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What is a digital hydraulic motor?

The digital low speed motors are eccentric shaft radial piston motors with an electronically controlled oil distribution system. Each cylinder has two electronically controlled two-stage valves. One connects the cylinder to high pressure and the other connects the cylinder to low pressure. This opens opportunities: valve timing can be adjusted to match the rotation direction; cylinders can be idled at minimum friction by opening the low pressure valves and closing the high pressure valves; cylinders can be locked by closing both high- and low pressure valves;  torque direction can be switched without switching pressure sides and it is even possible to pressurize a cylinder in the middle of a cycle.

Read more about digital HTLS motors.

Isn’t everybody going electric?

Everybody is talking about electrics and electrical drives are great solutions in many applications. Even hydraulic pumps are often driven by electromotors.

Some downsides of electrical drives on winches are the size and weight of motor+gearboxes and the high inertia of the electrical motor. A typical electrical combination of motor+gear weighs 6-8t/MW. Digital hydraulic low speed motors weigh 1,5-2t/MW. Billions of $ and € have been spend over many years to improve electrical drives and energy storage, yet the electrical technology still lags behind on weight to power and weight to torque ratios, not to mention on cost. During the same long period, only little resources have been spend on developing hydraulic technology, and accordingly there are some low hanging fruits to be taken. Those are the ones DIGIWIN focuses on, and it is expected to increase the winch range where hydraulic drives are the preferred choice.

Why will digital hydraulic winch drives be more efficient than other winch drives?

Digital low speed motors are more efficient than high speed motors and need no gearboxes. When a hydraulic or electrical high-speed motor drives a winch, a planetary gearbox is used to reduce output speed and increase torque. Depending on design and load, the typical loss in the gearbox is 1,5%-2,5% per gear stage. With two stages, 3-5% of all transmitted power is lost. In addition digital motors have very high efficiency, especially at partial load. Cylinders that idle generate hardly any losses, whereas a high speed hydraulic motor generates both leakage and friction from all cylinders, even at reduced displacement.

At the system level, no proportional valves or hydrostatic pumps are needed, and even load-lowering valves can be omitted. When lowering a load, the digital motor pumps oil from the low pressure side to the high pressure side and this oil can be used in other machinery or stored in hydraulic accumulators.

There must be some drawbacks…..

Of course there are. Though we like to consider them challenges and limitations. One performance limitation is that torque resolution is linked to the number of cylinders. A five cylinder motor only produces torque on two or three cylinders, and the step in torque by choosing one instead of two cylinders is big. With many cylinders, like on a winch with many motors, these effects are smoothed, and by switching valves in clever ways they become acceptable. Another challenge is the sound pattern. Digital HTLS motors are much less noisy than high speed hydraulic or electrical motors, but the sound from valve switching is hearable and ”different from what is usual”.

An often asked question is ”what about reliability?”. The number of small solenoid valves is high (two for each cylinder, 60 on the test winch). Though technology is proven, occurrences of valve failures and electronic failures must be foreseen. The method used to deal with this is ”robustness through redundancy”. Many small units are used to provide robustness as well as safety.

Throughout the project, handling of these limitations will be developed and demonstrated.

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