Proportional hydraulics Textbook v qA qB pA pB A B ∆pA ∆pB P T pP pT qP 094378 GB Order no.LB-TP701-GB Edition: 10/2002 Layout: 08. Scholz © Festo Didactic GmbH & Co., 73770 Denkendorf/Germany, 2002 Internet: www.com/didactic e-mail: did@festo.com The copying, distribution and utilization of this document as well as the communication of its contents to others without expressed authorization is prohibited. Offenders will be held liable for the payment of damages. All rights reserved, in particular the right to carry out patent, utility model or ornamental design registration.
Table of contents 1 Introduction to proportional hydraulics ____________________________ 5 1.1 Hydraulic feed drive with manual control____________________________ 6 1.2 Hydraulic feed drive with electrical control and switching valves_________ 7 1.3 Hydraulic feed unit with electrical control and proportional valves _______ 8 1.4 Signal flow and components of proportional hydraulics _______________ 10 1.5 Advantages of proportional hydraulics ____________________________ 12 2 Proportional valves: Design and mode of operation _________________ 15 2.1 Design and mode of operation of a proportional solenoid _____________ 15 2.2 Design and mode of operation of proportional pressure valves _________ 20 2.3 Design and mode of operation of proportional flow restrictors and directional control valves _______________________________________ 23 2.4 Design and mode of operation of proportional flow control valves ______ 26 2.5 Proportional valve designs: Overview _____________________________ 28 3 Proportional valves: Characteristic curves and parameters ___________ 29 3.1 Characteristic curve representation _______________________________ 29 3.2 Hysteresis, inversion range and response threshold__________________ 30 3.3 Characteristic curves of pressure valves ___________________________ 32 3.4 Characteristic curves of flow restrictors and directional control valves _______________________________________ 32 3.5 Parameters of valve dynamics ___________________________________ 38 3.6 Application limits of proportional valves ___________________________ 42 4 Amplifier and setpoint value specification _________________________ 43 4.1 Design and mode of operation of an amplifier_______________________ 45 4.2 Setting an amplifier ____________________________________________ 50 4.3 Setpoint value specification _____________________________________ 53 5 Switching examples with proportional valves ______________________ 57 5.4 Energy saving measures ________________________________________ 65 © Festo Didactic GmbH & Co. • TP701 3 Table of contents 6 Calculation of motion characteristics of hydraulic cylinder drives______ 71 6.1 Flow calculation for proportional directional control valves ____________ 75 6.2 Velocity calculation for an equal area cylinder drive disregarding load and frictional forces_____________________________ 77 6.3 Velocity calculation for an unequal area cylinder drive disregarding load and frictional forces_____________________________ 81 6.4 Velocity calculation for an equal area cylinder drive taking into account load and frictional forces _______________________ 88 6.5 Velocity calculation for an unequal area cylinder drive taking into account load and frictional forces _______________________ 94 6.6 Effect of maximum piston force on the acceleration and delay process ________________________________________________ 101 6.7 Effect of natural frequency on the acceleration and delay process _____ 105 6.8 Calculation of motion duration __________________________________ 109 4 © Festo Didactic GmbH & Co. Introduction to proportional hydraulics Hydraulic drives, thanks to their high power intensity, are low in weight and require a minimum of mounting space. They facilitate fast and accurate control of very high energies and forces.
The hydraulic cylinder represents a cost-effective and simply constructed linear drive. The combination of these advantages opens up a wide range of applications for hydraulics in mechanical engineering, vehicle construction and aviation. The increase in automation makes it ever more necessary for pressure, flow rate and flow direction in hydraulic systems to be controlled by means of an electrical control system. The obvious choice for this are hydraulic proportional valves as an interface between controller and hydraulic system.
In order to clearly show the advantages of proportional hydraulics, three hydraulic circuits are to be compared using the example of a feed drive for a lathe (fig.1): • a circuit using manually actuated valves (fig.2), • a circuit using electrically actuated valves (fig.3), • a circuit using proportional valves (fig.1: Hydraulic feed drive of a lathe © Festo Didactic GmbH & Co. Introduction to proportional hydraulics 1.2 illustrates a circuit using a hydraulic feed drive with manually actuated Hydraulic feed drive with valves. manual control • Pressure and flow are to be set during commissioning. To this end, the pressure relief and flow control are to be fitted with setting screws.
• The flow rate and flow direction can be changed during operation by manually actuating the directional control valve. None of the valves in this system can be controlled electrically. It is not possible to automate the feed drive.2: Hydraulic circuit diagram of a manually controlled feed drive 6 © Festo Didactic GmbH & Co. Introduction to proportional hydraulics 1.2 In the case of electro-hydraulic systems, the directional control valves are controlled Hydraulic feed drive with electrically.3 shows the circuit diagram of a feed drive using an electrically electrical control and actuated directional control valve.
The operation of the lathe can be automated by switching valves means of actuating the directional control valve via an electrical control system. Pressure and flow cannot be influenced during operation by the electrical control system. If a change is required, production on the lathe has to be stopped. Only then can the flow control and pressure relief valve be reset manually.3: Hydraulic circuit diagram of an electrically controlled feed drive © Festo Didactic GmbH & Co.
Introduction to proportional hydraulics Automation of pressure and flow control is only possible to a limited extent with electro-hydraulic control systems using switching valves. Examples are • the connection of an additional flow control by means of actuating a directional control valve, • the control of flow and pressure valves with cams.4 the hydraulic circuit diagram of a feed drive is shown incorporating Hydraulic feed drive with proportional valves. electrical control and proportional valves • The proportional directional control valve is actuated by means of an electrical control signal. The control signal influences the flow rate and flow direction.
The rate of movement of the drive can be infinitely adjusted by means of changing the flow rate. • A second control signal acts on the proportional pressure relief valve. The pressure can be continually adjusted by means of this control signal. The proportional directional control valve in fig.4 assumes the function of the flow control and the directional control valve in fig.
The use of proportional technology saves one valve. The proportional valves are controlled by means of an electrical control system via an electrical signal, whereby it is possible, during operation, • to lower the pressure during reduced load phases (e. stoppage of slide) via the proportional pressure relief valve and to save energy, • to gently start-up and decelerate the slide via the proportional directional control valve. All valve adjustments are effected automatically, i.
without human intervention. 8 © Festo Didactic GmbH & Co. Introduction to proportional hydraulics B A A B Y1 Y2 P T P Y3 T P P M T Fig.4: Hydraulic circuit diagram of a feed drive using proportional valves © Festo Didactic GmbH & Co. Introduction to proportional hydraulics 1.5 clearly shows the signal flow in proportional hydraulics.
Signal flow and components in proportional • An electrical voltage (typically between -10 V and +10 V) acting upon an electrical hydraulics amplifier. • The amplifier converts the voltage (input signal) into a current (output signal). • The current acts upon the proportional solenoid. • The proportional solenoid actuates the valve.
• The valve controls the energy flow to the hydraulic drive. • The drive converts the energy into kinetic energy. The electrical voltage can be infinitely adjusted and the speed and force (i. speed and torque) can be infinitely adjusted on the drive accordingly.
Electrical Proportional Proportional Controller Drive amplifier solenoid valve Proportional technology components Fig.5: Signal flow in proportional hydraulics 10 © Festo Didactic GmbH & Co. Introduction to proportional hydraulics Fig.6 illustrates a 4/3-way proportional valve with the appropriate electrical amplifier.6: 4/3-way proportional valve with electrical amplifier (Vickers) © Festo Didactic GmbH & Co. Introduction to proportional hydraulics 1.5 Comparison of switching valves and proportional valves Advantages of proportional The advantages of proportional valves in comparison with switching valves has hydraulics already been explained in sections 1.4 and are summarised in table 1. Advantages of electrically actuated proportional valves compared with switching valves Adjustability of valves – infinitely adjustable flow and pressure via electrical input signal – automatic adjustment of flow and pressure during operation of system Effect on the drives automatable, infinite and accurate adjustment of – Force or torque – Acceleration – Velocity or speed – Position or rotary angle Effect on energy consumption Energy consumption can be reduced thanks to demand- oriented control of pressure and flow.
Circuit simplification A proportional valve can replace several valves, e. a directional control valve and a flow control valve Table 1.1: Advantages of electrically actuated proportional valves compared with switching valves 12 © Festo Didactic GmbH & Co. Introduction to proportional hydraulics Comparison of proportional and servohydraulics The same functions can be performed with servo valves as those with proportional valves. Thanks to the increased accuracy and speed, servotechnology even has certain advantages.
Compared with these, the advantages of proportional hydraulics are the low cost of the system and maintenance requirements: • The valve design is simpler and more cost-effective. • The overlap of the control slide and powerful proportional solenoids for the valve actuation increase operational reliability. The need for filtration of the pressure fluid is reduced and the maintenance intervals are longer. • Servohydraulic drives frequently operate within a closed loop circuit.
Drives equipped with proportional valves are usually operated in the form of a control sequence, thereby obviating the need for measuring systems and controller with proportional hydraulics. This correspondingly simplifies system design. Proportional technology combines the continuous electrical variability and the sturdy, low cost construction of the valves. Proportional valves bridge the gap between switching valves and servo valves.
© Festo Didactic GmbH & Co. Introduction to proportional hydraulics 14 © Festo Didactic GmbH & Co. Proportional valves: Design and mode of operation 2.1 Depending on the design of the valve, either one or two proportional solenoids are Design and mode of used for the actuation of an electrically variable proportional valve. operation of a proportional solenoid Solenoid design The proportional solenoid (fig.
1) is derived from the switching solenoid, as used in electro-hydraulics for the actuation of directional control valves. The electrical current passes through the coil of the electro-solenoid and creates a magnetic field. The magnetic field develops a force directed towards the right on to the rotatable armature. This force can be used to actuate a valve.
Similar to the switching solenoid, the armature, barrel magnet and housing of the proportional solenoid are made of easily magnetisable, soft magnetic material. Compared with the switching solenoid, the proportional solenoid has a differently formed control cone, which consists of non-magnetisable material and influences the pattern of the magnetic field lines. Mode of operation of a proportional solenoid With the correct design of soft magnetic parts and control cone, the following approximate characteristics (fig.) are obtained: • The force increases in proportion to the current, i. a doubling of the current results in twice the force on the armature.
• The force does not depend on the position of the armature within the operational zone of the proportional solenoid. © Festo Didactic GmbH & Co. Proportional valves: Design and mode of operation 1 Electrical connection 7 Armature 2 Non-magnetisable inner ring control cone 8 Barrel magnet 3 Core magnet 9 Plain bearing 4 Guide rod (stem) 10 Housing 5 Stop/Guide disc 11 Compensating spring 6 Exciting coil 12 Venting screw 1 12 2 3 11 4 10 9 8 7 6 5 Force F Current I I0 0.25 I0 Armature position x Operational range (typically: approx 2mm) Fig.1: Design and characteristics of a proportional solenoid 16 © Festo Didactic GmbH & Co. Proportional valves: Design and mode of operation In a proportional valve, the proportional solenoid acts against a spring, which creates the reset force (fig.
The spring characteristic has been entered in the two characteristic fields of the proportional solenoid.