Devices for Limiting the Power Output of a Hydraulic Assembly

The present invention relates to a device for limiting the power output of a hydraulic assembly comprising plural hydraulic driven motors, hydraulic pumps for driving said driven motors, a pump driving motor and control means controlling the hydraulic flow from each pump to the driven hydraulic motor or motors coupled therewith.

In a hydraulic assembly of the above-mentioned type, the pumps of the driven hydraulic motors are coupled with the output shaft of the driving motor via a gearbox, and if the hydraulic power output exceeds the available power output of the driving motor, the latter will be overloaded, resulting in a decrease of speed or even a full stop of the driving motor. If the driving motor is an electric motor, the full stop may result in the destruction of the motor. For this reason, the pumps have to be so dimensioned that their combined maximum power does not exceed that of the driving motor, even if it is only in exceptional cases that all pumps are run simultaneously at maximum power. This means that the available power of the driving motor cannot be optimally utilized. It is, of course, a disadvantage that the pumps for reasons of safety have to be dimensioned to be able to meet with a situation that arises relatively seldom, and it has long been desired that it should be possible automatically to limit the power output of the pumps so that the sum of the maximum power output may safely exceed that of the driving motor. In this manner, it would be possible to run one or more--but not all--of the pumps at maximum power, and as soon as the total power output approaches the available power output of the driving motor, the power output would be reduced to a value immediately below the available power output, without incurring the risk of overloading.

It is the object of the present invention to satisfy these requirements by providing an extremely simple, inexpensive and reliable device.

To this end, a controller is adapted to supply each control means with a signal multiplied by a signal proportional to the setting of said control means, for obtaining the flow control signal of said control means, and a subtractor is connected to a reference value setting device and to a transducer sensing the speed of the driving motor for obtaining a difference signal that can be applied to said controller for controlling the output signal thereof to said control means.

According to a preferred embodiment of the invention, the controller is of the PID (proportional plus floating plus derivative) type and may consist, like said control means and said subtractor, of electrical means.

The invention will be described in more detail in the following, reference being had to the accompanying drawing which illustrates an embodiment of the invention in the form of a circuit diagram.

The drawing shows a number of hydraulic motors 10 which may be either piston and cylinder assemblies or torque motors. The hydraulic motors 10 are coupled each with one pump 11 via a solenoid valve 12. Although each hydraulic motor shown in the drawing is provided with a pump, several such motors are frequently coupled with a common pump. The pumps are operated by a driving motor 13 which may be either a diesel engine or an electric motor. As shown by a dash and dot line 21, all pumps are connected to the output shaft of the driving motor 13 via a gearbox (not shown). For controlling the supply of pressure medium from the pumps 11 to the hydraulic motors 10, each pump 11 has a control means 14 which, in the drawing, is shown as a potentiometer connected to an amplifier 15 which in turn is connected to the solenoid valve 12. It will be appreciated that it is possible, by changing the setting of the potentiometer, to control the supply of pressure medium by the solenoid valve 12. All control means 14 are connected to a controller 16 which in turn is connected to a subtractor 17. The components 16 and 17 are preferably enclosed in a common casing, as shown at 18.

A reference value setting device (not shown) supplies the subtractor with a signal R corresponding to a predetermined nominal value. An actual value setting device 20 preferably a transducer is disposed on the driving motor 13 for sensing the speed of said driving motor and supplying the subtractor 17 via a line 19 with a signal A responsive to said actual value. In the subtractor, the reference value signal R is compared, in conventional manner, with the actual value signal A, and the resulting signal D is applied to the controller 16. The controller 16 is of the PID (proportional plus floating plus derivative) type and is adapted to supply a signal K which is applied to the control means 14. The signal supplied by said control means 14 is a product of the controller signal K and the setting of the control means.

The device according to the invention operates as follows.

The reference value setting device is preset to a value corresponding to the lowest acceptable speed of the driving motor 13, and the signal R corresponding to this value and the signal A responsive to the motor speed are continuously compared in the subtractor 17. As long as the actual value is greater than the reference value, the signal D from the subtractor 17 does not actuate the controller 16 transmitting the signal K which normally equals one. However, when the actual value falls below or, because of the derivative function of the controller 16, approaches the reference value, the controller 16 responds and the signal K is reduced and obtains a value below one, as a result of which the control means signal reduces the flow and thus the power output so that the speed of the driving motor is stabilized at the reference value.

As will be apparent from the above, the present invention provides an extremely simple and reliable device for limiting the power output, said device being based upon a simple speed measurement instead of the far more complicated measurement of the total power output. Furthermore, it is possible, by means of a slightly more complicated controller 16 to connect the power stages in such a manner that, as the speed decreases, an increasing number of power stages participate in the control.