Straight to the Motor Mainstream

In the past, implementation issues dampened some of the interest in linear motors

 

One of the clearest signs of maturing technology is vendor emphasis on ease-of-use and implementation. Linear motors and controllers are at that stage. As a result, vendors are now more interested in helping machine builders use linear motors and less concerned about the general promotion of linear motor technology.

Most machine builders know linear motors can provide a host of benefits as compared to rotary motors. Those benefits include greater accuracy, repeatability, and speed. Accuracy for linear motors is ultimately determined by linear encoder feedback accuracy, and linear encoders can resolve distances down to 10 µm over 300 mm.

Repeatability for linear motors will typically be within a few encoder counts (about 1 µm). Speeds to 500 cm/sec. are possible, and acceleration to 10 g is achievable, says John Mazurkiewicz of Baldor Electric (http://www.baldor.com). Move times and settling times are also less than with rotary motor and force converter systems.

A linear motor doesn't require components to convert rotary motion to linear, so linear motion systems have a smaller form factor, fewer parts, and are easier to install than rotary motors. These benefits are increasing the use of linear motors in new industrial machinery and robot applications, and are also causing the replacement of rotary motors and force converters with linear motors in existing applications.

In the past, implementation issues dampened some of the interest in linear motors. Communication was one of those issues. Linear motor manufacturers are now recognizing the importance of integrating linear motor controllers with the rest of the machine control system, and they now offer products that incorporate industry standards. "Our DSC and DSB controllers offer compatibility with industry-standard fieldbus architectures," says Arthur Holzknecht, sales and marketing manager with Etel (http://www.etelusa.com).

Etel's DSC controllers have optional interfaces for CAN-Bus, Profibus, and Macro-Bus. The DSB series controllers also offer a SERCOS interface. "The linear motor is only part of the story," adds Holzknecht. "You need a high-performance motion controller that integrates easily into existing plant communication systems to realize all the performance benefits of direct drive."

Linear motion systems based on rotary motors and force converters have long been available from vendors as packaged systems. Machine builders were able to specify requirements, and vendors could either meet these requirements with a standard unit or supply customized solutions.

Linear motor suppliers are moving in the same direction, and their ultimate aim is to have machine builders order complete linear motion systems as a packaged solution. "Our goal is to help linear motors move down the bell curve of linear motion applications," explains Jack Marsh, senior staff engineer with Trilogy Systems (http://www.trilogysystems.com). "Trilogy can supply its motors as components in kit form or as part of a complete positioning system."

Baldor and other motor vendors can supply linear motors already built into a linear stage assembly--a single or multiple-axis mechanical system that positions a payload. It includes a linear motor, bearings, an encoder, limit switches, a cable carrier, and bellows.

Another way that vendors are making linear motors easier to implement is by eliminating the need for encoder feedback. "Trilogy's linear-encoder module technology uses the magnet field already present in the motor to provide position feedback and is available for all our motor types," adds Marsh. "This increases the value of our positioning systems by eliminating the cost and complexity of an external linear encoder."

Other vendors are following suit with different methods for eliminating encoder feedback. "Our TLA product line offers unique encoder-less feedback when combined with our electronics," says Holzknecht. "This saves the cost of the encoder and allows the axis to operate in harsher environments."

Machine builders can face significant costs protecting linear motion systems from high temperatures. Suppliers are reducing these costs and shortening design times by supplying linear motor assemblies that can operate in high-heat environments. Etel's LMK product line offers IP65 protection via a high-efficiency liquid cooling system. This cooling system eliminates heat transfer to the machine structure and includes an encapsulated magnet way.

Linear motors have unquestioned performance benefits, but some industrial OEMs can't take advantage of these benefits because of cost. The economical Glide series of slotless motors from Trilogy Systems uses an open magnet track and an ironless-type coil with backiron to provide flux return. These motors are not as efficient as ironless or ironcore motors, but they are lower in cost and have fewer cogging problems.

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