C
ontrolling flow of a fluid-power system does not necessarily mean regulating gpm from a valve. Flow rate can be specified three different ways, so it is important all in-volved in a project to be aware of how flow is to be specified or measured:Volumetric flow rate, Qv, expressed in units of in3
/sec or min — or cc/sec or cc/min in SI metric measure — is used to calculate the linear travel speeds of piston rods or rotation speed of motor shafts.
Weight flow rate, Qw, expressed in units of lb/sec or lb/min, is used to cal-culate power using CU units of mea-sure.
Mass flow rate, Qg, expressed in units of slugs/sec or slugs/min for CU measure — or kg/sec or kg/min in SI metric measure — is used to calculate
variable orifice, Figure 1 (b). Both types are non-compensated flow con-trol devices.
Flow regulators — This device, Figure 2, which is slightly more sophis-ticated than a fixed orifice, consists of an orifice which senses flow rate, as a pressure drop across the orifice; a com-pensating piston adjusts to variations in inlet and outlet pressures. This com-pensating ability provides closer con-trol of flow rate under varying pressure conditions. Control accuracy may be 0.5%, possibly less with specially cali-brated valves which operate around a given flow-rate point.
Bypass flow regulators — In this flow regulator, flow in excess of set flow rate returns to reservoir through a bypass port, Figure 3. Flow rate is con-trolled by throttling fluid across a vari-able orifice regulated by the compen-sator piston. The bypass flow regulator creates lower energy losses in a system. Pressure-compensated, variable flow valves — This flow control is equipped with an adjustable variable orifice placed in series with a compen-sator. The compensator automatically adjusts to varying inlet and load pres-sures, maintaining an essentially con-stant flow rate under these operating
inertia forces during periods of
acceler-ation and deceleracceler-ation.
Because they control the quantity of fluid that flows through the valve per unit of time, the same control valves are used for all three types of flow rates.
Control of flow rate with valves
There are eight types of flow-control valves:
Orifices — A simple orifice in the line, Figure 1(a), is the most elemen-tary method for controlling flow. Note that this is also a basic pressure control device. When used for flow control, the valve is placed in a series with the pump. An orifice can be a drilled hole in a fitting, in which case it is a fixed orifice; or it may be a calibrated needle valve, in which case it functions as a
Eight basic configurations of
valves can be used to control
flow rate according to fluid
volume, weight, or mass.
Flow-control
valves
(b) (a) Variable orifice Fixed orifice InletBypass Regulated flow Handwheel
Fig. 1. Simple fixed orifice (a) and vari-able orifice (b) flow controls.
Fig. 2. Flow regulator adjusts to variations in inlet and output
conditions to accuracy’s of 0.5%, Fig-ure 4. PressFig-ure-compensated, variable flow control valves are available with integral free-reverse-flow check valves and integral overload relief valves.
Pressure- and temperature-com-pensated, variable flow valves — Be-cause viscosity varies with tempera-ture, so does the clearance between a valve’s moving parts. For this reason, output of a flow control valve may tend to drift with temperature changes. An attempt has been made to compensate not only for such temperature varia-tions, but pressure variations as well, Figure 5. Temperature compensators adjust the control orifice setting to off-set the effects of viscosity changes caused by temperature fluctuations of the fluid. Pressure compensators adjust the control orifice for pressure changes, as described above.
Demand-compensated flow con-trols — Flow concon-trols are available to bypass excess system flow to a sec-ondary circuit, Figure 6. Controlled flow rate is ported to the primary cir-cuit. Bypass fluid can be used for work functions in secondary circuits without affecting the primary one. There must be flow to the primary one. There must be flow to the primary circuit for this type of valve to function: if the primary circuit is blocked, the valve will cut off flow to the secondary circuit.
Priority valves — A priority valve, Figure 7, is essentially a flow control valve which supplies fluid at a set flow rate to the primary circuit, thus func-tioning like a pressure-compensated flow control valve. Flow in excess of that required by the primary circuit by-passes to a secondary circuit at a pres-sure somewhat below that in the pri-mary circuit. Should inlet or load
Drain Adjustable orifice Temperature-sensitive element Fixed orifice
Secondary circuit Primary circuit
Pilot pressure
Pilot pressure Circuit 1 Circuit 2
Fig. 4. Pressure-compensated, variable flow control valve adjusts to varying inlet and load pressures.
Fig. 5. Pressure-, temperature-compensated, variable flow con-trol valve adjusts concon-trol orifice settings to offset effects of vis-cosity changes.
Fig. 6. Demand-compen-sated flow control by-passes full pump output to tank during idle portion of work cycle.
Fig. 7. Priority valve sup-plies fluid at a set rate to a primary circuit.
Adjustable bypass orifice
Solenoid
Fig. 8. Deceleration valve slows load by be-ing gradually closed by action of cam mounted on cylinder load.
pressure (or both) vary, the primary cir-cuit has priority over the secondary as far as supplying the design flow rate is concerned.
Deceleration valves — A decelera-tion valve, Figure 8, is a modified 2-way, spring-offset, cam actuated valve used for decelerating a load driven by a cylinder. A cam attached to the cyl-inder rod or load closes the valve grad-ually. This provides a variable orifice which gradually increases backpres-sure in the cylinder as the valve closes.
Some deceleration valves are pres-sure-compensated.
This force-balance concept of con-trol function also applies to flow rate control valves.
Flow control methods
There are three basic ways to control flow: meter-in, meter-out, and bleed-off.
Meter-in control — The circuit in Figure 9 illustrates meter-in control. The flow control valve is placed in
se-ries with the directional control valve
in the cylinder’s high pressure line. Thus, flow control valve A meters the amount of fluid entering the cap end of the cylinder. This type of control is best suited for resistive loads, where it is es-sential to control the speed at which a cylinder extends.
Meter-out control — Here, the flow control valve is placed in the cylinder
return line, Figure 10. The valve
con-trols the rate of fluid flow from the cyl-inder to tank and is best used with
over-running loads. The valve controls the
rate at which fluid leaves the head end of the cylinder. Thus, it controls the speed of the piston rod and load. Also, because it is placed in the return line, the overrunning load cannot force the piston rod to move at higher speed than that set by the flow control valve.
Bleed-off control — This flow con-trol device is placed in parallel with the cylinder, bypassing a part of the pump output flow to tank over the flow con-trol valve, Figure 11. The flow concon-trol valve can be sized to handle bleed-off flow only, rather than entire pump out-put. Because the bleed-off valve is mounted in parallel (not in series) with the active elements, this flow control valve does not introduce a pressure drop into the active part of the circuit. Inlet pressure will be actual load pres-sure rather than the prespres-sure of the re-lief valve setting. Figure 11 illustrates how a typical bleed-off circuit might be installed.
Other flow controls
Flow-dividers — A flow-divider valve is a form of pressure-compen-sated flow control valve which receives
one input flow and splits it into two
output flows. The valve can deliver equal flows in each stream or, if neces-sary, a preset ratio of flows. The circuit in Figure 12 shows how a flow divider could be used to roughly synchronize two cylinders in a meter-in configura-tion. Note that like all pressure- and flow-control devices, flow dividers op-erate over a narrow bandwidth rather than at one set point. Thus, there are likely to be flow variations in the sec-ondary branches, and for this reason, precise actuator synchronization can-not be achieve with a flow-divider valve alone. Load Vp Load Vp Load Vp
Fig. 9. In meter-in control circuit, flow control valve is connected in series with direc-tional flow control valve.
Fig. 10. In meter-out control circuit, flow control valve is installed in cylinder return line.
Flow dividers can also be used in meter-out circuit configurations. Bleed-off does not affect the perfor-mance of a flow divider valve. Flow di-viders can also be “cascaded,” that is, connected in series, to control multiple actuator circuits, Figure 13.
Rotary flow dividers — Another technique for dividing one input flow into proportional, multiple-branch out-put flows is with a rotary flow divider, Figure 14. It consists of several hy-draulic motors connected together me-chanically in parallel by a common shaft. One input fluid stream is split into as many output streams as there are motor sections in the flow divider. Since all motor sections turn at the same speed, output stream flow rates are proportional and equal to the sum of displacements of all the motor sections. Rotary flow dividers can usually han-dle larger flows than flow divider valves.
The pressure drop across each motor section is relatively small because no
energy is delivered to an external load, and is usually the case with a hydraulic motor. However, designers should be aware of pressure intensification gener-ated by a rotary flow divider. If, for any reason, that load pressure in one or more branches should drop to some lower level or to zero, full differential pressure will be felt across the motor section(s) in the particular branch(es). The sections thus pressurized will act as hydraulic motors and drive the maining section(s) as pump(s). This re-sults in higher (intensified) pressure in these circuits branches. When specify-ing rotary flow dividers, system de-signers must be careful to minimize the potential for pressure intensification. Rotary flow dividers can also integrate multiple branch return flows into a sin-gle return flow.
Pump control of flow rate — Pump control of flow rate presupposes the use of a variable-displacement pump. Non pressure-compensated pumps require an auxiliary control to stroke the
pump-ing element to vary the pump’s dis-placement. These auxiliary controls are available in hydraulic, pneumatic, me-chanical, and electrical versions to match the needs of most control appli-cations. Though pressure-compensated pumps are usually considered to be pressure control devices, designers must remember that flow control is achieved by reducing the displacement of the pump when a set pressure level is reached. Thus, a change in flow rate is involved. If this change occurs while the actuator is still moving, it will result in a change in actuator speed.
The purpose of flow control is speed control. All the devices discussed in this section control the speed of the ac-tuator by controlling the flow rate. Flow rate also determines rate of en-ergy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (stroke) equals the work done on the load. The energy transferred must also equal the work
Combining ports
Control orifice Restriction area Dividing port Flow divider Cylinder 1 Cylinder 2
Fig. 12. Linear type flow divider splits input flow into two out-put flows.
Fig. 13. Flow dividers can be cascaded in series to control multiple actuator circuits.
Proportional flow control valves combine state-of-the-art hydraulic valve actuation with modern, sophisti-cated electronic control. These valves are helping fluid power designers to simplify hydraulic circuitry by reduc-ing the number of components a system may require while, at the same time, substantially increasing system accu-racy and efficiency.
An electronically controlled, propor-tional flow control valve modulates hy-draulic fluid flow in proportion to the input current it receives. The valves can easily control cylinders or smaller hy-draulic motors in applications which require precise speed control, con-trolled acceleration and deceleration, or remote electrical programming. Most proportional flow control valves are pressure-compensated to minimize flow variations caused by changes in inlet or outlet pressure.
An electronically actuated, propor-tional valve consists of three main ele-ments:
● a pilot or proportional solenoid ● a metering area (where the valve
spool is located), and
● a linear variable differential
trans-former (LVDT) electronic feedback device.
Valve operation begins when it re-ceives a signal from an outside control-ling device such as a computer, pro-grammable controller (PC), traditional logic relay, or potentiometer. The con-trol device delivers electrical signals to the valve driver card, which, in turn, signals the valve pilot. Depending on
the nature of the signal the valve pilot receives, it generates a magnified force which acts on the appropriate end of the valve spool to start it moving.
As hydraulic force acts on the spool end, the spool shifts, gradually opening a flow path that lets fluid supplied by the pump flow to the appropriate actua-tor port. The important feature of this proportional valve is that all elements are proportional; thus, any change in input current changes force signals pro-portionately as well as the distance the valve spool will shift, the size of the flow path, the amount of fluid flowing through the valve, and finally the speed at which the actuator moves.
As the spool shifts, its motion is de-tected and monitored very accurately by a LVDT. This data is fed back to the driver card where it is continuously compared with the input signals from the controller. If the two differ, the driver adjusts spool position until the two signals match.
Pressure-compensated proportional flow control valves
Pressure-compensated proportional flow control valves are 2-port valves in which the main control orifice is ad-justed electronically. Similar to con-ventional pressure-compensated flow control valves, a pressure-compensated
Q1 Q2 Q2 Q3 Q4 Vp pAp = F Q = Vd t Q S
Fig. 14. Rotary flow divider sists of several fluid motors con-nected with parallel.
Fig. 15. Actuator speed determines rate of speed transfer, which is a function of flow rate. For the equations in the drawing, p is fluid pressure, Apis
piston area, and F is applied force; Q is flow rate, Vdis cylinder
dis-placement, and t is time per stroke; vpis
velocity of the piston, and S is cylinder stroke.
done. Actuator speed determines the rate of energy transfer (i.e., horse-power), and speed is thus a function of flow rate, Figure 15.
Directional control — Directional control does not deal primarily with energy control, but rather with direct-ing the energy transfer system to the proper place in the system at the proper time. Directional control valves can be thought of as fluid
switches which make the desired “contacts,” that is, they direct the high-energy input stream to the actua-tor inlet, and provide a return path for the lower-energy oil.
That is an important function can be inferred from the scores of different directional control valve configura-tions available in the marketplace. Moreover, it is of little consequence to control the energy transfer of the
sys-tem via pressure and flow controls, if the flow stream does not arrive at the right place at the right time. Thus, a secondary function of directional con-trol devices might be defined as the timing of cycle events. Since fluid flow can be throttled in a directional control valve, some measure of flow rate or pressure control can also be achieved with these valves.
proportional flow control valve main-tains constant flow output by keeping the pressure drop constant across the main control orifice. The proportional valve, however, is different in that the control orifice has been modified to work in conjunction with a stroke con-trolled solenoid.
In a 2-port pressure-compensated proportional flow control valve, an electrically adjustable control orifice is connected in series with a pressure re-ducing valve spool, known as a
com-pensator or hydrostat, Figure 16. The
compensator is located upstream of the main control orifice and is held open by a light spring. When the input signal to the solenoid is zero, the light spring force holds the main control orifice closed. When the solenoid is energized, the solenoid pin acts directly on the control orifice, moving it downward against the spring, to open the valve and allow oil to flow from port A to port B.
At the same time, the LVDT pro-vides the necessary feedback to hold position. In this case, the LVDT pro-vides feedback to maintain a very accu-rate orifice setting.
Pressure compensation is achieved by supplying a pilot passage from the front of the control orifice to one end of the hydrostat, A2, and feeding a pilot passage beyond the control orifice to
the opposite end of the hy-drostat, A3, assisted by the force exerted by the spring. Load-induced pressure at the outlet port or pressure devia-tions at the inlet port are thus compensated by the hydro-stat, providing constant output flow.
The amplifier provides time con-trolled opening and closing of the ori-fice. For reverse free-flow, check valve
C, built into the valve, provides a flow
path from port B to A. Proportional flow control valves are also available with ei-ther linear or progressive flow charac-teristics. The input signal range is the same for both. However, the progressive flow characteristic gives finer control at the beginning of orifice adjustment.
In case electrical power or feedback is lost, solenoid force drops to zero and the force exerted by the spring closes the orifice. When feedback wiring is connected incorrectly or damaged a LED indicates the malfunction on the amplifier card.
Proportional flow logic valves
Proportional flow control logic valves are basically electrically ad-justable flow controls that fit into a standard logic valve cavity. The cover and cartridge are assembled as a single unit, with the cover consisting of a pro-portional force solenoid and a pilot
controller, Figure 17.
When an electrical signal is fed into an electronic amplifier, the solenoid and controller adjust the pilot pressure supplied from port A to change spool position. An LVDT then feeds back the position to the amplifier to maintain the desired orifice condition for flow from port A to port B. The proportional logic valve is available with either linear or progressive flow characteristics which are adjusted by a 0 to 6-V, 0 to 9-V, or differential ±10-V command signal.
Because the valve remains relatively unaffected by changes in system pres-sure, it can open and close the orifice in the same length of time. This maximum time can be changed on the amplifier card by adjusting a built-in ramp generator.
The amplifier can be used in several ways. An external potentiometer can make the orifice remotely adjustable while maximum spool acceleration is still limited by this internal ramp; or a limit switch can be added to turn the ramp on and off. In case of power fail-ure, the element will return to its nor-mally closed position.
Fig. 17. Cross-sectional view of proportional flow logic valve. P1 A2 A B P2 A3 C P3 Hydrostat Amplifier Cover Pilot control Sleeve Main piston Window opening A B X Proportional selenoid Spring chamber LVDT
Fig. 16. Operating circuit diagram for pressure-compensated flow-control valve.
