Abstract:
The ram of the press is accelerated to an acceleration-terminating point whereat it attains a speed providing for the required energy of impact upon the work. During the acceleration, the velocity of the ram is measured, as well as the deviation of the actual point of attaining the required speed relative to the preset acceleration-terminating point. Depending on the value of this deviation, either the extent of the acceleration path is varied, with the acceleration value maintained constant, or else the acceleration rate is varied, with the acceleration path length remaining permanent. To effect control by varying the acceleration path length, the control system is provided with specific electric circuitry controlling a transmitter generating a signal sent to a solenoid-operated valve, to vary the latter&#39;s position so as to vary accordingly the approach portion of the ram&#39;s stroke, and thus to vary the acceleration path. To effect control by varying the acceleration rate, the control system likewise is provided with specific electric circuitry controlling the value of the pressure of the liquid supplied from a hydraulic accumulator into the hydraulic drive of the press.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to working by forging and pressing, and, more particularly, it relates to a method of controlling hydraulic screw presses and apparatus capable of performing such a method. 
     Over the recent years, presses of the hydraulic-screw type have been finding an ever wider field of applications, owing to their offering an advantage over other previously known pressing machinery, residing in their inherent capacity of having the impact energy regulated within a relatively wide range, i.e. of metering-out the impact energy. 
     This advantage provides for more accurate dimensions of the shaped work, and also reduces the wear of the tooling and prolongs the life of the pressing machines. 
     While effecting the operation of a hydraulic screw press, in most cases the ram is driven, first, through the approach path or portion of its stroke, whereafter the ram is accelerated to a required speed, with the acceleration terminated upon the ram having attained the required speed corresponding to the required impact energy. 
     One of the disadvantages of this known control method is inadequate accuracy of metering-out the impact energy, which is explained by the fact that upon the working fluid distributing device having been turned off following the acceleration, the speed of the ram decreases. This takes place on account of the cutting-off of the supply of the working fluid being effected upon the ram attaining the required speed at a certain distance from the point of the engagement between the tool and the work. 
     The ram has to pass this distance by the momentum gained (i.e. by the kinetic energy stored by the movable portions of the press), while the working fluid is not supplied into the hydraulic drive cylinder of which the movable element follows the motion of the ram. This results in the suction being created, and the liquid stream breaking somewhere in the line communicating the distributing device with the cylinder, whereby the ram is noticeably braked over this portion of its path. 
     Consequently, the speed or velocity of the ram at the moment of delivering an impact upon the work would differ from the preset one, this actual final speed of the ram being dependent predominantly on the length of its path after the termination of the supply of the working fluid. 
     However, as experiments have proved, the point in the path whereat the ram attains a required speed is not a permanent one, but is prone to shift in accordance with various factors such as variation of the viscosity of the working fluid, varying press lubrication conditions, pressure fluctuations in the pressure accumulator, and so on. This results in a varying distance between the point of deenergization of the drive and the point of impact, whereby the final speed of the ram varies. 
     Hence, the attained impact energy differs from the preset one, and the accuracy of metering-out the energy is inadequate. 
     The above described method is performed by apparatus including a speed sensor or transmitter connected via mechanical gearing to the ram of the press, and a speed comparison unit having its inputs connected to the master speed control and the speed sensor. The output of the speed comparison unit is connected to the solenoid controlling the distributing device in the hydraulic line connecting the accumulator with the hydraulic drive cylinder of the press. 
     However, this apparatus is not proof against the shortcomings of the abovedescribed method. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of controlling a hydraulic screw press, which should provide for attaining a higher accuracy of metering-out the impact energy. 
     It is another object of the present invention to provide an apparatus providing for metering-out the impact energy of a press with a high accuracy. 
     It is yet another object of the present invention to enhance the accuracy of the dimensions and the quality of the shaped work. 
     It is still another object of the present invention to reduce the wear of the tooling and to prolong the service life of a press. 
     These and other objects are attained in a method of controlling a hydraulic screw press, including accelerating the movable portions of the press to attain the required impact energy to shape a work, measuring the speed of the motion of the movable portions, and, upon having attained by such acceleration the speed corresponding to the preset impact energy, terminating the acceleration. In the method of the present invention, a point of terminating the acceleration is preset, and, by following the speed value, the deviation of the point of attaining the preset speed from the preset acceleration-terminating point is determined. Value of the deviation is used to vary at least one of the parameters of the motion of the movable portions, to attain the required speed at the impact point. 
     It is expedient in the control process to maintain the acceleration rate constant over the acceleration path and to vary the length of this acceleration path. 
     In this embodiment, the movable portions are first driven at a permanent speed through the approach portion or path, and then accelerated at a constant acceleration rate to the required speed. In accordance with the invention, depending on the deviation of the point of attaining the preset speed from the preset acceleration-terminating point, the length of the approach path is varied, with the length of the acceleration path varying accordingly. 
     Alternatively, also in accordance with the invention, the acceleration-terminating point is preset, and, by following the speed value, the deviation of attaining the preset speed from the preset acceleration-terminating point is determined. This obtained value of the deviation is used to vary the acceleration rate of the movable portions over the acceleration path, with the length of the acceleration path being constant. 
     In this case, in order to vary the acceleration rate, the pressure of the working fluid of the hydraulic screw press, acting on the movable portions being accelerated, is varied. 
     An apparatus for controlling a hydraulic screw press with the acceleration effected over a variable-length path, as hereinabove described, includes a hydraulic accumulator and a working liquid supply pump, connectable through a solenoid-controlled valve with the hydraulic cylinder of the hydraulic drive of the ram of the press. A sensor is responsive to the ram passing the termination point of the approach path, and is connected to an actuator for displacing said sensor. The actuator is connected to the controlled valve connecting the accumulator and the hydraulic cylinder. A sensor is responsive to the ram speed. A master control of the ram speed is provided. The ram speed sensor and master control are connected to a speed comparison circuit adapted to send a signal to the valve to cut off the connection between the accumulator and the hydraulic cylinder. In accordance with the invention, a system controls the actuator of the sensor responsive to the ram passing the termination point of the approach path. The system includes a sensor responsive to the ram passing the preset acceleration-terminating point, a sensor which senses displacement of the ram and a logic control circuit connected to the outputs of said sensors and to the output of the speed comparison circuit. The logic control circuit inhibits the sending of a signal from the ram displacement sensor when signals are sent simultaneously from the speed comparison circuit and from the acceleration-terminating point sensor. The logic control circuit passes the signal of the ram displacement sensor when only one of the above two signals is sent. 
     It is expedient that the logic control circuit should include means responsive to the direction of the actuation of the approach path termination point sensor. 
     It is further expedient that the logic control circuit should include three logical AND elements or gates of which the first AND element has its respective inputs connected to the ram displacement sensor and the sensor responsive to the ram passing the preset acceleration-terminating point. The second AND element has its respective inputs connected to the ram displacement sensor and the speed comparison circuit. The third AND element has its respective inputs connected to the speed comparison circuit and the sensor responsive to the ram passing the preset acceleration-terminating point. The logic control circuit also includes two inhibition gates of which the inhibiting inputs are connected to the output of the third AND element. The pass-no pass inputs of the inhibition gates are connected to the outputs of the first and second AND elements. The outputs of the inhibition gates are connected to a ring-type commutator controlling the stepping motor of the actuator which controls the displacement of the approach path termination sensor. 
     An apparatus for controlling a hydraulic screw press wherein acceleration is effected at a variable acceleration rate, as described hereinabove, includes a hydraulic pressure accumulator connectable through solenoid-controlled valves with the hydraulic cylinder of the hydraulic drive of the ram of the press, a ram speed sensor and a ram speed master control. The ram speed sensor and master control are connected to a speed comparison circuit adapted to send a signal to a solenoid-controlled valve to cut off the communication between the accumulator and the hydraulic cylinder. In accordance with the present invention, a system controls the pressure in the hydraulic line connecting the accumulator and the hydraulic cylinder. The system comprises a sensor of the ram passing the preset acceleration-terminating point, a ram speed sensor and a logic control circuit connected to the outputs of said sensors and the speed comparison circuit. The logic control circuit produces a signal proportional to the required variation of the pressure to attain the preset impact energy. The signal is supplied to an input of the pressure control circuit and another input of said circuit is connected to a pressure transducer or sensor. The output of the pressure control circuit is connected to a solenoid-controlled valve adapted to connect the hydraulic accumulator alternatively to the pump and to a drain line, respectively, to raise and lower the pressure in the hydraulic line. 
     It is expedient that in this embodiment the logic control circuit should include means responsive to the sense of the variation of the pressure in the hydraulic line. The logic control circuit includes three AND logic elements or gates, with the first AND element having its respective inputs connected to the speed displacement sensor and the sensor of the ram passing the preset acceleration-terminating point. The inputs of second AND element are connected to the ram displacement sensor and the speed comparison circuit. The inputs of the third AND element are connected to the speed comparison circuit and the sensor of the ram passing the preset acceleration-terminating point. The logic control circuit also includes two inhibition gates of which the respective inhibiting inputs are connected to the output of the third AND element and the other inputs are connected, respectively, to the outputs of the first and second AND elements. The outputs of the inhibition circuits are connected to a reversible counter of which the output is connected to the first input of a digital-to-analog converter. The other input of the digital to analog converter is connected to the output of the third AND element and the output of said converter is connected to the pressure control circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 shows a chart illustrating the method of controlling a hydraulic screw press by varying the acceleration path length; 
     FIG. 2 shows a chart illustrating the method of controlling a hydraulic screw press by varying the acceleration rate of the movable portions thereof; 
     FIG. 3 is a block diagram of an embodiment of the apparatus of the invention for controlling a hydraulic screw press with the acceleration path length being variable, and includes the hydraulic screw press per se; 
     FIG. 4 is a block diagram of an embodiment of the logic circuit of the apparatus of FIG. 3; 
     FIG. 5 is a block diagram of another embodiment of the apparatus of the invention for controlling a hydraulic screw press with the pressure value being variable; and 
     FIG. 6 is a block diagram of a logic circuit of apparatus of FIG. 5. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to the method of the invention by referring to the chart in FIG. 1 wherein the variation of the ram speed of a hydraulic press in the course of the working stroke thereof is plotted. 
     The ram stroke is commenced at point &#34;O&#34; and is effected at a constant speed to point &#34;A&#34;. This portion of the stroke is called the approach path and is effected by supplying the working fluid into the cylinder of the press directly from the pump, as further explained hereinafter in connection with the apparatus embodying the method. At point &#34;A&#34; the supply of the working fluid from the pressure accumulator is turned on, whereby a constant driving effort is provided by the pressure of the fluid onto the movable member of the hydraulic cylinder. Consequently, the ram of the press is driven at a constant acceleration rate, which means that the ram speed increases from point &#34;A&#34; to point &#34;B&#34;. This portion of the stroke is hereinafter called the acceleration path. 
     Upon the ram attaining the preset speed at point &#34;B&#34;,  the supply of the working fluid from the pressure accumulator into the hydraulic cylinder is discontinued, and the remaining portion &#34;BC&#34; of the stroke to point &#34;C&#34; of engagement with the work, where the shaping of the work is started, is travelled by the ram due to gained momentum. 
     The point &#34;B&#34; is preselected at a specific small distance &#34;BC&#34; from point &#34;C&#34;, so that the ram travelling over the distance should not be influenced noticeably by the braking action due to the discontinuation of the working fluid supply to the hydraulic cylinder. 
     If it is assumed, however, that the preset speed v is attained at point &#34;B 1  &#34;, spaced from point &#34;C&#34; by a distance &#34;B 1  C&#34; in excess of &#34;BC&#34;, this means that the ram has gained or stored the required amount of energy before reaching point &#34;B&#34;. Then the distance &#34;B 1  B&#34; can be considered the value of deviation of the point of attaining the preset speed. Then, depending on this value, the length of the acceleration path is reduced by shifting the acceleration starting point &#34;A&#34; toward the point &#34;B&#34; in the working-stroke direction, i.e. the length of the approach path is increased by ΔS 1  equalling &#34;B 1  B&#34;. 
     If it is assumed, on the other hand, that the preset speed is attained at point &#34;B 2  &#34; closer to point &#34;C&#34;, this means that the required energy is attained after the ram has reached the point &#34;B&#34;. The distance &#34;B 2  B&#34; is the deviation by which the length of the acceleration path is, according to the method proposed, increased by shifting point &#34;A&#34; in the direction against the working stroke of the ram (i.e. toward point &#34;O&#34;). The approach path is thus reduced by the deviation value ΔS 2 . 
     Thus, the aim of attaining the preset speed at a predetermined point is realized by varying or adjusting the acceleration path length. 
     The same aim can be attained, however, by varying another parameter of the motion, i.e. the value or rate of acceleration. This case will be discussed with reference to the chart in FIG. 2. 
     The stroke of the ram is initiated at point &#34;C&#34; by turning on the supply of the liquid from the accumulator, whereby acceleration occurs, and the speed of the ram starts rising from the very beginning. Upon the ram attaining the required speed at point &#34;D&#34;, a speed sensor produces a signal to cut off the supply of the working fluid into the hydraulic cylinder, and the acceleration is terminated, whereafter the ram strikes the work at point &#34;E&#34;. The distance &#34;ED&#34; is selected on the described principle. 
     If the preset speed is attained at point &#34;D 1  &#34;, which means that the required energy has been stored before the ram reaches point &#34;D&#34;, the acceleration rate of the movable parts is varied accordingly, i.e. reduced by reducing the pressure in the accumulator by a value ensuring that no deviation &#34;D 1  D&#34; occurs during the next successive stroke. 
     Similarly,, the pressure is increased if the preset speed is attained at point &#34;D 2  &#34;. 
     The aforedescribed modifications of the method of the invention may obviously be employed in various combinations. 
     A basic premise of the invention is to preset a point at a specific distance from the impact point, which distance should positively not practically affect the speed by reducing same, and to positively set this point by measuring the speed and varying the parameters of the ram motion. 
     The apparatus of the invention is capable of effecting the control by varying the approach path length, as been described with reference to FIG. 1. The hydraulic screw press 1, illustrated generally in FIG. 3, comprises a hydraulic cylinder 2, a screw 3, a flywheel 4 and a ram 5. The hydraulic screw press 1 is shown purely schematically because its exact structure is well known to any person competent in the art and has no direct bearing on the essence of the present invention. 
     The apparatus for controlling this hydraulic screw press includes a hydraulic pressure accumulator 6 and a pump 7 communicating with the hydraulic cylinder 2 via a solenoid-controlled valve 8. Furthermore the pump communicates with the hydraulic accumulator via a check valve 9 in order to provide pressure adjustment. 
     The control apparatus further includes an electric control system 10 which provides the control functions for varying the acceleration path length whenever necessary. The system 10 includes a speed sensor 11, e.g. a tachogenerator connected via mechanical gearing with the ram 5, and a master speed control 12, e.g. one including a voltage divider. The speed sensor 11 and the master speed control 12 are connected to a speed comparison circuit 13, e.g. of the null-detector type, well known in the art of automatic control. The system 10 also includes a sensor or transmitter 14 of the ram passing the approach path termination, e.g. in the form of any suitable limit switch, either of a contact or non-contact type, associated with an actuator or drive 15 for setting it to a desired position. 
     The approach path termination sensor 14 is connected to the solenoid-controlled valve 8, so that when said sensor sends a signal, said valve is turned on to connect the hydraulic accumulator 6 with the hydraulic cylinder 2. 
     The system also includes a ram displacement sensor 16 responsive to the value of the displacement of, or the distance travelled by, the ram 5, in the form of any suitable known linear displacement transducer, e.g. a ring-type pulsed transducer connected via mechanical gearing to said ram. An acceleration terminating point sensor 17 of the ram passing the preset acceleration terminating point, may comprise any suitable track switch of any known type. The outputs of the sensors 16 and 17 are connected to a logic control circuit 18 which controls the actuator 15 of the approach path termination transmitter 14. The logic circuit 18 passes the signal from the ram displacement sensor 16 to the actuator 15 when signals from the speed comparison circuit 13 and from the acceleration-terminating point sensor 17, respectively, are provided at different moments. The actuator 15 may comprise a reversible motor. If the signals from the aforedescribed two elements are provided simultaneously, the logic control circuit 18 inhibits the transmission of a signal from the sensor 16 to the motor 15. 
     One possible embodiment of the logic control circuit 18 is illustrated in FIG. 4. The logic control circuit includes three logic and elements or gates 19, 20 and 21. The respective inputs of the first AND element 19 are connected to the ram displacement sensor 16 of the and to the acceleration terminating point sensor 17. The inputs of the second AND element 20 are connected to the ram displacement sensor 16 and the speed comparison circuit 13. The inputs of the third AND elements 21 are connected to the speed comparison circuit 13 and to the acceleration terminating point sensor 17. The logic control circuit also includes first and second inhibition gates 22a and 22b, respectively. The inhibiting inputs of the inhibition gates 22a and 22b are connected to the output of the third AND element 21. The pass-no pass inputs connected, respectively, to the outputs of the inhibition gates 22a and 22b are of the first and second AND elements 19 and 20. The outputs of the inhibition circuits 22a and 22b are connected to a ring-type switch or commutator 23 adapted to switch over the windings of the actuator 15 consisting of a stepping reversible motor which drives the approach path termination sensor 14 (FIG. 3) to a desired position. 
     The aforedescribed embodiment of the apparatus of the invention operates, as follows. Pressurized working fluid is supplied by the pump 7 through the solenoid-controlled valve 8 to the hydraulic cylinder 2. In this manner, the approach path of the ram 5 is commenced. Upon the ram 5 actuating or driving the approach path termination sensor 14, said sensor produces a signal for the solenoid-operated valve to switch over to supply the pressurized working fluid into the hydraulic cylinder 2 from the hydraulic pressure accumulator 6. The acceleration path of the ram 5 is commenced. While the ram 5 is accelerating, a growing output signal is sent by the ram speed sensor 11 to the input of the speed comparison circuit 13, while the ram displacement sensor 16 sends a pulsed signal, proportional to the displacement or path travelled by the ram 5, to the logic controlled circuit 18. When the ram 5 attains the preset speed, an output signal is sent by the speed comparison circuit 13 to the solenoid-controlled valve 8, to cut off the supply of the working fluid from the hydraulic accumulator 6 to the hydraulic cylinder 2. The acceleration of the movable portions of the press is terminated, and the further progress of the ram 5 to the point of the impact against the work is effected by the momentum gained, i.e. by the kinetic energy stored. 
     The following situations are possible. 
     (1) The ram 5 attains the preset speed at the point where the acceleration terminating point sensor 17 indicates that the ram passes the preset acceleration terminating point). 
     In this case the respective output signals of the speed comparison circuit 13 and the acceleration terminating point sensor 17 are simultaneously supplied to the respective inputs of the third AND element 21, whereby said third AND element passes an output signal to the inhibiting inputs of the inhibition circuits 22a and 22b. The pulses from the ram displacement sensor 16 are not transmitted to the inputs of the ring-type commutator 23 controlling the rotation of the stepping motor 15. Consequently, there is no correction of the position of the approach path termination sensor 14, since no such correction is actually required. (2) The ram attains the preset speed before the operation of the acceleration terminating point sensor 17. 
     If this is the case, the signal of the speed comparison circuit 13 supplied to the respective input of the second AND element 20 does not interfere with the transmission of pulses from the transmitter 16 to the pass-no pass input of the inhibition circuit 22b. No signal is sent to the inhibiting input of the inhibition circuit 22 from the third AND element 21, because said third AND element receives only one signal from the speed comparison circuit 13. Consequently, pulses from the ram displacement sensor 16 are fed to the &#34;plus&#34; input of the ring commutator 23 which distributes these pulses among the windings of the stepping motor 15. The stepping motor 15 often rotates and shifts the approach path termination sensor 14 in the direction of matching the moment of attaining the preset speed with the preset point. As the ram moves further on, acceleration terminating point sensor 17 operates, and the other signal is fed to the input of the third AND element 21, so that said third AND element passes an output signal to the inhibiting inputs of the inhibition gates 22a and 22b. The supply of pulses to the ring-type commutator 23 is terminated, and the stepping motor 15 stops. The position of the approach path termination sensor 14 is such that at the next successive cycle of operation of the press, the ram would attain the preset speed at the preset acceleration-terminating point. 
     (3) The ram attains the preset speed after the operation of the acceleration terminating point sensor 17. 
     The signal of the acceleration terminating point sensor 17 is fed to the first AND element 19, and pulses from the ram displacement sensor 16 are passed to the input of the first inhibition gate 22a. Since at this moment there is no signal coming from the speed comparison circuit 13 to the input of the third AND element 21, no signal is supplied to the inhibiting inputs of the inhibition gates 22a and 22b, whereby pulses from the output of the first AND element 19 are transmitted to the ring-type commutator 23 to rotate the motor 15 and to displace the approach path termination sensor 14 in the direction opposite to the one of the previously described situation. Meanwhile, the ram 5 moves on with its speed increasing, and when it attains the preset value, the speed comparison circuit 13 provides an output signal to the respective input of the third AND element 21. The third AND element 21 passes a signal to the inhibiting inputs of the inhibition gates 22a and 22b, whereby the motor 15 is stopped at the moment when the approach path termination sensor 14 has been set to a position whereat the ram would attain the preset speed at the preset point upon its next-successive stroke. 
     Another embodiment of the apparatus of the invention capable of performing the disclosed method of control effects the control action by varying the supply pressure, as hereinbefore described with reference to FIG. 2. 
     The block diagram of the apparatus shown in FIG. 5 contains predominantly the same or similar elements as the apparatus illustrated in FIG. 3. The difference between the apparatus of FIGS. 3 and 5 is in that an electric pressure control circuit 10&#39; incorporates means for varying the pressure in the hydraulic supply line between the accumulator and the hydraulic cylinder, including a pressure control circuit 24. The inputs of the pressure control circuit 24 are connected to the output of the logic control circuit 18&#39; and the pressure variation transducer 25. The output of the pressure control circuit 24 is connected to a regulator including a solenoid-controlled valve 26 adapted to establish communication between the pump 7 and the hydraulic accumulator 6, should it be necessary to raise the pressure in the line through which the working fluid is supplied to the hydraulic drive, or between the hydraulic accumulator 6 and a drain line, when said pressure is to be reduced. The circuit of FIG. 5 has no approach path termination sensor 14 (FIG. 3) with its associated actuator. 
     The logic control circuit 18&#39; of the embodiment of FIG. 5 also differs from the logic control circuit 18 of FIG. 3. 
     The block diagram of the circuit 18&#39; is illustrated in FIG. 6. The circuit includes the same logic elements 19, 20, 21, 22 which have been described, and a reversible pulse counter 27. The inputs of the pulse counter 27 are connected to the outputs of the first and second inhibition circuits 22a and 22b . The output of the pulse counter 27 is connected to the first input of a digital to analog converter 28 of the code-to-voltage type. The second input of the digital to analog converter 28 is connected to the third AND element 21 and the output of said converter is connected to the pressure control circuit 24. 
     The apparatus of FIG. 5 operates as follows. With the press energized for operation, the solenoid-controlled valve 8 is turned on to connect the hydraulic cylinder 2 to the hydraulic accumulator 6, whereby acceleration of the ram 5 is commenced. As the ram 5 accelerates, the ram speed sensor 11 sends an increasing signal to the respective input of the speed comparison circuit 13, while the ram displacement sensor 16 feeds to the logic control circuit 18&#39; a pulse signal proportional to the value of the displacement or path travelled by the ram. 
     Upon the ram 5 attaining the preset speed, the output signal of the speed comparison circuit 13 is fed to the valve 8 and said valve cuts off the supply of the pressurized working fluid from the hydraulic accumulator 6 to the hydraulic cylinder 2. The acceleration of the movable portions of the press is terminated, and the further movement of the ram to the point of delivering an impact upon the work is effected by the momentum gained, i.e. by the kinetic energy stored. 
     The following situations may occur. 
     (1) The preset speed is attained by the ram at the moment of operation of the acceleration terminating point sensor 17 responding to the ram passing the acceleration terminating point. 
     In this case signals from the transmitter 16 fed to the respective inputs of the first and second AND elements 19 and 20 (FIG. 6) are not transmitted to the inputs of the reversible counter 27, because with signals coming simultaneously to the inputs of the third AND element 21 from the speed comparison circuit 13 and from the acceleration terminating point sensor 17, the AND element 21 passes an output signal to the inhibition circuits 22a and 22b to inhibit the transmission of the pulses. 
     Hence, the system 10&#39; develops no correction signals. 
     (2) The ram attains the preset speed before the acceleration terminating point sensor 17 sends an output signal. 
     If this is the case, the output signal of the speed comparison circuit 13 fed to the respective other input of the second AND element 20 permits the transmission of pulses from the ram displacement sensor 16 to the respective input of the second inhibition circuit 22b, while no signal is fed to the inhibiting input of said second inhibition circuit 22 from the AND element 21, because only the signal from the speed comparison circuit 13 is fed to the respective input of said third AND element. Consequently, pulses from the ram sensor displacement 16 are fed to the direct-count input of the reversible counter 27, which counts the pulses. As the ram moves on, the acceleration terminating point sensor 17 responds, and the second input signal is sent to the third AND element 21, whereby said third AND element passes an output signal to the inhibiting inputs of the inhibition gates 22a and 22b. 
     The supply of pulses to the reversible counter 27 is discontinued, with the counter having registered a number corresponding to the motion of the ram from the point of attaining the preset speed to the point of operation of the acceleration terminating point sensor 17. Such number is being converted in the digital-to-analog converter 28 into a voltage value proportional to the required pressure variation and is fed to the input of the pressure control circuit 24 when said converter receives the output signal from the third AND element 21. The moment this voltage value of the corresponding polarity is fed to the input of the pressure control circuit 24, said circuit provides at its output a signal for turning on the control valve 26 so that the hydraulic accumulator is connected to drain, whereby the pressure therein drops until the signal from the pressure transducer 25 equals the signal from the DA converter 28. The moment the equality is achieved, the valve 26 is turned off, with the pressure having been reduced to a value ensuring that in the next successive cycle of the press operation the ram would attain the preset speed precisely at the preset acceleration terminating point. 
     (3) The ram attains the preset speed after the operation of the acceleration terminating point sensor 17. 
     The signal from the acceleration terminating point sensor 17 is fed to the first AND element 19, whereby pulses are fed from the transmitter 16 to the input of the first inhibition circuit 22a. At this moment no signal is sent to the input of the third AND element 21 by the speed comparison circuit 13, and the pulses thus travel from the output of the first AND element 19 to the count-down input of the counter 27. The counter 27 counts the pulses down. As the ram moves on, its speed continues to increase, and when it attains the preset value, the speed comparison circuit 13 produces an output signal which is fed to the respective input of the third AND element 21, whereby the inhibiting inputs of the inhibition gates 22a and 22b receive a signal, and the counting of the pulses is interrupted. At the same time, the output of the third AND element 21 is fed to the input of the DA converter 28, opening up the transmission of the voltage signal to the pressure control circuit 24. The output signal of the pressure control circuit turns on the control valve 26, so that the pump 7 is connected to the accumulator 6. The pressure in the accumulator rises. The moment the signal from the pressure transducer 25 equals the output of the DA converter 28, the pressure control circuit 24 turns the valve 26 off, and the pressure build-up is terminated, with the pressure having been built up to a value ensuring that in the next successive operating cycle the ram would attain the preset speed at the preset point.