Selection factors proportional valves Steve Skinner, Eaton Hydraulics,

Selection factors proportional valves Steve Skinner, Eaton Hydraulics,

Selection factors proportional valves Steve Skinner, Eaton Hydraulics, Havant, UK Copyright Eaton Hydraulics 2000 Selection factors proportional valves Configuring the piston Nominal flow Performance Control / regulation Respone Linearity Repeatability Reproducibility Hysteresis Pressure gain Recovery of piston Configuring the piston Piston (symmetrical) input and output

A A B T P B T P 20N T The piston is shown in the more current a proportional The piston is shown in the more current a proportional valve. For a given aperture, limiting the flow is the valve. For a given aperture, limiting the flow is the same on all ranges. In the case of an actuator with same on all ranges. In the case of an actuator with equal active areas (such as a motor), this enables a equal active areas (such as a motor), this enables a control on both the input to the output. In the case of an

control on both the input to the output. In the case of an actuator to different active surfaces (cylinder), the order actuator to different active surfaces (cylinder), the order is essentially, in the direction of displacement, on the is essentially, in the direction of displacement, on the entry or exit. entry or exit. Output piston A A B P T T B P 28S

T The output piston has notches at A-level The output piston has notches at A-level spans T and BT. The limitation of the flow is spans T and BT. The limitation of the flow is relatively low in litters PA and PB (as in a relatively low in litters PA and PB (as in a conventional distributor switching). conventional distributor switching). The output piston may be used in the case of The output piston may be used in the case of cylinders and engines. It is often used for cylinders and engines. It is often used for controlling negative charges. controlling negative charges. Input piston A A B T

B P T 22A P T The KDG4V distributor 3S is available only The KDG4V distributor 3S is available only input piston. It has notches in litters PA and input piston. It has notches in litters PA and PB. PB. Asymmetric piston A A B P

T T B P 20N10 T To ensure optimal actuators different active To ensure optimal actuators different active control surfaces, most proportional valves are control surfaces, most proportional valves are available with asymmetric piston. In this case, the available with asymmetric piston. In this case, the limitation of the flow from P to A or to B P is lower limitation of the flow from P to A or to B P is lower than P to B or from B to T. If, is used, for example than P to B or from B to T. If, is used, for example a piston in a cylinder 2:1 2: 1, the same command a piston in a cylinder 2:1 2: 1, the same command can be input and output. can be input and output.

Symmetrical piston The advantages of an asymmetric The advantages of an asymmetric piston can be illustrated with a piston can be illustrated with a simple example of an uncharged simple example of an uncharged cylinder controlled by a proportional cylinder controlled by a proportional valve and a pressure compensator. valve and a pressure compensator. 2 : 1 A B P T 8 bar Symmetrical piston Upon retraction of the piston Upon retraction of the piston

rod, the pressure compensator rod, the pressure compensator maintains a constant pressure maintains a constant pressure drop of 8 bar between ports P drop of 8 bar between ports P and B of the proportional valve. and B of the proportional valve. 2 : 1 A B P P = 8 bar 8 bar P T Symmetrical piston As the flow from A to T is As the flow from A to T is

double that of P to B (relative to double that of P to B (relative to a cylinder of the active surfaces a cylinder of the active surfaces of 2:1), the pressure drop from of 2:1), the pressure drop from A to T is four times that of B to A to T is four times that of B to P (32 bar in this case). P (32 bar in this case). 2 : 1 A B P = 32 bar 8 bar P T Symmetrical piston 1) against a pressure of 32 1) against a pressure of 32

bar on the side of the front bar on the side of the front face of the piston requires a face of the piston requires a pressure of 64 bar on the pressure of 64 bar on the side of the rod. side of the rod. 2 : 1 32 bar 2) As a result it takes a 2) As a result it takes a mini-mum pressure of 72 mini-mum pressure of 72 bar at the port P of bar at the port P of connec-tion to move the connec-tion to move the actuator in the absence actuator in the absence of load. of load. 64 bar

A B 72 bar P = 32 bar 8 bar P T Asymmetric piston If the symmetrical piston is If the symmetrical piston is replaced by an asymmetric replaced by an asymmetric piston (2:1), the pressure piston (2:1), the pressure drop imposed by the drop imposed by the balance of pressure will also balance of pressure will also 8 bar. 8 bar.

2 : 1 A B Piston 2:1 P = 8 bar 8 bar P T Asymmetric piston A 2:1 to a 2:1 piston A 2:1 to a 2:1 piston associated cylinder then associated cylinder then results also have a pressure results also have a pressure drop of 8 bar A to T. drop of 8 bar A to T. 2 : 1

A B Piston 2:1 P = 8 bar 8 bar P T Asymmetric piston The against-pressure of 8 The against-pressure of 8 bar on the side of the front bar on the side of the front face of the piston 16 face of the piston 16 requires the side bar of the requires the side bar of the rod, and therefore a rod, and therefore a minimum pressure of 24 bar minimum pressure of 24 bar

at the port P. at the port P. The use of a reduced and The use of a reduced and 72 bar to 24 bar the 72 bar to 24 bar the pressure required at the pressure required at the piston orifice asymmetric P. piston orifice asymmetric P. 2 : 1 8 bar 16 bar A B 24 bar Piston 2:1 P = 8 bar 8 bar

P T Selection options in the piston A T P 20N A T A B T B P 22A

T The choice of the limiting The choice of the limiting characteristic of the piston characteristic of the piston speed is determined by the speed is determined by the type of actuator and the type of actuator and the nature of the feedstock. nature of the feedstock. T T B P 28S A T B

P 20N10 T Central positions of the piston A B A B A B P T P T

P T 2C 33C 5C Several options for selecting the central position of the piston are also Several options for selecting the central position of the piston are also available with most proportional valves. The right choice depends mainly available with most proportional valves. The right choice depends mainly on the load characteristic and auxiliary valves in the system. If used, for on the load characteristic and auxiliary valves in the system. If used, for example, a proportional valve to prevent movement of an actuator, it is example, a proportional valve to prevent movement of an actuator, it is possible to use a plunger 2C. If used, however, a non-return or possible to use a plunger 2C. If used, however, a non-return or electromagnetically controllable control valve, a piston 33C of the type can electromagnetically controllable control valve, a piston 33C of the type can be more appropriate. be more appropriate. Nominal flow

Nominal speed of the piston QR 2) Various volumetric flow rates can be obtained by 2) Various volumetric flow rates can be obtained by varying the size, shape and number of slots of the varying the size, shape and number of slots of the piston. piston. l/min A B P T P= 5 bar 1) The nominal flow rate is defined by a piston flow that passes a 1) The nominal flow rate is defined by a piston flow that passes a range of the piston (maximum aperture) to a pressure drop of 5 range of the piston (maximum aperture) to a pressure drop of 5

bar. bar. QR 3 l/min 3) valves at nominal flow rate of 3) valves at nominal flow rate of between 3 and 550 l / min are currently between 3 and 550 l / min are currently available. available. 550 l/min Flow calculation A1 V1 Q1 QR = ? The right choice of the size of a The right choice of the size of a proportional valve is important proportional valve is important because a too small distributor does

because a too small distributor does not provide the required speed of not provide the required speed of the actuator. Too distributor may the actuator. Too distributor may result in a very low piston opening, result in a very low piston opening, which is difficult to control. which is difficult to control. Flow calculation A1 A2 F P1 P2 PS PT As a proportional valve As a proportional valve

limits the flow to both the limits the flow to both the inputs or outputs of the inputs or outputs of the actuator, the choice of the actuator, the choice of the correct size however, is correct size however, is not as simple as in the not as simple as in the case of a conventional case of a conventional valve switching. valve switching. Flow calculation A1 A2 P1.A1 P1 F P2.A2

P2 . 1 = P2.A . 2 + F PP1.A A = P 1 1 2 A2 + F PS PT To determine the pressure drop To determine the pressure drop across the valve, it is necessary to across the valve, it is necessary to estimate the pressures P1 and P2. estimate the pressures P1 and P2. This can be done starting from the This can be done starting from the balance of forces on the piston of balance of forces on the piston of the jack ...

the jack ... Flow calculation A1 A2 F P1 Q Q. Q A2 A1 P2 . 1 = P2.A . 2 + F PP1.A A = P 1

1 2 A2 + F P Q2 PS PT P2 PT A2 = PS P1 A1 2 And ... flow ratio and losses at the And ... flow ratio and losses at the metering orifice. metering orifice. Sizing software The simplest approach, The simplest approach, however, is to use software however, is to use software

performing all the steps of performing all the steps of the calculation. the calculation. performance Valve performance against non-response 2) When, in a valve-free 2) When, in a valve-free against reaction, energizes against reaction, energizes the electromagnet, there is the electromagnet, there is generated a magnetic force generated a magnetic force that pushes the piston in the that pushes the piston in the valve body in opposition to valve body in opposition to the force of a spring. the force of a spring. 1) In the selection of the 1) In the selection of the optimum operating speed of optimum operating speed of a proportional valve, it is

a proportional valve, it is also necessary to consider also necessary to consider the performance of the the performance of the valve, such as its ability to valve, such as its ability to maintain the piston in the maintain the piston in the required position of the required position of the forces in spite of flow. forces in spite of flow. P Q Valve performance against non-response 3) When, however, the 3) When, however, the valve is traversed by the valve is traversed by the fluid forces of flow is also fluid forces of flow is also exerted on the piston in exerted on the piston in

opposition to the magnetic opposition to the magnetic force. force. 1) The piston opens the 1) The piston opens the passage until the magnetic passage until the magnetic force and the force of the force and the force of the compressed spring are in compressed spring are in balance. balance. P 2) fully open, the flow 2) fully open, the flow through the valve is through the valve is

proportional to the square proportional to the square root of the pressure root of the pressure difference across the valve. difference across the valve. Q Valve performance against non-response 1) The forces of flow has 1) The forces of flow has the effect of closing the the effect of closing the part in the piston, ... part in the piston, ... P Q

2) ... which leads to a 2) ... which leads to a different curve of the different curve of the calculated and a calculated and a limitation of the limitation of the maximum possible maximum possible throughput. throughput. Valve performance against non-response As soon as the speed As soon as the speed

limit is reached, the curve limit is reached, the curve can either be vertical or can either be vertical or even regressive. In other even regressive. In other words, an increase in words, an increase in pressure drop then leads pressure drop then leads to a reduction in flow to a reduction in flow through the valve. through the valve. A vertical curve, A vertical curve, however, means that a however, means that a variation of the pressure variation of the pressure drop con-tained no drop con-tained no change in flow, for change in flow, for example in the case of a example in the case of a pressure compensating

pressure compensating effect. effect. P Q Valve performance against non-response P Performance curve indicated Performance curve indicated thus defines the boundary of thus defines the boundary of the range of user-tion of a the range of user-tion of a valve for a maximum input valve for a maximum input signal; it is, for example, not signal; it is, for example, not possible to work outside this possible to work outside this range. range.

Q Valve performance against non-response Valves against reaction Valves against reaction also limits performance, also limits performance, but these are generally but these are generally superi-eures to those of superi-eures to those of an equivalent against an equivalent against valve without feedback. valve without feedback. P Q Valve performance against non-response

1) When the flow forces 1) When the flow forces push the piston toward push the piston toward the closing position, the the closing position, the signal of the sensor signal of the sensor against the feedback against the feedback indicates that the piston indicates that the piston has changed position. has changed position. The intensity of the The intensity of the injected current in the injected current in the electromagnet is thus inelectromagnet is thus increased automatically. creased automatically. 2) This allows the piston 2) This allows the piston to oppose the forces of to oppose the forces of flow and thus permits flow and thus permits higher at higher flow

higher at higher flow rates through the valve. rates through the valve. P Q Valve performance against non-response However, it may happen However, it may happen that the forces of flow that the forces of flow become too large for the become too large for the solenoid, and the valve solenoid, and the valve will begin to close as will begin to close as before. before. P Q

Valve performance against non-response The range-formance of a The range-formance of a valve against the reaction valve against the reaction is greater than that of an is greater than that of an equivalent valve against equivalent valve against without feedback control without feedback control and enables higher and enables higher speeds. speeds. P Q Control / regulation

Order Ramp 1) In a control system, generates an amplifier from an input 1) In a control system, generates an amplifier from an input current applied voltage to the proportional valve, which current applied voltage to the proportional valve, which regulates the power and speed of the actuator and control its regulates the power and speed of the actuator and control its speed. speed. Amperage voltage flow Position acceleration speed 2) A change in the load of the actuator or the system pressure, however, 2) A change in the load of the actuator or the system pressure, however, may vary the speed of the actuator. may vary the speed of the actuator. A throttle control and braking of the actuator can be obtained using the

A throttle control and braking of the actuator can be obtained using the ramp generator of the amplifier which varies the amplitude of the ramp generator of the amplifier which varies the amplitude of the variation of the output current. variation of the output current. 3) In order to control the position of the actuator with a control system, may be 3) In order to control the position of the actuator with a control system, may be used a limit detector, which cuts the voltage applied to the amplifier (the input used a limit detector, which cuts the voltage applied to the amplifier (the input signal and selects and zero speed of the actuator). signal and selects and zero speed of the actuator). The exact position at which stops the actuator depends, however, a series of The exact position at which stops the actuator depends, however, a series of factors (load inertia, the response time of the valve, speed, etc..). The factors (load inertia, the response time of the valve, speed, etc..). The accuracy of the position and therefore limited, and overrun is not corrected. accuracy of the position and therefore limited, and overrun is not corrected. Order Ramp Amperage Rate of change force

pressure Force voltage A control pressure can be obtained by use of a pressure limiter or A control pressure can be obtained by use of a pressure limiter or regulator. In this case, the input voltage applied to the amplifier regulator. In this case, the input voltage applied to the amplifier generates a corresponding output current applied to the proportional generates a corresponding output current applied to the proportional valve to thereby regulate the pressure at the actuator. Generator ramps valve to thereby regulate the pressure at the actuator. Generator ramps integrated in the amplifier can s' used for controlling the rate of increase integrated in the amplifier can s' used for controlling the rate of increase or decrease of the pressure. or decrease of the pressure. Speed control Ramp acceleration PID + Amperage

_ pressure speed voltage speed voltage voltage A speed control requires a sensor which outputs a signal proportional to the speed of A speed control requires a sensor which outputs a signal proportional to the speed of the actuator against feedback. To generate an output current (or flow at the actuator) the actuator against feedback. To generate an output current (or flow at the actuator) when the error signal is zero (= against feedback input), the amplifier must be an when the error signal is zero (= against feedback input), the amplifier must be an integrator. Maintaining the speed (under different loads, etc.). Is much more accurate integrator. Maintaining the speed (under different loads, etc.). Is much more accurate than in the case of a control system. than in the case of a control system. As a control, a control of the acceleration and braking can be obtained using the As a control, a control of the acceleration and braking can be obtained using the ramp generator integrated in the amplifier. ramp generator integrated in the amplifier.

Position control Ramp 1) A position control requires a sensor which outputs a 1) A position control requires a sensor which outputs a signal proportional to the position of the actuator against signal proportional to the position of the actuator against feedback. feedback. PID + Amperage _ flow speed Position voltage Position voltage voltage

2) In most applications, the amplifier simply 2) In most applications, the amplifier simply generates a current output that is proportional to the generates a current output that is proportional to the positional deviation (less against feedback input). positional deviation (less against feedback input). The position accuracy is much greater than in a The position accuracy is much greater than in a control system. control system. 3) In some situations, however, it may use the integrator and differentiator amplifier to improve 3) In some situations, however, it may use the integrator and differentiator amplifier to improve system performance. However, such applications require a relatively high level of experience. system performance. However, such applications require a relatively high level of experience. The ramp generator integrated in the amplifier may also be used for controlling the speed of The ramp generator integrated in the amplifier may also be used for controlling the speed of movement of the actuator from one position to another. movement of the actuator from one position to another. Force control Ramp Rate of change force PID

+ Amperage pressure Force Force _ voltage voltage voltage For the force control, may use either a pressure sensor or a force sensor for the For the force control, may use either a pressure sensor or a force sensor for the provision of the feedback signal against. If the command is provided by a spool provision of the feedback signal against. If the command is provided by a spool valve, the amplifier typically generates an output signal including both a valve, the amplifier typically generates an output signal including both a proportional component and an integral component. proportional component and an integral component. As before, the ramp generator used to control the rate of change of the cylinder

As before, the ramp generator used to control the rate of change of the cylinder force. force. Response step response flow entry output The response time of the valve is an The response time of the valve is an important factor in high performance important factor in high performance systems. This may indicate the systems. This may indicate the behavior of the valve to a change in behavior of the valve to a change in level of the input signal, which in level of the input signal, which in some cases can have the shape some cases can have the shape shown here. The problem is,

shown here. The problem is, however, to determine the point at however, to determine the point at which the input signal and the output which the input signal and the output signal are equal. signal are equal. time step response entry output 100% 90% flow In other cases, the output may exceed In other cases, the output may exceed the input, and it is again difficult to the input, and it is again difficult to determine the extent to which input determine the extent to which input

and output signal are equal. To avoid and output signal are equal. To avoid this problem, the response time is this problem, the response time is usually indicated as the time required usually indicated as the time required for the output to reach 90% of final for the output to reach 90% of final value. value. T time Flow (P to B) Flow (P to A) Frequency Response 100% entry output 75%

50% 25% time It is sometimes more appropriate to measure the performance of It is sometimes more appropriate to measure the performance of the valve by examining its behavior to a sinusoidal curve applied to the valve by examining its behavior to a sinusoidal curve applied to the input. This is often obtained by oscillating the piston of the valve the input. This is often obtained by oscillating the piston of the valve of roughly 25% to around 50% open position. At low frequencies, of roughly 25% to around 50% open position. At low frequencies, the output of the valve can almost exactly follow the input. the output of the valve can almost exactly follow the input. Flow (P to B) Flow (P to A) Frequency Response entry 100% output

75% A2 50% 25% 360 A1 L 1) When the input frequency increases, the output amplitude decreases; 1) When the input frequency increases, the output amplitude decreases; the output starts to lag behind the input signal. the output starts to lag behind the input signal. Amplitude response(dB) = 20 log Phase delay () = L time ( ) A2 A1 2) Both of these effects are described by the two variables in

2) Both of these effects are described by the two variables in response amplitude and phase delay. response amplitude and phase delay. Frequency Response Phase delay () Amplitude (dB) 1) Amplitude and phase delay are 1) Amplitude and phase delay are measured for a series of frequencies and measured for a series of frequencies and shall refer in a diagram with logarithmic shall refer in a diagram with logarithmic scale. It is known Bode diagram. scale. It is known Bode diagram. -3 bandwidth 1 10 135

100 Frequence (Hz) 2) The frequency at which the output is reduced by 3 dB 2) The frequency at which the output is reduced by 3 dB (50%) is known as bandwidth. The cutoff frequency is the (50%) is known as bandwidth. The cutoff frequency is the frequency corresponding to a phase lag of 90 . One or more frequency corresponding to a phase lag of 90 . One or more of these values are normally used to determine the response of these values are normally used to determine the response of the valve. of the valve. 90 45 1 10 100 Frequence (Hz) Frequence break Linearity

Linearity Outflow QMAX E In control systems, in particular, it is often In control systems, in particular, it is often important to have a linear relationship important to have a linear relationship between input signal and the valve output between input signal and the valve output flow. The linearity of the valve quantifies flow. The linearity of the valve quantifies the quality of compliance with this the quality of compliance with this condition. condition. E x 100 Linearity (%) = QMAX Input signal

Repeatability Repeatability QMAX E Outflow Signal Repeatability (%) = successive applications a single signal (same valve) E x 100 QMAX The repeatability accuracy The

repeatability accuracy measurement with a valve which measurement with a valve which generates a desired output is generates a desired output is applied and removed when the applied and removed when the same signal. This is important in same signal. This is important in control systems in order to ensure control systems in order to ensure the smooth operation of a the smooth operation of a machine. machine. Reproducibility Reproducibility QMAX E Outflow

Signal Reproducibility (%) = successive applications a single signal (different valves) E x 100 QMAX Reproducibility is a similar Reproducibility is a similar measure, but refers to the same measure, but refers to the same signal applied to the various signal applied to the various valves. valves.

Hysteresis Hysteresis QMAX Outflow Hysteresis describes the difference Hysteresis describes the difference between the input signal increasing between the input signal increasing and decreasing input required to and decreasing input required to obtain a desired output signal. obtain a desired output signal. E Input signal IMAX Hysteresis (%) = E IMAX

x 100 Pressure gain Pressure gain The gain pressurizing valve The gain pressurizing valve measures the speed with which the measures the speed with which the piston opens and closes the work piston opens and closes the work ports of the valve when the piston ports of the valve when the piston moves to one side or the other of moves to one side or the other of the central position. the central position. Pressure gain Leaving its central position, the Leaving its central position, the piston gradually opened piston gradually opened

communication (A) to (P) and communication (A) to (P) and closes (T). This is the opposite to closes (T). This is the opposite to the hole (B). the hole (B). AA TT BB PP TT Pressure gain Once the piston has traveled a certain distance Once the piston has traveled a certain distance (typically equal to 3-4% of its total course), the (typically equal to 3-4% of its total course), the orifice (A) is the maximum pressure, and the port orifice (A) is the maximum pressure, and the port (B) to the reservoir pressure. In many control (B) to the reservoir pressure. In many control systems requires a high pressure gain for the valve

systems requires a high pressure gain for the valve to respond to also correct errors very small. to respond to also correct errors very small. AA P BB TT PP P S TT P PS 1 2 3 4 % entry Recovery of piston

Recovery of piston zero overlap positive overlap In some cases, it is possible to specify the In some cases, it is possible to specify the nature of recovery proportional piston. A nature of recovery proportional piston. A piston positive overlap reduces leakage in piston positive overlap reduces leakage in central position, a piston recovery often central position, a piston recovery often giving a negative system extremely fast giving a negative system extremely fast response (pistons zero overlap exist only response (pistons zero overlap exist only in theory). in theory). negative overlap po sit i ve

ove rla p neg ati v e ov erla p Outlet pressure Recovery of piston Input signal positive overlap negative overlap The nature of recovery influences the pressure gain, as can be seen here. A positive overlap leads to a dead The nature of recovery influences the pressure gain, as can be seen here. A positive overlap leads to a dead zone, wherein the outlet area does not change as the input signal does not exceed a certain threshold. In zone, wherein the outlet area does not change as the input signal does not exceed a certain threshold. In control systems, valves positive overlap (to reduce leakage) associated with electronic compensation to reduce control systems, valves positive overlap (to reduce leakage) associated with electronic compensation to reduce the actual dead zone is often used. Control valves, they normally use pistons negative overlap for maximum

the actual dead zone is often used. Control valves, they normally use pistons negative overlap for maximum control. control. Application Instructions yes poss. not control shocks Acceleration. order speed regulation speed regulation position regulation pressure This diagram shows the typical application areas for various types of proportional valves. This diagram shows the typical application areas for various types of proportional valves. As always, there is, again, exceptions. VALVE As always, there is, again, exceptions. Selection factors proportional valves

Steve Skinner, Eaton Hydraulics, Havant, UK Copyright Eaton Hydraulics 2000

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