Mobile control

Mobile control
Content

Wherever it’s necessary to dig, lift or tow, that’s where mobile working machines are needed. A look inside will show that hydraulic power is behind the many different movements. But how are these motions coordinated, and which mobile control concept is suitable for which application?

Mobile hydraulic systems have to be not only rugged and low in maintenance. They also have to satisfy the special needs of mobile use. In mobile work vehicles these systems may occupy only a minimum of space and must weigh as little as possible, but should nonetheless deliver high power and performance. The same is true for the controls. Mobile controls, when compared with their industrial counterparts, have to carry out a large number of functions, in limited space, and at low weight.

Fundamentally, two different hydraulic circuits are used in mobile control concepts: open and closed circuits. The closed circuit comprises essentially a pump, a motor and two pipes that join them. This configuration is suitable for individual tasks such as propulsion power or a winch drive. A separate circuit is required for every additional task.

The highly varied functionality of mobile working machines makes the open circuit especially suitable here. The pump draws the fluid from a tank as needed and moves it into a pipe which, by way of one or more multi-port valves, serves many different using units.

In everyday use, three types of controls will be employed in open circuits:

 
 

Throttle control

 
 

Applications

Large diggers, 100 tons and upwards, used in mining and as large railroad cranes – as well as in the most basic small implements such as small diggers and small drills.

 
 

Functioning

A pump moves hydraulic fluid at a constant rate. If the path to the cylinder is closed and that to the tank open, then the piston will not move, since the fluid takes the path of least resistance and returns to the tank. The path to the tank can then be throttled down to the same extent that the valve to the cylinder is opened.

When the back pressure reaches a certain level, the piston and the load will begin to move. In this system, one that is dependent on the force exerted by the load, the operator will have to control the speeds of the using units by skillfully manipulating the throttles. Modern systems improve performance with a variable-displacement pump that delivers volumetric flow proportional to current needs.

 

Performance picture

Economical use at full-load operation. Considerable losses in the fine control range since the fixed-displacement pump always delivers the entire flow volume and more than the maximum pressure required to move the load.

 
 

Advantages/Limitations

(+) Simple and rugged

(+) Load feedback

(–) The start of motion depends on load pressure and fluid flow

(–) Depends on operator skill

 

Load sensing control

 

Applications

This control concept is utilized in almost all mobile hydraulic applications.

 
 Load sensing control

Functioning

A variable-displacement pump delivers only the volume required at any given moment. Located upstream from the throttle is an individual pressure compensator that will automatically reduce incoming flow in dependency on the working pressure.

This compensator ensures that – where several using units are connected in parallel and where these are subjected to different load levels – the pressure differential (∆p) and thus the volumetric flow to the units served remains constant, regardless of the loading level.

 
 Load sensing control

Performance picture

Economical in use, particularly when operating at partial load.

 
 

Advantages/Limitations

(+) Start of motion independent of load pressure

(+) Parallel actuation, independent of the load pressure, through to the “saturation point” (full use of maximum potential fluid flow)

(+) Individual load-sensor pressure limiters, adjustable for each using unit

(-) Regulated system and thus subject to oscillation, meaning increased effort when commissioning the prototype

 

Flow-sharing control valves

 

Applications

This flow-sharing control concept is used for diggers, wheeled loaders and cranes in which the installed power is to be kept low.

 
Flow-sharing control valves

Functioning

Here, too, a variable-displacement pump delivers just the volumetric flow needed at any movement. Using the working pressure as its lead variable, a pressure compensator located downstream ensures that the pressure differential is kept constant, regardless of the volumetric flow and the pressure exerted by the load.

The pressure differential across the measurement aperture, in contrast to load-sensing control, is uniform for all using units. This means that all the using units will lose speed uniformly if the fluid delivery volume is insufficient.

 
Flow-sharing control valves

Performance picture

Economical in use, particularly when operating at partial load.

 
 

Advantages/Limitations

(+) Advantages the same as for the load sensing system

(+) Parallel operation, independent of load pressure and fluid flow, even if there is insufficient flow

(-) Only a single pressure level, regulated by load senor, can be set for all the using units