The use of a digital cylinder drive for exoskeletons was proposed in the recent year as a means to save weight, installation space, and energy. Also the hydraulic actuation of the switching valves of the digital drive was brought into discussion, in particular by a so called hydraulic digital counter concept. That concept was originally invented to realize a digital hydraulic amplifier. It uses a mechanical input and feedback, transfers this into binary switching states of the valves which switch the chambers of the digital cylinder either to tank or system pressure. It is now reconsidered with a hydraulic input, a special design of the valves appropriate for the sub-kilowatts power ranges of exoskeletons, and a special control strategy. The configuration of this system with a true binary counting property requires the input pressure thresholds to grow exponentially with the number of binary stages of the drive. That limits the number of feasible stages to approximately five or six, complicates the mechanical parts of the valves which realize the hysteretic response to the pilot pressure, and increases the hydraulic power for valve actuation. To overcome these problems the system is configured as a quasi binary counter which jumps over certain digits if the input flow is monotonous, thus, requires a reversal of input flow to realize those digits. A theory and a numerical model of the counter in this configuration and a proper counting control strategy of the hydraulic input are presented. With this much higher number of digits can be realized and valve design and configuration are eased.

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