A Basic Study on the Response Dynamics of Pulse-Frequency Controlled Digital Hydraulic Drives

Abstract

Various control strategies in digital hydraulics have been proposed and studied so far. In hydraulic switching control Pulse Width Modulation (PWM) of one or two switching valves was mostly considered. This paper deals with Pulse Frequency Control (PFC) which — opposite to PWM — uses the pulse repeating frequency and not the pulse width as control input. PFC may be to be preferred if the hydraulic switching device can realize a very particular pulse in a quite favorable way. This paper studies the influences of the flow rate pulse shapes and of the pulse frequency on the overall system dynamics. Based on a dimensionless mathematical model of a simple linear hydraulic drive and on elementary performance requirements (e.g. overshooting and pressure pulsations) dimensioning rules are derived. In addition to a repeated pulsing single or just a few pulses are investigated. It turns out that particular single or twin pulses can realize stepping motions of the drive without subsequent pulsations. In this way a hydraulic stepping drive can be realized. In case of repeated pulsing, high pulsing frequencies, in particular frequencies well above the natural frequency of the drive system, reduce oscillations considerably. Such frequencies may be realized either by one high frequency pulse device or by several pulse devices which are arranged in parallel and are operated in a phase shifted mode.

High dynamic digital control for a hydraulic cylinder drive

“…For smaller steps the method coincides with the double pulse strategy according to literature, as in Gradl and Scheidl. 15 The minimum steps can be realized arbitrary consecutively, which represents a sort of stepper drive. The step size depends on the size of the switching valve and on the load pressure.…”

“…Smaller steps can be realized with the double pulse strategy as presented, for instance, in Gradl and Scheidl. 15 In Figure 3 the minimum HDDC step is opposed with a smaller step according to the double pulse concept. Combining both concepts, the HDDC and the double pulse strategy result in arbitrary step sizes.…”

“…A basic study of this concept for hydraulic systems can be found, for instance, in Gradl and Scheidl. 15 The PFC is designed to the dynamics of the drive in order to keep the unwanted oscillations low. Therefore, the pulses must be produced in a certain phase relation with regard to the natural frequency of the drive.…”

Digital Hydraulic Technology for Linear Actuation: A State of the Art Review

“…A disadvantage of the concept is that the valve has to switch constantly in order to achieve any flow level different from the maximum. Furthermore, to achieve good control, the frequency of the valve should preferably exceed the natural frequency of the system by a factor of up to 20 as stated by Gradl et al [36]. Gradl et al proposed that several valves can be operated in parallel with a phase shift allowing higher frequencies to be achieved.…”

Performance of an Energy Efficient Low Power Stepper Converter

“…Such pulsed control is likely to provoke unwanted oscillation. They can be counteracted by a proper double pulse techniques as shown by Gradl and Scheidl [18], by passive hydraulic filtering investigated by Haas et al [19,20], or by a phase shifted operation of several HSD's connected on one cylinder drive in parallel, as has been demonstrated for the hydraulic buck converter by Kogler [21].…”