While constant-velocity control is essential for many advanced industrial processes, traditional motor designs give rise to ‘cogging forces’ that generate ripples in the motor velocity.
Kovery Inc — based in Gyeonggi-do, South Korea — has designed a series of advanced linear motors, with integrated encoders supplied by Renishaw
that solve this problem (a linear motor is equivalent to a rotary synchronous motor with a ‘rolled-out’ stator and rotor, which means that instead of producing a torque for rotation, it produces a thrust force along its length).
One option for eliminating cogging is to use a slotless (ironless) flat motor. However, while this provides excellent control, it is at the expense of higher thrust outputs, but Kovery has developed a range of motors that minimise cogging forces without sacrificing thrust.
Kim Houng-joong, Kovery’s president, said: “The linear motors on the market come in various forms; and while each design has its benefits there are always trade-offs. The ‘unique’ linear motors developed by our company are the first to arrange the permanent magnets in the vertical plane, so that they pass between the pole pieces of the forcer coils.
“This ensures uniform magnetic flux and an independent magnetic circuit for each magnet track, allowing the motor strength to be increased simply by increasing the number of tracks. The design effectively reduces the normal attractive force between the slotted iron cores and the magnet tracks without sacrificing thrust.”
He added: “In other words, the overall thrust of the motor is increased, with the maximum being at least twice that of a conventional linear motor. Other advantages include reduced weight, increased design freedom, simple assembly, and a low cost of ownership.”
Kovery uses a range of Renishaw optical encoders for its linear motors, including the QUANTiC encoders with a 0.2µm resolution. Also used are Renishaw’s Evolute absolute optical encoder with RTLA50-S linear scale.
Mr Houng-joong added: “Kovery’s linear motors offer a range of specifications, and there are more than a dozen stroke lengths alone. Sometimes, it is necessary to provide customised products with strokes up to several metres. We also expect that market demand for longer-stroke linear motors will increase.
The QUANTiC encoder’s RTLC40 steel tape scale is supplied on a convenient reel, which allows us to cut the exact length required for each motor, improving our operation flexibility.
“Our customers are mostly precision equipment manufacturers that have demanding quality requirements. The linearity, straightness and squareness of each linear motor is measured using a Renishaw XL-80 laser interferometer. Dynamic measurements are also performed, including analysis of the velocity ripple. For product testing, the XL-80 system is quick to set-up and easy to use.”