Hydraulics 3 - Flow control

Flow control controls the speed of Hydraulic end actuators by varying the flow - thus altering response in the system.
The basic flow controls are as follows:
Non pressure compensated flow controls
These systems result in pressure loss across variable elements , which have to be accounted for in system design
1.Metered flow control :This is a basic flow control with a simple throttle valve  , either as a meter - in or meter out (throttle valve control/ throttle cum check valve  either in / out). These controls result in wide fluctuations in end outputs due to large variations in controls of flows and need  control over the flow varying element or a variable  orifice .

2.Bleed off controls: These controls incorporate a flow diversion back to low pressure circuit/ return - controlled by a flow varying element .This control also suffers from inconsistencies due to lack of compensation of flow loss

Pressure Compensated Flow Controls
This system compensates for pressure changes with variation of load
This can be achieved by a feedback mechanism to automatically cater for these variations.These can be incorporated in metered flow systems to achieve the desired precision.
They can be divided as Bypass type :

Restrictor type:

Limiting type controls

Pressure and Temperature compensation controls
This includes a temperature compensating rod with a variable throttle control , and this compensation is usually set based on the flow designed . The spool is a restricted flow type and the throttle of temperature control is a v- notch design on an aluminum alloy rod  , which varies the flow when  temperature increases , thus compensating for  changes in viscosity of oil to perform the job on actuator thus maintaining constant output of end actuator.

Proportional Flow controls are the remote application of the volume controls , output varying with the magnitude of input signals - either electrical , pneumatic or  hydraulic.

Throttle motor actuated flow control :

Flapper- nozzle pilot control

 Proportional solenoid throttle control.: Unlike conventional on/ off solenoids , the voltage can be varied on solenoids to vary the spool openings, this gives a wider control over processes.

 Direct operated proportional valve

 Pilot operated proportional valve

Pilot operated valve with feed back

They are further divided into Symmetric and assymetric spools- whose field application is decided based on depending on  multi speed / torque applications.

 

The forces acting on the spool during operation ,by virtue of flow through metered notches produces additional force due to Bernouli's effect . The flow through restriction tends to create a pressure drop , while pressure on the other side is constant. this tends to shift the spool towards the area of throttling, reducing flow.Hence spool position has to be corrected by solenoid overcoming the bernouli's force and spring forces. The variations in system operation tend to vary the forces and make the control dynamic.
Hence a control system with a dynamic controls is required when operating with proportional valves .

The control system for these valves are divided into two types- namely - feed back control and Non- feed back control

Feed back control proportional  two way solenoid control

Non- feed back control proportional two way solenoid control

Feed back control proportional  one way solenoid control - spring to return

Non-feed back control proportional  one way solenoid control - spring to return

Basic control System for proportional valves

1.Dead band compensation

This type of control is required  when the spool attains the position , but keeps hunting over the position due to variations of flow.If feed back were to act accordingly , then it creates imbalances in the system operation , resulting in large scale drops in pressure. Hence a control, which has a delayed operation is employed - called dead band as shown.

 Hysterisis is the effect of wear and tear in spool , combined with oil quality to a certan extent. This causes offset to spool movement as shown below.

 Dither is a high frequency low amplitude AC applied to offset  hysterisis  over the existing signal.

Ramp function : It is used to control the speed of operation of solenoids. Decided by the amplifier settings.

Feed back control :
The feed back from the proportional solenoid can be as a proximity sensor for end positions, pressure transducer / LVDT( as shown below - where the core displacement varies secondary voltage) , Drive enabled potentiometer for intermittent positions.

 Pulse width modulation is employed when an infinitely variable dc voltage is used and the amplifier has to vary the voltage with respect to the system settings. this intermittent actions cause heating of amplifier . hence a pulsed width concept is employed, where the voltage is pulsed and time determines the mean voltage at solenoid.

 

 A typical amplifier circuit is shown as under

 

Hydrostat in system

Hydrostat module is used to sense pressure when valve is used .
This module can be described as follows for the proportional valve as described below: :

  A hydro stat is added to extra ports C1, C2 , X and Y of the proportional valve

Internal drilling at the main spool pressure at A is sensed at C1, port B is sensed at C2.Hydrostat senses pressure either at A or B depending on the spool movement.Pressure compensation is achieved by either P to A or P to B with a constant drop maintained across the spool.