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09/11/2009
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Justin Cunningham finds out the advantages of using genuine linear motors
 Although linear motors are not a particularly new development, they are offering distinct advantages when very high dynamic motion is required. Companies like LinMot and Copley all produce tubular linear motors, but the main difference with Festo's newest development is the force for volume that it delivers. Festo's tubular linear motors use north and south magnet stacks which have coils wound around them in three phases, essentially making it a fixed coil moving magnet linear motor.
"When the designers first sat down to develop a tubular linear motor their long term goal was to develop something that could fit inside an actuator body to make it useful for industrial applications," says Nigel Dawson, product manager at Festo UK. "In order to do this they had to make it very compact in its cross section. "The coils are produced in a flat flexible membrane and then are rolled around the tube. Then, the whole thing is filled with resin to seal it between the external and internal tubes."
The linear motors also have no external magnetic force as the tube itself creates a magnetic return path. This allows the motor to be used in applications where there is iron ferrite dust or swarf. This is also useful in areas where it might be used with other instruments that might be subject to interference from a magnetic field such as hall sensors. Linear motors have, historically, suffered from a drop in force when they get close to the end of the stroke. This is because the force of a linear motor is dependent on how many coils are in contact with the magnetic field and toward the end of a stroke, the magnets actually begin to go outside this. "We have no drop in force across the full stroke length," says Dawson. "They have a peak force of 141N to 202N. So the ability to have some decent force, although nothing like a pneumatic cylinder, it is quite useful as it has very high dynamics and acceleration is 125m/s2."
During repeated high duty cycles linear motors tend to get very hot and are prone to overheat, especially when running at optimum dynamic loads. To help keep the motors cool Festo has incorporated two air ports on either end of the profile to allow for air cooling. There is a considerable cost difference between a pneumatic actuator and a linear motor. While the linear motors are designed to be drop in replacements with the same hard points as pneumatic variants, the application often define which method of linear motion is most suitable. "We have lots of almost identical type actuators with different motion technologies; pneumatic, lead screw, ball screw, the linear motor, and even servo controlled pneumatic," says Dawson.
"The linear motor is the top end in terms of cost and technology. So to use it as a direct replacement has the disadvantage of cost and also force. It will never produce - size for size - as much force as a pneumatic actuator, lead screw or ball screw. But, what they lose on force they make up for in accuracy and also dynamics. "It is in high speed dynamic applications where the advantage of repeatable accuracy profiled movement is the driving factors." Going forward Festo is looking to develop a hybrid linear motor to combine the advantage of pneumatics, which is power, with the advantage of a linear motor, which is high dynamics and accuracy in a small space envelope.
"One concept is to have them in series with one rod acting on the other rod, so you have a pneumatic element and a linear motor element in the same actuator," says Dawson. "The other concept is to have a flatbed linear motor and to have the linear motor and magnets sat outside the aluminium profile. Inside the aluminium profile you effectively have a rod and a cylinder. "Both of these concepts are in test and development. This could be a standard option on linear motors so you could effectively have a pneumatic assist with out a compromise to space."
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Author
Justin Cunningham
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