a way that the program pulses are counted by the counter in accordance with the position of the “\(T_{\text{sign prog.}}\)” flip-flop. In exactly the same way, the feedback pulses are counted in accordance with the position of the “\(T_{\text{sign fb.}}\)” flip-flop. Thus, at every instant, the counter produces the difference between the displacement specified by the program and the actual displacement of the actuator. This mismatch signal is issued by the counter in the form of a four-bit binary number (positive or negative) and is fed to a digital-to-current converter. The mismatch signal, amplified by the summing amplifier and the EMU, acts on the motor.

The displacement of the actuator is measured by a photoelectric sensor. A shaping circuit is used to form feedback pulses of standard shape. Its functions also include determining the sign of the feedback.

The operation of the synchronization circuit and the reversible counter is considered below.

Synchronization circuit and reversible counter

The counter of the digital tracking system is assembled according to a serial circuit and contains five digits (Fig. 4). The connection between the counter digits is effected through controlled valves. With the valves “\(K_+\)” open, the counter operates for addition; with the valves “\(K_-\)” open, it operates for subtraction.

Fig. 4.

“\(K_-\)”—for subtraction. The voltage controlling the valves comes from the output of the synchronization circuit. The synchronization circuit also serves to separate the program and feedback pulses in time by an interval greater than the resolving time of the counter. It consists of the multivibrator “\(M\),” the electronic delay relays “\(RZ_1\)” and “\(RZ_2\),” two

Fig. 5.

trigger “\(T_1\)” and “\(T_2\),” and logic elements “\(\mathrm{OR}_1\)”–“\(\mathrm{OR}_5\),” “\(\mathrm{I}_1\)”–“\(\mathrm{I}_4\).”

The operation of the entire circuit is controlled by multivibrator “\(M\).” Its frequency is chosen from the condition

\[ 5 f_{\mathrm{pr.\ max}} < F_M < \frac{1}{5 t_{\mathrm{res}}}. \]

where \(F_M\) is the frequency of the multivibrator;

\(f_{\mathrm{pr.\ max}}\) is the maximum possible repetition frequency of the program pulses;

\(t_{\mathrm{res}}\) is the resolving time of the counter.

The timing diagram of the operation of the elements of the synchronization circuit is shown in Fig. 5. Depending on the direction of motion, the program pulse is applied either to the “\(+\)” input or to the “\(-\)” input of the “\(T_{\mathrm{zn.\ pr.}}\)” trigger and sets it to the corresponding state. The same pulse, through the “\(\mathrm{OR}_1\)” circuit, is applied to input “1” of trigger \(T_1\) and sets it to state “1.” The high positive voltage from output “1” of this trigger opens valve “\(K_1\),” through which delay relay “\(RZ_1\)” is connected to the multivibrator. With the next positive pulse from the output of multivibrator “\(M_{\mathrm{pr.}}\),” relay “\(RZ_1\)” is started. The time delay of “\(RZ_1\)” is chosen according to the condition

\[ \frac{1}{4} T_M < T_{R.Z} < \frac{1}{2} T_M . \]

By this same positive pulse of the multivibrator, coincidence circuits “\(\mathrm{I}_1\)” and “\(\mathrm{I}_2\)” are switched on. Depending on the state of trigger \(T_{\mathrm{zn.\ pr.}}\), a high voltage appears either at the output of circuit “\(\mathrm{I}_1\)” or “\(\mathrm{I}_2\).” Through the circuits “\(\mathrm{OR}_3\),” “\(\mathrm{OR}_4\),” this voltage is applied to the counter valves. At the end of the delay of “\(RZ_1\),” trigger “\(T_1\)” is set to the “0” state. At this time trigger “\(T_1\)” delivers, through the “\(\mathrm{OR}_5\)” circuit, one pulse to the counter input. At this time either the “\(K_+\)” or the “\(K_-\)” valves are open, depending on the state of the “\(T_{\mathrm{zn.\ pr.}}\)” trigger. Thus, the program pulses will be counted in accordance with the state of the “\(T_{\mathrm{zn.\ pr.}}\)” trigger.

The second half of the circuit (\(T_2, RZ_2, \mathrm{I}_3, \mathrm{I}_4, \mathrm{OR}_1, T_{\mathrm{zn.\ f.b.}}\)) for feedback signals is controlled by the voltage of the multivibrator from output “\(M_{\mathrm{f.b.}}\)” and operates analogously.

Synchronization circuits constructed according to the principle described above contain a minimum of electronic equipment and can operate at frequencies up to several kc.

The article describes a digital control system for a milling machine. Similar systems, with some insignificant modifications, can be used to control lathes, turret lathes, boring machines, drilling machines, grinding machines, and the most varied other metal-cutting equipment.

The use of digital control systems in factory practice raises the culture of production, yields a large economic effect, and is a major step toward the complete automation of universal metal-cutting equipment.

Received by the editors
in May 1962.