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Proportional hydraulic operation : Scal and recovery

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.

Choosingvalves 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- Recovery and scal

2 symbolisation distributeur proportionnel 1

By moving the spool to the left, it gradually opens the passage from P to A & B to T.

By moving the spool to the right, it gradually opens the passage from P to B & A to T.

The flow through the valve depends in part on the stroke of the spool.

Depending on the proportional distributors used, the spool when it is in the central position can have different overlaps which influence the scal in flow and pressure.

3 recouvrement tiroir

Fig. A : Positive recovery

At rest, the edges of the spool (item 1) cover the bearing surface (item 2) and completely close the passage to the oil.

The curve indicates the flow scal of the valve. That is, the increase in flow for a certain change in the input signal with a given pressure difference.

Because of the overlap, the input signal must reach a minimum value for a flow to pass through the spool.

This type of spool is used in speed regulation.

 

Fig. B : Zero recovery

At rest, the edges of the spool (item 1) are flush the bearing surface (item 2) and close the passage to the oil.

The curve indicates the flow scal of the valve. That is, the increase in flow for a certain change in the input signal with a givenpressure difference.

With zero overlap, a low input signal allows flow to pass through the spool. The proportionality of the input signal with respect to the flow rate scal is true over the entire area of ​​movement of the spool. The movement of the drawer is very sensitive on either side of the central position.

This type of spool is used for position regulation.

Fig. C : Negative recovery

At rest, the edges of the spool (item 1) do not cover the bearing surface (item 2) and do not completely close the oil passage.

The curve indicates the flow gain of the valve. That is, the increase in flow for a certain change in the input signal with a given pressure difference.

There is an initial flow when the spool is in the center position.

This type of spool is used in pressure regulation.

4 tiroir fente de progressivite

The spools can have progressive slots which allow the passage of the oil to be opened more progressively.

Depending on their shapes and the pressure difference across the spool, the relationship between spool movement and flow is not always linear.

5 gain de pression

Pressure scal: The pressure scal corresponds to the ratio between the output pressure and the input signal.

The cover of the spool influences the scal in pressure

The pressure scal is plotted with the valve orifices plugged.

With a zero overlap spool (curve B), full pressure is reached when the input signal reaches 3 to 4% of the maximum value.

With a negative overlap spool (curve C), the spool has to travel a longer stroke and results in a less steep curve.

With a spool with positive overlap, the pressure begins to rise when the spool reaches the uncovered zone (curve A).

2 - The different settings

6 les reglages

I max: This setting is used to reduce the maximum hydraulic parameter when the setpoint is maximum.

I max = scale = gain = scal

I limitation: Internal adjustment for multivalve cards which allows not to exceed a maximum intensity.

I min: This setting adjusts the amount of jump. This has the effect of reducing the sensitivity of the joystick.

I min = jump = BIAS

Pre-magnetization: Small current present even if there is no setpoint

Pre magnetization = I polarization = I base

Ip min: This adjustment is used to adjust the percentage of the setpoint necessary to perform the jump (1 to 3%). “Adjusting the start of the jump” .

Ip max: This is the inverse of I max, it allows the setpoint to be reduced to have the current at 100%.

Zero adjustment: For servo valves and servo distributors, it helps to avoid drift when there is no setpoint.

Ramps

7 rampe

The ramp allows the valve to reach the set point gradually to provide flexibility to the hydraulic installation.

In general, the maximum adjustable ramp is 5s.

The electronic cards can be multi-ramp. That is, it is possible to set different ramps.

 

3 - Important concepts

Resolution / sensitivity: Variation of the control signal from which an evolution is observed without change of direction, from which the variation in flow or hydraulic pressure can be measured. It is expressed in % of the rated current.

Threshold voltage / inversion magnitude: Variation of the control signal from which an evolution is observed with a change of direction, the variation of flow rate or hydraulic pressure of which can be measured. It is expressed in % of the rated current.

Reproducibility / fidelity / repeatability: Ability to systematically reproduce the same hydraulic output quantity (pressure or flow) for the same control signal applied several times in succession. It is expressed in % of the hydraulic parameter.

Hysteresis: Difference in control signal necessary to have the same hydraulic output quantity (flow or pressure) between an increasing and decreasing signal. It is expressed in % of the rated current.

Linearity  : Ability to respect a linear law between the quantity measured and the theoretical display. It is expressed in % of the hydraulic parameter.

Ex: sensor 0-400 bars (0-10V) linearity of +/-0.5% or +/- 2 bars.

 

 

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