Digital Displacement – its time has come

First conceived in the early 1990s, the concept of fully digital hydraulic pumps offered a future where radial piston machines, built with computer-controlled valves, would offer radical improvements in energy efficiency and control – for a range of applications, writes Dr Niall Caldwell, Managing Director of Artemis Intelligent Power.

Now this vision is approaching commercial reality – and today Artemis Intelligent Power is developing working prototypes for off-road vehicles, trains and other sectors, with products reaching the marketplace in the next few years.

It is an idea whose time has come.

Indeed, its potential has been long-recognised.

Here we publish my PhD thesis, submitted to the University of Edinburgh in 2007. This covers early work done between 1999 and 2006 to demonstrate the feasibility of applying Digital Displacement machines to off-highway vehicle applications.

I will be presenting a summary of this work at the Fluid Power and Motion Control Conference, September 12-14 2018, Bath, UK.

The full thesis can be downloaded here.

Digital Displacement® pumps and motors are a new type of hydraulic machine, in which fluid commutation and displacement control are achieved by solenoid-actuated valves under the command of a microprocessor, rather than mechanical means. The thesis is that radial piston machines, built according to this principle, offer energy efficiency and control advantages over variable stroke axial piston pumps, when applied to hydrostatic vehicle transmissions.

Experimental results on the efficiency of prototypes are analysed and compared to published results from swashplate machines, showing an improvement in energy efficiency. Loss models are proposed and compared with experiment.

A Digital Displacement motor suitable for propelling a vehicle is described and the design and development of the mechanics, electro-magnetics and embedded software are described.  Experimental results are also presented, illustrating the performance of a demonstrator vehicle driven by the motor, in particular demonstrating the closed-loop regulation of vehicle speed using motor displacement control.

A demonstrator vehicle is described which features a hydrostatic transmission using both a Digital Displacement pump and an axial piston motor. Experimental results of pump performance are presented with specific focus on vehicle propel. A control technique is described which increases the sensitivity of the pump at low speeds. Results are presented of tests on the prototype transmission system, focussing on the time-domain system dynamics. A computer simulation model of the vehicle is presented, and results compared to experiment.

Submitted to The University of Edinburgh in 2007.