How to Understand Variable Pump Controls. Brendan Casey Marian Tumarkin

Transcription

1 How to Understand Variable Pump Controls Brendan Casey Marian Tumarkin

2 How to Understand Variable Pump Controls 2 Copyright 2007 Brendan Casey & Marian Tumarkin All rights reserved. No part of this electronic book may be reproduced or transmitted in any form or by any means, electronic, mechanical, photocopying, and recording or otherwise, without the prior, written permission of the publisher. The contents of this book reflect the author s views acquired through his experience in the field under discussion. The information in this document is distributed on an As is basis, without warranty. Every effort was made to render this book free from error and omission. However, the author, publisher, editor, their employees or agents disclaim liability for any injury, loss, or damage to any person or body or organization acting or refraining from action as a result of material in this book, whether or not such injury loss or damage is in any way due to any negligent act or omission, breach of duty, or default on the part of the author, publisher, editor or their employees or agents. First published in 2006 by HydraulicSupermarket.com PO Box 1029 West Perth WA 6872 Australia info@hydraulicsupermarket.com Web: About the Authors Brendan Casey is the founder of HydraulicSupermarket.com and the bestselling author of 'Insider Secrets to Hydraulics' the most comprehensive guide to reducing hydraulic equipment operating costs ever published. A fluid power expert with an MBA, he has more than 18 years experience in the design, maintenance and repair of mobile and industrial hydraulic equipment. Dr Marian Tumarkin has over 35 years of experience in the field of Fluid Power with a Ph.D. from the National Academy of Science in Moscow. He is an accomplished scholar in the field with over 50 published papers and 10 patents to his name. A highly experienced Fluid Power engineer, Marian has designed electrohydraulic systems for Russian and Australian Air Forces, Australian and American automotive industries, as well as developing special purpose machines. In this role, he was responsible for concept design and problem solving, calculations and component selection, system testing and troubleshooting. Marian has extensive teaching experience both in Europe and Australia, delivering basic and advanced Fluid Power subjects to undergraduate and postgraduate students as well as engineers and technicians.

3 How to Understand Variable Pump Controls 3 PUMP DELIVERY CONTROL DEFINITIONS Pump delivery control is the most effective way to minimize wasted energy and reduce power consumption in hydraulic systems. Two main methods of control can be used: Delivery control of the fixed-displacement pump by changing the speed of the pump shaft Delivery control of the variable-displacement pump by changing pump displacement. The first method is in an early stage of development and currently does not have wide industrial applications. In the future it could become a popular method of pump delivery control. The second method is widely used. Adjustment of pump displacement is accomplished in vane and radial piston pum by varying the eccentricity, and in an axial piston pump, by adjusting the angle of the swash plate. For example, in a variable-displacement vane pump, control pressure moves the stroke ring, which changes the Displacement Stroke ring s eccentricity, and therefore, control ring pump delivery Fig The types of control methods are similar for different pum and for different manufacturers. Therefore, to simplify explanation, different types of displacement control are considered below with regard to a swash-plate axial piston pump. Fig Variable displacement vane pump Main components of a variabledisplacement, swash-plate axial piston pump are shown in Fig Pump 2 Swash plate 3 Control piston 4 Pressure here reduces pump displacement 5 Pressure here increases pump displacement Pump delivery 5 4 Fig Swash plate variable displacement pump 3 2 1

4 How to Understand Variable Pump Controls 4 A variable pump regulator is essentially a system of modular valves installed on the pump Fig Control Piston The regulator circuit diagram uses common symbols for modular valves. The pump module includes pump and control piston Fig Fig Rexroth pump A4VSO Fig Pump module with single control piston Two control pistons can be used to simplify control Fig Fig Pump module with two control pistons The pump module is an actuator (Fig. 5.6) for a closed-loop proportional control system. Control input (Pressure Pc) CONTROL PISTON Swash Plate Angle PUMP Control output (Pump Delivery ) Fig Block diagram of the pump module From a functional point of view, there are three common types of control. Each of these presents a variety of options. The control system function is usually described by a characteristic curve, which is the pump pressure-delivery graph.

5 How to Understand Variable Pump Controls 5 1. PRESSURE CONTROL. The regulator automatically adjusts pump delivery to limit output pressure according to the setting. Any pressure p 1, p 2, etc inside of the limit (p set p) can be set using manual or remote hydraulic adjustment Fig Q p2 p1 Fig Pressure control graph p Q Q1 Q2 p Fig.5.8. Flow control graph 2. FLOW CONTROL. The regulator provides constant pump delivery according to the setting. Any flow rate Q 1, Q 2, etc below maximum pump delivery Q can be set by manual adjustment of a variable throttle Fig POWER CONTROL. The regulator constantly multiplies pressure and flowrate and compares the result with the preset value of power P set. If the pump output power exceeds the set value, the regulator reduces pump delivery. It means that for any particular pump delivery Q 2 (see Fig. 5.9) pressure p 2 can be only p 2 P setting Q 2 Structure of pump delivery regulator is shown in Fig The functions of the regulator are: Measure the system parameter (pressure, flow rate, or hydraulic power) Compare it with the setting value Give the command (control pressure p c ) to the control piston Hydraulic System Q1 Q2 Q3 p1 1 p2 2 p3 Fig Power control graph Measurement More displacement 3 Setting Control pc Fig Structure of the regulator

6 How to Understand Variable Pump Controls 6 Currently, the majority of regulators use proportional spools for their operation. So, a proportional spool creates the control pressure p c, which changes the angle of the swash plate and therefore, delivery of the pump. Block diagram of the regulator (Fig. 5.11) describes its components from the point of view of closed-loop control: Controller compares system parameter (pressure, or flow rate, or hydraulic power) with setting and generates control signal (pressure p c ) proportional to the error (difference) Actuator (pump module) generates control output (pump delivery Q p ) to the object (Hydraulic System) Feedback measures controlled variable (pressure, flow rate, or hydraulic power) and provides signal to controller Setting + Force PROPORTIONAL SPOOL Pc ACTUATOR OBJECT () - Controlled Variable Fig Block diagram of the pump delivery regulator Important features of the regulator are: Controlled variable is measured directly (system pressure) or with a dedicated feedback device (flow rate, or hydraulic power) The control output of the regulator is pump delivery - for any controlled variable Two or more variables can be controlled by the regulator, but not simultaneously because of the same control output Setting options can be manually, directly on the pump, or remotely from the control panel

7 How to Understand Variable Pump Controls 7 PRESSURE AND FLOW CONTROL PRESSURE CONTROL (PRESSURE COMPENSATOR) Function of the pressure regulator is to control system pressure p s according to the setting p Fig With p s below setting p 1, pump delivery Q p is maximum from point 1 to point 2. Decrease in maximum flow rate with increasing pressure is the result of increasing internal leakage in the pump. Regulator controls pump delivery and therefore system pressure p s from point 2 p1 through point 3 to point 4. Variation in the controlled pressure (non-vertical lines) is Fig Pressure control graph the result of regulator error Principle of regulator operation is shown in System pressure 4 2 pc 3 5 Fig Pressure control 1 1 Fig The system pressure p s acts through connection (4) on the proportional spool (3) against spring (5). Position of the spool is proportional to system pressure. Control pressure p c is proportional to spool displacement. Therefore, position of the control piston (1) is proportional to system pressure. Setting of the spring (5) determines the starting point of spool movement. Below this setting the spool (3) stays in the left position, connecting the right chamber of the control piston (1) with the tank, p c = 0, and pump displacement is maximum The pressure regulator is a closedloop proportional control system with system pressure feedback through the pipeline 4 Fig Q Pressure module Pump module Fig Pressure control circuit Setting Pc (line 4) ACTUATOR Controlled Variable Fig Block diagram of the pressure regulator Hydraulic circuit diagram for variable displacement pump with pressure control is shown in Fig. 5.15

8 How to Understand Variable Pump Controls 8 REMOTE PRESSURE CONTROL Remote pump delivery control is required in two situations: Frequent adjustment of the setting parameters is desired Manual adjustment during system operation is required. Remote pressure control arrangement is shown in Fig System pressure p s is determined by the setting of the pilot relief valve (7), located in a convenient position, remote from the pump. When pressure p s is below the setting of valve (7), proportional control valve (3) gives the command for maximum pump displacement. When pressure p s reaches the setting of relief valve (7), it opens, allowing the proportional spool to move to the right, which reduces pump displacement and, therefore, flow rate Q. Q 7 3 x 5 Remote setting (7) Fig Remote pressure control Setting Pc (line 4) ACTUATOR Controlled Variable Remote pressure control acts in parallel with main setting see block diagram in Fig Fig Block diagram of pressure regulator with remote control 9 x 5 Q 3 Remote pressure control can be used in a twopressure (high-low) circuit by adding an ON - OFF solenoid DCV (9) between the pump module and the remote relief valve (7) Fig When the valve (9) is de-energized, pressure p s is determined by the setting of the spring (5). When the valve (9) is energized, pressure p s is reduced to the setting of the remote relief valve (7). Fig Remote two-pressure control

9 How to Understand Variable Pump Controls 9 FLOW CONTROL (LOAD SENSING) Function of the flow regulator is shown in Fig Pump delivery is controlled according to setting Q 1 from point 1 through point 2 to point 3. Small decrease of the set flow rate (non-horizontal lines) is the result of regulator error. From point 3 to 5 the system relief valve limits maximum pressure. The controlled flow does not depend on the pump drive speed. Q Fig Flow control graph 6 5 pc p 4 7 Principle of regulator operation is shown in p= p p S Fig The pressure drop across an adjustable throttle (6) in the pump output line acts on the proportional spool, to alter control pressure p c and adjust the pump displacement to maintain: p = const and therefore Q = const P Fig Flow control The flow regulator is a closed-loop proportional control system with flow feedback via the throttle (6) and two pipelines (4) and (7) Fig Any interference, for example remote pressure control in pipeline (7), would disrupt feedback and override the primary function of the regulator. Setting p Pc (lines 4,7) ACTUATOR Controlled Variable X Fig Block diagram of the flow regulator Flow module Pump module Hydraulic circuit diagram for variable displacement pump with flow control is shown in Fig Fig Flow control circuit

10 How to Understand Variable Pump Controls 10 REMOTE FLOW CONTROL Manual remote flow control arrangement is shown in Fig Instead of an adjustable throttle (6) in Fig. 5.20, pressure drop is created by metering manual 4/3 DCV (7). The pressure lines between DCV and cylinder are connected via shuttle valve (8) to the pump flow control module (5). When the operator moves the spool in the DCV say 20% of its stroke, the slot on the spool land creates pressure p= p p S drop, which acts on the proportional spool of the flow control module (5). The flow control module varies pump displacement to maintain pressure drop p = const according to its setting and therefore, flow rate and cylinder speed remain constant Q p Fig Manual remote flow control x In applications where pump drive speed is constant, another remote flow control arrangement is to control pump displacement directly - Fig. 5.24: 2 Control pump to supply pressure to regulator independently of system pressure 5 Pressure reducing valve for remote control of the position of the proportional spool (6) 8 Mechanical feedback to maintain position of the control cylinder (9) proportional to the position of the proportional spool (6) SwP Swash plate displacement B P 1 X2 SwP Fig Remote displacement delivery control Setting Remote setting (5) Pc (link 8) ACTUATOR Controlled Variable SwP Fig Block diagram of the displacement regulator Displacement remote control is the closed-loop proportional control system with mechanical feedback Fig This control method is not as accurate as flow control (load sensing). Therefore, its main application is for high flow rates, where pressure drop across an adjustable throttle results in significant power loss.

11 How to Understand Variable Pump Controls 11 COMBINED PRESSURE AND FLOW CONTROLS Both pressure and flow control modules can be used together Fig From point 1 to point 3 flow regulator operates to control set flow Q 1, and from point 3 to point 5 pressure regulator overrides flow control to control set pressure p 1. Q p1 Fig Pressure-flow control graph Q Principle of the combined regulator operation is shown in Fig Flow control module and pressure control module are installed in series. If flow rate Q increases above its setting, the proportional spool in the flow control module will reduce control pressure p c and, therefore, pump displacement. pc However, if system pressure p s increases above its setting, the proportional spool in the pressure control module will further reduce the pump displacement. Fig Pressure-flow control Q Flow module Hydraulic circuit diagram for variable displacement pump with combined control is shown in Fig Pressure module Pump module Fig Pressure-flow control circuit

12 How to Understand Variable Pump Controls 12 EXAMPLE 5.1. Hydraulic system for molding press To increase efficiency of the hydraulic system for a molding press using a multipump circuit, consider possible solution of the problem by using pump delivery control. To provide fast approach with low pressure and slow final pressing with high pressure, a variable displacement pump with pressure-flow control is installed in the system Fig. 5.29: 6 Adjustable throttle 8 Safety relief valve 9 Solenoid On-Off valve 15 Flow control module 16 Pressure control module 17 Mechanical stop 10 1 S1 S The mechanical stop (17) is adjusted to achieve the required cylinder speed during step 1 (fast approach). S3 9 6 p1 15 During step 2 (final pressing) the solenoid S 3 is energized, and the flow control module (15) provides the required flow rate Q = 6 L/min (1.6 GPM). 8 1 Q p Pressure control module (16) maintains the required pressure for step 3 (holding) Fig Hydraulic circuit for molding press with variable displacement pump and pressure-flow control The efficiency of the described system during step 2 is similar to the multi-pump system. However, the system with variable displacement pump has the following advantages: Pump delivery for step 1 can be tuned exactly according to the required cylinder speed (up to maximum pump delivery) Additional functions for pump delivery control can be used. For example, in the system shown in Fig the pressure control module (16) must have a pressure setting above the maximum pressure required during step 3 (holding) to provide pressure drop across the throttle (6) during step 2. If this condition is unacceptable, additional remote pressure control can be installed to enable setting of the holding pressure exactly to specification.

13 How to Understand Variable Pump Controls 13 POWER CONTROL Power control sets a power limit that is a combination of pump displacement and operating pressure p s * V g = constant. With constant drive speed, it means 2 Power control p s * Q p = const Fig The power control provides a curve with hyperbolic 3 characteristic. Increasing of system pressure Flow control p s causes decreasing of Q p (points 1 2 3). Any point at parabolic curve follows the rule: p Q = p Q = p Q = const Fig Power control graph This control has two important advantages: 1 Pressure control It significantly reduces the size of the prime mover It provides a smooth transition from high flow (point 1) to low flow (point 3) Note. Regulator dynamic response, when moving from point 1 to point 3, will be close to the curve If pressure and flow control is used, it is difficult to predict step response. In the worst case scenario, the movement can be Principle of operation of the power regulator is shown in Fig System pressure p s acts on the plunger (4) and creates a force on the proportional spool (6) via mechanical link (8). The force acts on the proportional spool against spring (5). Therefore, spool displacement and control pressure p c are proportional to the force pc For the same system pressure p s the amount of force acting on the spool (6) depends on the position of the control piston (3): the larger swash plate displacement, the larger the force. SwP Fig Rexroth LR2 power control Finally, the swash plate position SwP (i.e. pump displacement V g ) is proportional to the control pressure p c. This provides output hydraulic power proportional to the product of p s and pump displacement V g. Setting of the spring (5) determines static position of the spool (6), pressure p c, swash plate position SwP, and pump delivery Q P.

14 How to Understand Variable Pump Controls 14 Power regulator is a closed-loop proportional control system with the feedback signal proportional to product of system pressure p s and pump displacement V g Fig Drive speed Setting Pc CONTROL PISTON SwP PUMP xvg Vg Controlled Variables Fig Block diagram of the power regulator Pressure control module can be included to control maximum system pressure p s Fig Pressure control module (7) overrides power control, when the set pressure is reached. pc 7 Block diagram of the regulator is shown in Fig Fig Rexroth LR2D power and pressure control PRESSURE Setting Pc CONTROL PISTON SwP PUMP xvg Vg Controlled Variables POWER Fig Block diagram of the power regulator with pressure control

15 How to Understand Variable Pump Controls 15 EXAMPLE 5.2. Hydraulic system for molding press Hydraulic system with variable displacement pump (Example 5.1) can be further developed by using power control with additional control functions Fig Control pump 5 Control pressure reducing valve 9 Control ON-OFF solenoid valve 14 Hydraulic stroke limiter (remotely controls maximum pump delivery) 15 Pressure control module 17 Mechanical stroke limiter 10 9 p2 Q2 S3 S pr S pc 17 Fig Hydraulic system for molding press with power control, pressure control, and stroke limiters The following settings are applied: Power control module is adjusted to limit hydraulic power to 7 kw (10 HP) Mechanical stop (17) is adjusted to provide pump delivery 72 L/min (19 GPM) for step 1 Pressure control module (15) is adjusted to limit maximum system pressure for step 3 Reducing valve (5) is adjusted to provide pump delivery 6 L/min (1.6 GPM) for step 2 During step 1 (fast approach), pump delivery (power) control provides stepless deceleration of cylinder speed as load-induced pressure increases without power losses. During step 2, solenoid (9) is energized, and pump delivery provides accurate cylinder speed control without power losses. During step 3, pressure control module (15) reduces the pump displacement to minimum required to maintain the holding pressure. The advantages of power control in the described application are obvious. Disadvantages of power control: Higher initial cost of the regulator Higher cost of maintenance

16 How to Understand Variable Pump Controls 16 ELECTRO- CONTROL Pump delivery control can be part of an electro-hydraulic proportional control system. Such arrangement can be used for both functions of closedloop proportional control: A follow-up system (servo system), where the electrical input signal is constantly changing, and the output (controlled variable) is controlled to follow the input (Fig. 5.36) Control input PUMP DELIVERY Controlled variable Fig Block diagram of a servo system A regulator, where the function of the system is maintaining the controlled variable at a set value, and electrical control signal changes setting (Fig. 5.37) Setting PUMP DELIVERY Controlled variable Fig Block diagram of a regulator To simplify analysis, consider possible options for electro-hydraulic control of the system pressure based on the variable displacement pump. All options are applicable for control of many different parameters (flow, power, etc). Consider remote pressure control of the variable displacement pump (see Fig. 5.16). This is a hydraulic closed-loop system with function to control system pressure according to the setting, i.e. pressure regulator Fig The closed loop here is: proportional spool pump displacement system pressure feedback line x Manual Input RELIEF VALVE (4) PROPORTIONAL SPOOL (3) (12) Pc CONTROL PISTON (2) SwP PUMP (1) Output - Force CYLINDER 1 2 Fig Remote pressure control OPTION 1.

17 How to Understand Variable Pump Controls 17 Electro-hydraulic pressure regulator can be configured by replacing the conventional pressure relief valve (4) in Fig with a proportional pressure relief valve (5). The regulator provides reasonably accurate control of the hydraulic system pressure according to setting of the input electrical signal Fig x 2 delivery control piston 3 proportional spool 5 proportional pressure relief valve 12 pressure feedback 1 2 Fig Electro-hydraulic pressure regulator Block diagram of the regulator is shown in Fig Input electrical signal can be generated by any programmer or manually. The main problem with this arrangement is double conversion of the signal: Input PROPORTIONAL VALVE (5) p5 PROPORTIONAL SPOOL (3) pc CONTROL PISTON (2) 1) Input electrical signal into proportional set pressure p 5 (12) SwP PUMP (1) 2) Set pressure p 5 into proportional control Output - Force CYLINDER pressure p c Fig Block diagram of pressure regulator Therefore, accuracy of such control is within 5%. Dynamic response is very slow with possible oscillation. Possible applications: Manual changing of the set pressure Electro-hydraulic sequential control, when dynamic response is not critical

18 How to Understand Variable Pump Controls 18 OPTION 2. For proportional follow-up pressure control with high accuracy and good dynamic response the solution is to use servo control with internal feedback through swash plate position transducer. In this arrangement the pump control includes an electro-hydraulic proportional valve (4) and pump displacement transducer (9) Fig Control pump to provide independent hydraulic supply for proportional valve (7) 7 Proportional valve 8 Pressure transducer for main feedback 9 Control piston displacement transducer 10 Electronic card to provide internal feedback via transducer u u s 10 9 Controller Programmer Fig Follow-up pressure control system Block diagram of the system is shown in Fig Input AMPLIFIER (10) PROPORTIONAL VALVE (7) Pc CONTROL PISTON (2) PUMP (1) TRANSDUCER (9) SwP TRANSDUCER (8) Output - Force CYLINDER Fig Block diagram of follow-up pressure control system From the point of view of pump displacement control this is the best option, because it provides precise and repeatable control of the swash plate angle independently of the force generated on the swash plate by the pump s rotating cylinder block. Moreover, fine tuning of the amplifier (10) will give smooth and fast dynamic response of the control system. Accuracy of the pressure control is achieved by main feedback (8) with the fine tuning of the controller. It is not affected by internal leaks in the pump, speed of pump rotation or any other system disturbances.

19 How to Understand Variable Pump Controls 19 OPTION 3. High accuracy of control is achieved by using main feedback of the controlled mechanical parameter (here, the force created by the cylinder) Fig The electronic controller gives the command to the proportional valve based on the error in the closedloop system (difference between input signal and actual output force) u s 9 u F 8 Controller Programmer Fig Follow-up force control system Block diagram of the system is shown in Fig Input AMPLIFIER (10) PROPORTIONAL VALVE (7) Pc CONTROL PISTON (2) PUMP (1) SwP TRANSDUCER (8) TRANSDUCER (9) Output - Force CYLINDER Fig Block diagram of follow-up force control system Output of the system depends on the main feedback transducer (8). It could be actuator force, or displacement, or speed, or torque of a hydraulic motor, etc Advantages: Direct control of the output mechanical parameters (force instead of pressure, speed instead of flow rate, etc) High accuracy of control Replacement of hydraulic regulator with electronic regulator Substantial reduction in the number of hydraulic valves and therefore, simplification of maintenance and troubleshooting

20 How to Understand Variable Pump Controls 20 FURTHER READING Advanced Hydraulic Control by Brendan Casey & Marian Tumarkin Available second half of 2007: