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This paper presents the development of an energy efficient low power stepper converter. A prototype with a hydraulic output power of ≈600 W was designed, manufactured, investigated and improved. The converter consists of a hydraulic cylinder piston unit controlled by a fast switching valve to displace a defined fluid quantum by the limited forward stroke of the piston in its cylinder. The displaced fluid generates a precise, incremental motion of a load cylinder which should be controlled. Energy saving is achieved by storing the pressure surplus intermediately in the kinetic energy of the piston to displace a part of the fluid quantum without hydraulic energy from the supply line. Energy recuperation can be done in a similar way. Simulations and experiments showed two main efficiency improvement measures of the first converter prototype. The weak points were the commercially available check valves and the used guidance system for the pistons. The second part of the paper reports about the development of a fast check valve and of a combined hydrostatic hydrodynamic bearing system based on the elastic deformation of plastics. The theoretical and experimental results show a significant improvement of the energy efficiency, the potential of this drive technology and further improvement potential. Expressed in terms of numbers an energy efficiency increase compared to a resistance control up to 30% and a maximum recuperation energy efficiency over 60% were measured.

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.

Servo cylinders with hydrostatic bearings are employed when ultimate speed, low friction, or high precision are required. These advantages are opposed by a considerable leakage loss and high costs. The latter are caused by the high component precision required in order to avoid excessive leakage and to obtain high stiffness of the bearing. In this paper an alternative concept to realize such bearings with a considerable cost reduction potential is investigated. The sleeve is made of Polyetheretherketon (PEEK). A hydrostatic difference pressure or / and a hydrodynamic pressure deforms the PEEK sleeve such that a conical stabilizing sealing gap is created. A possible mechanical design is shown and the characteristics are analysed. To study the characteristics of the bearing system the finite element suite Abaqus is used. A Reynolds User Element is developed and included into Abaqus for the simulation of the fluid structure interaction. The Reynolds equation is discretised by finite elements and solved simultaneously with the mechanical model. With the developed user element in Abaqus, static and quasi-static analyses of mechanical models (linear, non-linear or inelastic material behaviour) containing lubrications gaps can be performed efficiently. The preliminary results showed the feasibility of the concept and, generally, the potentials of plastics as a flexible material to employ elastic deformation for the creation of lubrication effects.

Stepper drives can realize quite precise, incremental motions without position sensors. Sensorless hydraulic motion control is strongly demanded by industry and, therefore, is an established idea in hydraulics for a while. Some concepts have been proposed in the past and a few of them have also been realized and applied in specific cases. But it is expected that digital hydraulics — due to its intrinsic discrete nature — can create new, more advantageous hydraulic versions of stepper drives. In this paper, a new stepper drive is presented and investigated. It creates the steps by fixed fluid quanta generated by a so called digital flow unit. That unit is realized by a hydraulic cylinder-piston unit which displaces a defined fluid quantum by each limited forward stroke of that piston. The unit is controlled by a fast switching valve which connects the piston areas alternately with the pressure-, tank- and output-line. The return motion is generated by a return spring. Energy saving is accomplished by storing the supply pressure surplus intermediately in the kinetic energy of the piston and converting that energy to displace part of the quantum to the consumer line without hydraulic energy from the supply line. Different detail concepts of this stepper drive are theoretically assessed. The transient behavior, the performance characteristics and the energy efficiency of a preferred concept are investigated by mathematical modeling and simulation. Furthermore, the main system parameters are identified and corresponding basic dimensioning rules are presented. In a second step, the influence of finite switching times of the valves, the hydraulic impedances of the various flow channels and of the dead volumes and the dynamical properties of the hydraulic cylinder attached to the device, are discussed.