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12/07/2006
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Dean Palmer reports on some of the latest linear motor products to hit the market that offer a cost effective replacement for traditional ball screws or belt drives
Linear motors, because of their simple two-part design and the inherently non-contacting nature of their operation, are beginning to replace traditional ball screws and belt drive applications.
A number of suppliers are using linear servomotor technology to replace ball screw drives. An example is Copley Motion Systems of Basildon, Essex. It recently launched a new type of modular linear drive unit with integrated position sensing, which can dramatically simplify the building of gantry and XYZ handling systems.
Using the company's IP67-rated industrial linear motor technology, linear stages can now be bolted together in minutes to produce a rugged, multi-axis configuration.
Compared to traditional ballscrew-based linear stages, Copley said its 'ServoTube'-based linear stage is much simpler, consisting of a single off-the-shelf unit with only two parts, the forcer (the coils) and the thrust rod (permanent magnets). Because direct drive actuation is used, a gearhead, servomotor and ballscrew do not have to be aligned and integrated, saving valuable engineering time.
Backlash between a screw and nut and gearhead teeth are avoided using tubular direct drive technology, resulting in excellent repeatability of just 12 microns. Eliminating the mechanical drive train also improves speed to 10m/s, which is more comparable to belt drives, but delivers a more dynamic response, more reliably and with higher accuracy.
External position encoders require precise alignment, extra wiring and mounting area, occupying valuable machine space. However, Copley has solved this problem by using an in-built position sensor on its ServoTube. This means mechanical integration is simplified, but it also enables the IP67-rated system to be used in dirty environments, such as bottling plants or food slicing machines.
According to Copley, saving space is another benefit of its tubular direct drive technology. Machine builders might find that they have to compromise on their design to accommodate a servomotor. For example, an XY system of 1m square might require an additional 20mm for the servomotor and gearhead. Whether affecting the external footprint or the internal operating space of the machine, eliminating this overhead is a clear advantage.
Large air gaps between the forcer and the thrust rod further simplify machine integration compared to some alternative tubular linear motors. Alternatively, load-bearing X- and Y-axes can be built from pre-assembled 'ServoTube' modules that typically use a single rail linear slide for support. The ServoTube's rugged aluminium extrusion has DIN standard T-slots so that a load can be mounted directly onto the forcer. Similarly, a Z-axis actuator can be bolted perpendicularly onto a Z-axis for instant configuration.
But that's not all. Linear gantry and handling systems require maintenance - whether inside a machine or easily accessible, a production line must be halted for lubrication and part replacement. The 'ServoTube' requires no lubrication and a typical configuration is non-contact, so it will not wear like gearheads and ballscrews. In fact, the only maintenance that may be required is the lubrication of any external bearings added to improve radial load handling.
As well as traditional moving forcer configurations, the 'ServoTube' is also manufactured in a novel, moving rod configuration called the 'ServoTube' actuator (covered by Eureka in its March 2005 issue). Often preferred for a Z-axis, the moving rod actuator uses a long life polymer bearing (developed by Igus) that requires no lubrication.
The reduction in engineering time resulting from the use of modular motor and actuator components has been well received by machine builders and systems integrators worldwide. ServoTube systems can be operated using standard sinusoidal drives.
Anorad's 'LEU' Series is a brushless motor with an overall width of just 18mm, yet it develops 18kg (39Ibs) of peak force, which, according to the company, makes the motor the smallest of its power available.
The LEU motors come in two coil heights, the 'LEU-15' (15mm) and the 'LEU-30' (30mm). The LEU Series has six coil sizes of 35mm, 65mm and 95mm in length for the LEU-15 and LEU-30 models. The motors also come in four magnet channels, in lengths of 60mm, 90mm, 150mm and 300mm.
Pointers:
* Anorad's LEU Series linear brushless servomotors have an overall width of just 18mm, yet develop 18kg (39Ibs) of peak force
* Copley Motion Systems' modular linear drive unit has integrated position sensing, so it can dramatically simplify the building of gantry and XYZ handling systems
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Author
Tom Shelley
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