Proportional hydraulic valve and PID

1 valves proportionnelles
 

The use of proportional control valves makes it possible to meet the most demanding hydraulic applications. Proportional controls provide flexibility and precision compared to All or Nothing controls.

Choosing valves with proportional control makes it possible to control accelerations and decelerations. This choice gives access to a wide range of settings (pressure or flow valves)

Proportional components feature ultra-low clearance sliders with sharp edges. Particular attention should be paid to oil pollution. Effective filtration must be implemented on the pressure in addition to the return filtration already present in All Or Nothing hydraulic systems.

 

1- Open loop

In a proportional open loop system, there is no actuator control.

The proportional system can be used to control the position, speed or force of a receiver.

 
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An electrical instruction controls an electronic card. The electronic card transforms the instruction into a control signal.

 

2- closed loop

In a proportional  closed-loop system , a sensor is used to control the actuator.

The sensor can monitor position, speed, force or pressure.

 

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An electrical instruction controls an electronic card. The electronic card transforms the instruction into a control signal. The sensor controls the actuator. The electronic card has a regulator called PID. The electronic board checks the difference between the setpoint and the measurement and adjusts the control signal.

 

3- PID regulator

The PID regulator is an electronic card which makes it possible to perform mathematical calculations on signals in order to adjust the control signal to the requested setpoint.

 
Pid 1
 

The electronic card makes the difference between the set point and the measurement made by the sensor.

The error signal enters a corrector which transforms it into an output signal.

Several types of correctors are possible:

Apps

Types of correctors

 

P

IP

PID

Pressure

Simple requirements

Adapted

Adapted

Debit

Not suitable

Adapted

Less suitable

Temperature

Simple requirements

Adapted

Adapted

Level

Adapted

Not suitable

Not suitable

Speed

Adapted

Adapted

Adapted

 

The proportional gain P : Speed

Gain p 1
 

2 - The different settings

The error signal is multiplied by a coefficient. The value of the command signal is proportional to the difference between the setpoint and the sensor measurement.

It can be seen that increasing the gain accelerates the step response time. (Order)

A gain that is too high will cause the set point to be exceeded significantly (Over shoot) and will increase the stabilization time.

For example we can make the comparison with a vehicle. The car is stationary and the driver wants to drive at 100 km/h. You have to press hard on the accelerator pedal to reach 100 km/h as quickly as possible.

 

Integral corrector I : Precision

Gain i 1
 

Ramps

The output signal is proportional to the integral of the error signal.

At steady state, a closed-loop system may exhibit errors due to changing external forces, for example, or imprecise commands.

The integral corrector eliminates these errors.

For example we can make the comparison with a vehicle. The car is stationary and the driver wants to drive at 100 km/h. You have to press hard on the accelerator pedal to reach 100 km/h as quickly as possible. If the acceleration is rapid, it is possible that the speed exceeds 100 Km/h. The driver who notices this releases the accelerator pedal to adjust the speed which risks falling below 100 Km/h. Integral corrector eliminates this "yoyo" effect.

 

The derivate corrector D : Stability, anticipation

Gain d 1
 

3 - Important concepts

It is the variation of the error.

The output signal is proportional to the derivative of the error.

In steady state, a closed-loop system can exhibit significant variations.

If the error is changing rapidly, the derivative is large.

If the error changes slowly, the derivative is low.

If the error is constant, the derivative is zero.

For example, we can make the comparison with a vehicle traveling at 100 km/h. When the car starts a slope, the speed of the vehicle decreases sharply and suddenly. There is a large drift between the setpoint and the measurement (speed). The setpoint must be corrected by pressing the accelerator harder.

 

PID Parameter Tuning Method

The system is adjusted in production.

  1. Set the Integral (I) and Derivative (D) value to zero.
  2. Increase the proportional gain (P) until the output oscillates.
  3. Increase the Integral gain until the oscillation stabilizes.

Adjust the gain of the Derivative so that the system is stable when the error evolves.

 

4- Hydraulic rigidity

Although it is often assumed that fluids are incompressible, in fact and under high pressures, hydraulic oil is relatively elastic.

When the load and speed of a receiver are large, the lack of rigidity can limit the performance of the entire hydraulic system.

Rapid accelerations and decelerations of loads can cause pressure peaks. If the hydraulic system is not stiff enough, it will cause load disturbances.

A cylinder with large dimensions in relation to the load to be moved, the distributor placed as close as possible to the receiver and rigid piping promotes good rigidity.

 

Hydraulic stiffness :

Ch= (E*Sp²) / (Vsp + Vl1) + (E*Sa²) / (Vsa + Vl2)

Ch: Hydraulic stiffness in N/m

E: Modulus of elasticity of the hydraulic fluid (1.4*107  Kg/cm.s²)

Sp: Piston area in cm²

Sa: Annular surface in cm²

Vsp: Piston chamber volume in cm3

Vsa: Annular chamber volume in cm3

Vl1: Piping volume on the piston chamber side in cm3

Vl2: Piping volume on the annular chamber side in cm3

 

5-Natural frequency

The natural frequency of a system composed of the load and the receiver can be compared to a mechanical system mounted on a spring.

With a shock, a certain speed, an inertia, the system can begin to oscillate.

The natural frequency of a system is the frequency at which it begins to oscillate naturally without any external excitatory or dissipative force.

Any mechanical system can oscillate at a frequency generally between 0.1 to 100 Hz.

Mechanical frictions are exciters, leaks will act as a damper (dissipator)

It is important to determine the natural frequency of a system in order to control it correctly and avoid the risk of resonance.

 

Natural frequency of a mechanical system :

Fp= 1 / 2π√  (Ch/M)

Fp: Natural frequency in Hz

Ch: Hydraulic stiffness in N/m

M: Mass in Kg

 

Natural frequency of a piston pump :

Fp= (N * Nb) / 60

Fp: Natural frequency in Hz

N: Rotation speed in rpm

Nb: Number of pistons

 

To avoid system instability, it is advisable to take a coefficient of 10. For example, for a natural frequency of a system at 30Hz, a valve with a frequency of 300Hz will be chosen.

6-Troublesshooting

Methodology :

  • Measure the input setpoint.
  • Measure the current  flowing through the coil.
  • Measure the feedback voltage of the LVDT sensor.

According to the results:

  • Check the power supply to the card.
  • Check the fuse.
  • Check the connections.
  • Check the ohmic value of the coil .
  • Check card validation.
  • Check the command (PLC, potentiometer, etc.)

In some cases, the proportional valve can be fitted with an emergency manual control which removes any doubt.

In the case of a closed loop, test the system in open loop. If the loop cannot be opened, an electronic test box can be very useful.

 
 

 
 

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