Jon Excell looks at the ServoRam and asks could electromagnetic actuators take the place of hydraulic cylinders in motion control?

Over 2000 years ago, Archimedes jumped in the bath, jumped out again, and ran naked through the streets shouting ‘Design Engineering’.

1800 years later, the Greek mathematician’s epiphany was the bedrock of a technology that ruled the world of motion control: the hydraulic piston.

But could hydraulics’ stranglehold be coming to an end? After all, the technology does have its drawbacks. It can be clumsy, it leaks pollutants, and it requires frequent maintenance.

According to British engineer Philip Denne, for certain applications there is an alternative.

Denne, who talks of ‘a strong imperative within industry to move away from hydraulics’ is the inventor of the ServoRam, a powerful, precise, energy efficient linear motor that looks like a hydraulic ram.

The ServoRam, originally developed for the ‘rough, tough, zero maintenance world of the entertainment simulator’ is a dual-action, free piston, linear, 3 phase, brushless servomotor. It has a single moving part capable of moving at speeds of up to 100m/s and positioning to an accuracy of a few microns. Piston travel lengths vary from 10mm to more than 100m and thrust is scalable from less than 10Newtons to more than 1 MegaNewton.

Denne claims that the actuator is clean, simple, silent, fast-acting, easy to control, precise, reliable, and free of stray electromagnetic fields. Also, unlike hydraulic rams, it doesn’t leak oil and it can be used safely indoors.

How does it work?

A magnet array mounted on a piston / armature moves on slide bearings inside a coil array cylinder. The armature is sealed, creating a pneumatic piston with an internal gas spring reservoir that can be tuned for maximum efficiency and used to counteract motion caused by the deadweight of a device to which it is attached.

The air inside the ram is controlled by two small poppet valves that allow more air to enter from an external supply or vent it to the atmosphere. The mass of air within the ram controls the compliance of the gas spring.

Unlike a piston, the device features an output shaft through one end of the cylinder, enabling it to be mounted by the other end to a universal joint so that it can pivot and swing in any direction.

According to Denne, the ServoRam differs from other cylindrical linear motors in a number of ways. As well as its combined electromagnetic and gas spring capability, ServoRam is a permanent magnet brushless three-phase servomotor (the most efficient form of electrical machine for positioning devices). ‘Some apparent competitors,’ says Denne ‘are just induction linear motors,’ adding that ‘unlike any other brushless three phase servomotor, ServoRam’s electrical and magnetic fields are totally enclosed at all times, making it suitable for use in places where iron dust is common.’ Denne also claims that because it’s sealed, it’ll work under water and resist steam hosing.

Finally, the application area of ServoRam is different from other cylindrical motors. It can, says Denne ‘easily meet specifications for large and beefy machines that are difficult – if not impossible – to meet by the use of other cylindrical linear motors.’Denne claims that its precision, strength and flexibility makes the device a perfect hydraulics replacement in numerous motion control applications. Entertainment rides, robotics, active car suspensions and machining hexapods (a six axis arrangement of linear actuators which manouever a tool around a metal part) are just a few of the applications he’s identified.


‘We expected that the core demand would be for applications in which the parameters included speeds that made fluid systems expensive or precision positioning that was unachievable because of control transport lags’ says Denne. ‘We have found that even designers of slow-moving brute-force machines want to move away from hydraulics. What attracts them to the ServoRam is the silence, or the sterility, or the simplicity, or the extreme precision, or the extremely rapid reaction to an external disturbance.’

So how has Denne’s invention gone down in the world of hydraulics?

Jack Penty of Denison Hydraulics describes it as a ‘development of the linear motor with some interesting additions’. He adds that ‘the guiding of the armature is improved being of a piston construction. However the ServoRam relies on magnetic flux to transmit the power from the fixed stator to the moving armature.

This will limit the forces which can be transmitted with a conveniently sized winding as compared with a hydraulic ram working at 1 Ton or even 2 Tons per square inch.’ Penty goes on, ‘having said that it cuts out all the pumps, valves, pipes and other equipment required to make a hydraulic servo system work. The ServoRam might have an overall efficiency better than the inefficient hydraulic servo system. Given the above limitations it will undoubtedly find applications if for no other reason than it won’t spill oil on the floor.’

Over at Hagglunds, another hydraulics specialist, Brian Holmes is less generous. ‘We did look briefly at the technology’ he says, ‘but it seems to be in very early stages and the size of the ServoRam is much larger than the hydraulic equivalent making the possibility of using it for that purpose impractical and uneconomic. I imagine for most other normal uses such as on mobile plant and general industrial machines the same problems will be evident.’


John Shepherd from JCB Research describes the device as ‘very impressive’ and says that he ‘remains interested in the technology’. ‘JCB did look for a suitable application,’ he adds, ‘but the big problem is that the power density is much lower than a hydraulic cylinder. For example the ServoRam we saw produced around 3 tonne, a hydraulic cylinder of similar proportions would produce around 25 tonne.’One source in the aerospace industry told us that there’s a great desire to unify systems within a plane, make everything electrical and do away with hydraulic systems. The big issue here, he said, is the power to weight ratio.

Caterpillar, Eaton Fluid Power, Parker Hannifin and Rexroth Bosch all declined to comment.

So how does Denne react to these comments?

‘There is really no comparison between hydraulic rams and electromagnetic rams in terms of thrust per unit weight or thrust per unit volume’ he says. ‘An electromagnetic ram that is about the same size as a 6000 psi hydraulic ram producing (say) 30,000 Newtons steady thrust would only produce a few thousand Newtons short-term peak thrust. I cannot believe that we will ever see this kind of electromagnetic ram in aerospace applications.’

He continues, ‘I do not believe either that electromagnetic rams will be used in place of hydraulics in (e.g.) earth-moving equipment, rail wagons or truck tippers. What electromagnetic rams can do, that fluid rams cannot do, is to move with extreme precision (hundredths of a micron) and at high speed (tens of metres/second) and with high efficiency (90% plus) and have a bandwidth of several hundred Hertz and have a large peak-to-mean thrust rating and be silent and be sterile and have only one moving part in the whole system.

The electromagnetic ram is not (in my opinion) destined to do away with the hydraulic ram technology – only to replace fluid ram technology when the application demands one or more of the unique and valuable properties of these electromagnetic machines. Denne is the founder and chief scientist of US-based Advanced Motion Technologies, which owns the patents to the ServoRam and is offering the technology for licensed manufacture world-wide.