Elektrohydraulische Pumpenregelung HPC von Rexroth

Pump control – simple or intelligent?

Control pumps have a fixed place in hydraulics. Your advantage: They only provide as much flow and/or power as is required for the specified movement task. But which pump control  is suitable for which application? Mechanical-hydraulic or electro-hydraulic pump control? What are the differences?

Pump control functions - Rexroth

Figure 1: Pump control functions

The advantages and disadvantages of the two pump control types can well be explained using the flow control of a deep drawing press as example. The hydraulic drive of the cylinder is based on a variable displacement pump working in an open circuit. The displacement is 250 cm3, the nominal pressure 350 bar. The mechanical input signal is hydraulically amplified. In this connection, the pump has three typical control tasks: Flow control (N and/or S function), power control (LR function) and pressure control (G function).

01) Mechanical-hydraulic pump control: simple, however limited

The mechanical input signal from the hand lever is hydraulically amplified. In this case, the flow is controlled by means of load-sensing. The pump swivel angle is adjusted independently of the load occurring at the actuator by means of a load sensing valve which is set to a Δp of 20 bar. So the velocity at the actuator remains constant.

The disadvantage: The throttling of the flow at the pump output goes along with a power loss which is completely converted into heat and increases the cooling demand. One advantage, however, is the easy set-up which does not even require a pilot oil pump as the adjustment energy is taken from the high pressure. Due to the continuous Δp of 20 bar, flow control is also possible at low pressures.

Power controllers increase the complexity

There is a need for an additional pilot oil pump if the deep drawing press – for example for safety-related reasons – requires a flow of zero in case of a low counter pressure (maximum of 4 bar). More components are necessary for realizing the power controller.

02) Electro-hydraulic pump control

Data recording and comparison by control electronics Compared to that, an electro-hydraulic system with only one fast high-response valve at the pump and amending control electronics is the more elegant solution. The regulated variables (path, force and velocity) correspond to the analog hydraulic variables flow and pressure.

The principle: A swivel angle sensor on the actuating piston and a separate and/or attached pressure transducer record the actual flow and pressure values. After comparison to the specified command values, the control performs all flow, pressure and torque limitation tasks and forwards a command value to the valve. Figure 2 shows different pump control systems which are autarkic subsystems and connected to the machine control via corresponding
interfaces.

Today, there is a whole range of motion controls and NC controls for hydraulic actuators available. It comprises single-axis controllers without control
cabinets where the electronic controls are integrated completely in the valve, up to multiple axis controllers with control cabinets for more complex tasks. In addition, intelligent pump controllers are improving the system performance. These control systems communicate via established field buses or Ethernet protocols with superior systems, and with these open standards it is possible to completely integrate them into Industry 4.0 architectures – this way, intelligent, networkable hydraulics are completely Industry 4.0 ready.

Pumpenregelsysteme mit offenem Kreislauf (li.) und mit geschlossenem Kreislauf (re.) von Rexroth

Figure 2: Pump control systems with open circuit (left) and with closed circuit (right)

Elektrohydraulische Pumpenregelung HPC (re.) und Axialkolbenpumpe mit HS5-Verstellung (li.) von Rexroth

Figure 3: Electro-hydraulic pump control HPC (right) and axial piston pump with HS5 control (left) from Rexroth

03) Decision-making aid: Selection of the pump control type

It first of all depends on the physical variable to be controlled whether the mechanical-hydraulic or the electro-hydraulic variant is finally the better choice for the relevant application. Flow and pressure can be controlled with both types. For limiting the torque, however, the mechanical-hydraulic method needs an additional power controller changing the flow with constant pressure and simultaneously increasing the complexity of the hydraulics. Here you can find the selction guide

Master-slave pump combinations

A master-slave pump combination interesting for many applications is only feasible with an electrohydraulic control; however, it allows for combined pump systems with special properties. If, for example, by an early swiveling out of the pump, the master pump provides a certain flow from a certain point in time, it can be positioned at the maximum swivel angle already upon start-up of the motor and deliver into the system, which again increases the velocity and precision of the application.

How dynamic and accurate should the pump be?

The required dynamics and precision are more decision-making criteria. If, for example, particularly high dynamics with up to 80 ms are required, a primarily controlled pump would be suitable. With regard to precision, electro-hydraulic control systems with a repetition accuracy of <= 0.2 % for the pressure and a linearity deviation for the swivel angle of <= 1 % show convincing results. Compared to that, mechanical-hydraulic controls achieve about +/- 1.5% repetition accuracy for the pressure and a linearity tolerance of 2.5 to 7 % of Vgmax. All values are valid for a constant operating temperature of 50°.

Conclusion

The strength of the mechanical-hydraulic pump control is its simplicity. It is, however, only convincing in correspondingly clear applications. With increasing requirements with regard to function, precision and energy efficiency, there is no alternative to electro-hydraulic control systems which allow for pressure and flow control with high control quality according to the demand. As digital control electronics with integrated Multi-Ethernet interface can moreover be integrated into most different structures, it moreover also masters the prerequisites for the increasingly demanded networking in the sense of Industry 4.0.Decision-making aid for the selection of the pump control type - Rexroth

Info graphics: Decision-making aid for the selection of the pump control type.

You want to learn more about an electro-hydraulic pump control: www.boschrexroth.com/hpc

 

About the Author:

Martin Endres is a specialist in sales, product management & marketing for industrial hydraulics at Bosch Rexroth AG. Here he took over the task "Motion Control for hydraulic drives". After studying electrical engineering in 1994, he worked as a system engineer in the field of external service and put into operation analogue and digital control systems for electrohydraulic drives worldwide. In 2005, he moved to sales product management.

2 comments on “Pump control – simple or intelligent?

  1. tieumagni christoophe

    Hello Mr Martin
    your abstract is very good

    Please i’m in africa and for we to cary out a pecialysed training to a specialysed manufacturer as Bosch is very hard . since the the required condition to atend any training session is subject to visa plus other rquirement .
    Please because of all these difficult process to fulfill , just let me get the title of a book both theorical and pratical concerning the hydraulic pump control (opend and closed ) and a mobile electronic . i aways plan to comme a Bosch germany but is a dificult issues

    Tank you for your cooperation

    Tieumagni christophe
    ETS GROS MOTEUR
    Rock drilling rigmaintenance engineer
    Yaoundé cameroon

    Reply
  2. Clifford Yeazel

    Hello, i was wondering if any schools or colleges in fl would be interested in a complete do over on a Genie 50/30 Tow able boom lift that caught fire while charging the 24 v system. I am in the tree business & do not have the ability to repair this equipment. I selflessly thought this would be a great project to help teach hands on functions of electro hydraulic. If you can assist me i would be forever grateful for the help. Thanking You in advance. Cliff Yeazel Sr

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *