The last time we investigated linear motors a few years back for use on our pick-and-place machines, we found a lot of system integration headaches to mix and match various components. We stayed away from them. Today it looks like there are more integrated systems available. Also, are there any design or selection guidelines to simplify matters?
—From November '10 Control Design
Integrated Linear Modules
Because linear motors combine great speed and high accuracy, they require tighter tolerances from measuring devices, rails and other components. Historically, companies in the general automation market that use pick-and-place machines needed to purchase the separate components from other manufacturers and then integrate them with the linear motor, which is a difficult engineering challenge.
Today, many suppliers, including Bosch Rexroth, offer preconfigured, integrated linear modules that include the linear motor, measuring system, bearings and other components; the module can be mounted directly onto the customer’s machine. Machine builders and end users know that the tolerances will be correct, the motor will work with the measuring system and the module will be aligned properly. For single-axis systems, this is now a viable option, with many variations of linear motor modules available. Multi-axis systems are somewhat more complex, although some suppliers are moving in this direction, too, offering packages with adapter brackets and other shared features so customers can choose from a family of standard, engineered parts for a pick-and-place robot.
Deciding whether to use linear motors in pick-and-place linear modules requires a clear understanding and definition of the application’s operational and engineering requirements. Linear motors offer a great deal of speed and precision and are used most in industries where the cost of such a performance premium is warranted, such as metrology or semiconductor applications. Indeed, pick–and-place machines with linear motors can achieve higher throughput while providing less wear and tear because there is no contact between components in a linear motor drive.
The use of linear motors requires systematic justification, including a thoughtful analysis of alternative technologies and lifetime total cost of ownership (TCO). The same considerations used in evaluating the differences between belt- or ball-screw-driven systems can also be applied to linear motors: the load of the application; the orientation, or plane of travel; the speed and acceleration of the load; the travel (product of twice the stroke length and the total number of cycles anticipated before motion component replacement); precision; environmental extremes; and the duty cycle.
For more information on how to design and engineer a linear motion system, consult the Bosch Rexroth white paper, "LOSTPED: Sizing and Selecting a Linear Motion System," at www.ControlDesign.com/LOSTPED.
Joel Galliher, Director—Mechatronic Systems,
Bosch Rexroth, www.boschrexorth-us.com
No Longer Exotic
Linear motors have become much more commonplace in industrial applications. Although they might have been considered “exotic” some years ago, they are now being used in applications as diverse as pick-and-place gantries, stages/tables for laser machining, and of course high-end applications in the semiconductor industry and R&D. The price vs. performance relationship, as well as the never-ending quest for higher accuracy and throughput, makes linear motors a viable option in many applications that previously were the domain of rotary-to-linear motion systems such as ball screws or belt drives.
The more common use of linear motors makes integration a much easier proposition, as many more integrators/engineers have become familiar with the requirements of employing them. In addition, there are vendors that can perform the entire integration using their own controls, drives and linear motors. So the specter of dealing with two or three different motion manufacturers and trying to coordinate the functioning of their disparate components is no longer true.
Linear motors (forcer and track) can be purchased and integrated into a custom housing or purchased as an integrated actuator that includes a bearing structure, encoder, cable management, limit switches and hardstops, in various travel lengths or for a variety of environments. In environments with debris, for example, linear motors can be enclosed in stages with all the same features as the unprotected actuator.
In conclusion, the expanding use, familiarity of integrators, vendor support, and options for a “one-stop shop” solution for linear motor-based motion systems means that they should be a viable option for your pick-and-place machines. However, as always, you must thoroughly examine the required operating specifications to make the ideal choice between available motion technologies.
Our Linear Motors Application Guide can be found at www.aerotech.com/products/PDF/LMAppGuide.pdf.
Joseph Profeta, Director Product Management,
Control Systems Group,