Various control strategies in digital hydraulics have been studied and published in the last years. Pulse Frequency Control (PFC) which – opposite to PWM – uses the pulse repeating frequency and not the pulse width as control input, is a fairly new control concept in digital hydraulics. PFC may be to be preferred if the hydraulic switching device can realize a very particular pulse in a favorable way, e.g. concerning energetic efficiency, simplicity and cost of components, or ease of component or control standardization.

This paper deals with the application of PFC to the control a hydraulic drive. It is assumed that a digital flow unit (e.g. digital pump) can realize only one particular flow pulse which can be repeated any time but not before the previous pulse is finished. As a consequence, the relative control resolution of a PFC-system is limited because of the fixed pulse quantity and the maximum repeating frequency. Since this pulsing may interact with the dynamics of the plant, the dynamics of the whole system with the PFC needs to be analyzed.

In a previous paper authors investigated a system comprising the digital flow unit and a hydraulic cylinder with an attached mass and a constant load. They found that oscillation can be largely reduced, if either the single pulse’s duration or the time span of two pulses is tuned to the eigenfrequencies of the drive. In this paper additional degrees of freedom in form of an elastic structure (e.g. an elastic beam) are added to the linear hydraulic drive which is pulse-frequency controlled. The dynamical response characteristics of such a multi degree of freedom system in PFC, the switching effort, and the achievable position accuracy in two different operating scenarios – a stepwise and a quasi continuous motion - are analyzed by mathematical modelling, simulation, analysis, and reasoning.

Earlier simulations as well as measurements have shown the potential of the Digital Hydraulic Power Management System (DHPMS). The machine can function as a pump, motor and transformer, and due to multiple independent outlets actuators with arbitrary pressure levels can be efficiently served. In addition, pre-compression and pressure release phases can be optimized for every point of the operation thanks to actively controlled on/off valves of the pumping pistons. Hence, the energy stored in compressed fluid is possible to be optimally utilized.

Displacement control using the DHPMS has also been studied recently. In the approach, the outlets of the DHPMS are connected directly to actuator ports without using any flow control valves. The direct control has been tested by measurements in the case of a lift cylinder of a mobile crane. A six-piston DHPMS with two independent outlets was used in the study and the results showed the technique to be valid despite of low number of the pumping pistons. Moreover, the system was capable to efficient energy-recovery. Controlling several actuators however, requires more DHPMS outlets.

In this study, a DHPMS with five outlets is modeled and a controller is created to directly control two actuators; a lift cylinder and tilt cylinder of a small excavator crane. A high pressure accumulator is attached to the fifth outlet and the accumulator energy is controlled to keep the prime mover energy at minimum. Simulation results show that the crane can be controlled smoothly also without position feedback of the cylinders. Even the chance in the load force direction does not deteriorate the velocity tracking as long as the cylinder back pressure is controlled. Furthermore, the system losses are significantly smaller than of that of the simulated LS-system.

By now, digital hydraulic concepts are widely accepted in the fluid power community.
However, the implementation of such solutions often requires an increased effort in control
design. Nonlinear model based control methods are particularly useful for digital fluid
power systems [1, 2].
This contribution is concerned with flatness based control for a class of simple digital
hydraulic drives based on an independent metering approach. As an example, a fixed-
displacement motor driving an inductive load with variable load torque is considered. The
motor is controlled by means of switching valves at both the inflow and the outflow port
allowing for driving as well as (non-regenerative) braking. Additionally, hydro-pneumatic
accumulators are connected to each port for pulsation smoothing.
For the resulting nonlinear multiple-input multiple-output problem, a flatness based
tracking controller involving a cavitation avoidance strategy is presented. The control
method proposed is applicable to both major digital hydraulic principles [3]: the fast
switching approach and the parallel connection approach. The design of a nonlinear load
observer is discussed, too. The control algorithms are illustrated by numerical simulation
results.

[1] Kogler H., ”The Hydraulic Buck Converter - Conceptual Study and Experiments,”
Dissertation, Johannes Kepler University, Linz, Austria, 2012.
[2] Hießl A., Plöckinger A., Winkler B., and Scheidl R., “Sliding Mode Control for
Digital Hydraulic Applications,” in Proc. 5th Workshop on Digital Fluid Power, Tampere,
DFP12. Finland, 2012, pp. 15–26.
[3] M. Linjama, “Digital fluid power – state of the art,” in Proc. 12th Scandinavian
Int. Conf. on Fluid Power, Tampere, SICFP12. Finland, 2011, pp 331–353.

At the 13th Mechatronics Forum International Conference in 2013 a novel Micro-Positioning System for a multi-spindle milling machine was presented. The purpose of this system is to compensate relative positioning errors of simultaneously operating spindles of multi spindle mill centres. In the first system, presented in 2013, a fast proportional control valve was used to fulfil the needs on reaction time and accuracy.

This paper reports about a digital hydraulic control concept for such micro-positioning drive replacing the proportional valve of the first system. The use of fast digital valves in combination with a standard industrial motion controller allows an increase of the accuracy compared to proportional valve control. The absolute position accuracy depends much more on the precision of the position sensor than on this drive’s performance limits and can be better than 1um. Besides the improved accuracy additional benefits of the digital drive system are: no drifting and energy savings because of a missing leakage and by far lower oil quality requirements.

Currently, the main market demands with regard to drive technology are energy efficiency, reduction of installation space and cost reductions. Depending on the application scenario, conventional throttle control based hydraulic systems (e.g. constant pressure supply and proportional valves as control element), often can not compete against electromechanical solutions with regard to the stated market demands. The use of variable-speed displacement unit drives (e.g. a fixed displacement unit being controlled by an electric motor and a frequency converter) can significantly improve the efficiency of the hydraulic system and therefore reduce the total cost of ownership as requested by the market. Nevertheless, such variable-speed displacement systems face challenges when the application demands cycles with little volume flow and high pressure levels at the same time. This results in an operating point with low rotational speed and a high driving torque, putting high loads on the components and thus generating heat and wear.

In 2011, Heino Försterling et al. presented a paper at DFP11 [Försterling, H., Stamm, E. and Roth, P. Tailored solutions limit complexity. The Fourth Workshop on Digital Fluid Power, 21st – 22nd September, 2011, Linz, Austria] stating the advantages (e.g. precise control and energy efficiency) of a 1bit digital hydraulics solution with ballistic-mode control.

The paper to be presented will demonstrate that the intelligent combination of the two system approaches (digital hydraulics throttle control, variable-speed displacement unit drive) is able to generate multiple application benefits. In such a system, the displacement unit drive is used to supply and control the high-speed process phase (high volume flows) as well as to charge a hydraulic accumulator. When entering the quasi-static process phase (e.g. applying a high force in a press application or fine-positioning resp. maintaining the reached target position for a longer time period), the digital hydraulics throttle control solely will be used to supply and control the hydraulic actuator. During this phase, the displacement unit drive is completely powered off, the needed hydraulic energy will be provided by the accumulator. The segmentation of the process phases and the use of the different control approaches in these phases results in the following advantages:
- High efficiency at high volume flows since a displacement unit is more efficient than throttle control
- Possibility to realize long pressure holding times without stressing the displacement unit / the electric motor
- The digital hydraulics on/off valve is a poppet valve and thus leakage free, making the quasi-static process phase very energy efficient.
- Very precise control (pressure, force, position) during quasi-static process phase through ballistic controlled digital hydraulic valve
- Considerable noise reduction during static process phase since displacement unit drive is switched off
- High overall system efficiency minimises the need for cooling capacity