Slow Electro Hydraulic Servo Pumps

1. STRUCTURE OF SLOW ELECTRO HYDRAULIC SERVO PUMPS

Electro hydraulic servo pumps represent one of the most important applications of the proportional electro hydraulic amplifiers. They permit big energy savings, they preserve for a long time the functional liquid properties and they simplify essentially the hydraulic schemes of complex functional cycles.

Classification of servo pumps depending on their capacity:

- high capacity (210  280 bar)

- medium capacity (50  100 bar)

- low capacity (50  100 bar)

This chapter presents the slow servomechanism with double normal-open proportional valves used to adjust the capacitance of inclined block industrial servo pumps.

Figure 1. Double normal-open proportional valve

The double normal-open proportional valve is compound of:

- coils

- body

- slide

reaction pistons

There are 3 specific connections specific for hydraulic cylinders with simple effect and elastic recovery

Most important application of these valves is the hydraulic command of the hydraulic proportional slides with central valve on springs.

2. WORKING METHOD

The slide has bevels that reduce to half the gradient flow in both ways.

The permanent contact between the slide and the plunger of the electro magnet is assured by a low rigidity spring.

The plunger is active only in the direction of the connection between the pump and the cylinder (P -> A). Because there is no command current a spring situated behind the electromagnet removes very quickly the plunger in order to permit the drainage of the hydraulic cylinder. The maximum pressure assured by the valve depends on the maximum force developed by the electromagnet and of the slide diameter. The hydraulic cylinder of the servo pump is directly connected to the proportional valve without needing hydraulic adjustments resistances of the system respond time. The non-reaction position electro hydraulic command has a proportional character and the tilting movement corresponding to the repression pressure of the servo pomp modifies the capacitance of the servo pomp randomly. Considering the power consumed from the pump to the driving motor the proportional command works like a capacity controller. The electrical position reaction assures the necessary precision preventing the under loading of the servomechanism. The pressure command of the hydraulic cylinder imposes an algorithm of adjustment of feed forward type.

3. MATHEMATICAL MODEL

The dynamic behavior of the command system in open loop results from the following differential equations:

- motion equations of the 2 slide valves denoted by A and B

- continuity equations corresponding to the variable volume chambers from the hydraulic cylinder

- motion equation of the hydraulic piston cylinder

If we work in closed loop we can add also the equation of the electronic comparator, error amplifier equation and of the tension-current convector .

Figure 1.2. Computing scheme of the command system in open loop