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Magnetic fluids give added attraction to pneumatic actuators

01 May, 2000

Magnetic fluids give added attraction to pneumatic actuators

A strange substance which changes from a liquid to a near-solid when placed in a magnetic field is being harnessed to produce a new breed of pneumatic actuators that offer position and velocity control with a precision comparable to that of more costly hydraulic actuators.

The actuators using this magneto-rheological (MR) fluid are said to be extremely accurate, affordable and durable. The initial products are based on existing actuators - both rotary and linear - to which MR brakes or clutches have been added to control or stop their movement. These are likely to be followed soon by purpose-designed MR actuators.

MR fluids are oil-based suspensions of microscopic ferrous particles. When a magnetic field is applied across them, the particles align themselves and resist flow, transforming the MR suspensions from fluids to near-solids in less than 30ms. Their viscosity is proportional to the strength of the applied field.

Magneto-rheological fluids should not be confused with electro-rheological (ER) fluids which exhibit similar effects when subjected to electric fields. North-Carolina-based Lord Corporation, which has developed the MR fluids, claims that the MR effect is 20-50 times stronger than the ER effect and says that its MR fluids are less sensitive to contaminants and to extremes of temperature than the ER varieties.

The MR fluids have the added attraction of working directly from low voltage power supplies. MR fluids were invented more than 50 years ago by a researcher working for the US Bureau of Standards, but it is only recently that Lord claims to have overcome drawbacks such as their abrasiveness and the tendency for the particles to precipitate out.

The variable viscosity of Lord`s Rheonetic fluid can be harnessed in three main ways:

  • a "valve" mode, with fixed magnetic poles, that can be used to produce dampers and shock absorbers with damping forces that vary in direct proportion to the current applied to an electromagnet in the piston;
  • a "direct shear" mode, with a moving pole, which is more suitable for clutches or brakes, with the MR material replacing metal-to-metal contact with controlled torque transmission provided by the variable viscosity of the fluid;
  • a "compressive squeeze" mode, which can be used to control small movements.

Lord, which has been developing its fluids for 15 years, has already used them in other applications, including truck seat suspensions that respond instantly to cushion shock and vibration produced by rough road surfaces.

The company has also applied its technology to prosthetic knees, washing machines and earthquake absorption systems. It now suggests that the MR-based pneumatic actuators could replace multiple "bang-bang" actuators as well as some electromechanical devices.

Three US actuator manufacturers - Turn-Act, PHD, and Parker Hannifin - are now starting to use the MR technology in their products. Each is applying the technology to a different type of actuator: Turn-Act to vane-type rotary actuators; PHD to rack-and-pinion actuators; and Parker to rodded linear actuators.

A fourth, as-yet unnamed, supplier will use the MR technology on rodless linear actuators. Although the deals are not formally exclusive, Lord wants to focus its efforts on supporting these early adopters.

"We want to support the four companies that have stepped out to adopt the technology," says Ed Houchin, Lord`s pneumatic actuator programme manager.

Turn-Act is the first manufacturer to reach the market with an MR-based actuator. It is using an MR brake on a range of range of rotary air-powered actuators that are said to provide precise positioning with pinpoint repeatability.

The company claims that its Comp-Act MR actuator combines the affordability of air-powered actuators with the control of electric servos. Adding an encoder and a PLC can provide the actuators with stepper-like behaviour, and provide characteristics such as controlled deceleration at the ends of rotation.

PHD is still finalising the controls for its rotary actuators, while Parker`s MR-enhanced linear actuators are undergoing field trials and are expected to go on sale during the summer. At the National Design Show in Chicago in March, Parker was showing prototypes which, it claimed, have the control characteristics of hydraulic or multi-actuator pneumatic systems, but are more precise and use standard components.

Parker is looking at using internally mounted linear potentiometers to sense continuous position, or externally mounted reed or Hall effect sensors to sense discrete positions.

All three manufacturers are using MR components to enhance the performance of existing actuator products, but Houchin predicts that, within a year, the first purpose-designed MR actuators will emerge. He says that the MR components will add about 30-40% to the cost of a "bang-bang" actuator but he points out that being able to reduce velocity into an endstop can eliminate the need for shock absorbers or for some actuators.

Houchin says it is impossible to put a value on the potential sales of the MR actuators because they are creating a new market for actuators with position and velocity controls. "You can`t do this today without servos," he says.

Lord is focusing on using the MR actuators to replace conventional bang-bang actuators, targeting the user "who wants to stop a rodded cylinder in mid-stroke", according to Houchin. But he adds that "technically, there is no reason for not replacing electromechanical actuators". He reckons that an MR-based pneumatic actuator would be about 75% the cost of an electromagnetic device.

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