Linear Motors and Controls
This section contains additional information related to Control Engineering 's April 1999 article on Direct-Drive Linear Motors and Controls. A table of linear motor products appears in the main article, while representative linear control products are tabulated at the end of this section.
Linear motors in machine tool systems require real development process
While it's getting simpler to use linear motors, their successful application in machine tool/computer numerical control (CNC) systems still requires a thorough development plan. Complete integration of machine and motor is vital. Dr. Bo Eidelberg, director of Linear Motor Business Development at Anorad Corp. (Hauppauge, N.Y.), provides one outlook on such a process. That outlook has widened strategically with the recent acquisition of Anorad by Rockwell Automation (Milwaukee, Wis.).
Dr. Eidelberg's seven-step development process starts with 'qualifying' the given application, and then proceeds to more detailed technical issues.
Step1 : Contact an application engineer-- Contact with a local Rockwell Automation representative starts the qualification process. The Rockwell engineer(s) will continue the process after the application qualifies. Machine tool builders with the following applications are automatically qualified: transfer lines, machining centers, boring, milling, drilling, grinding, sawing, laser cutting, water jet cutting, bending, punching, and EDM. Dr. Eidelberg notes the importance of getting the first steps right. He attributes an order of magnitude cost increase to a corrective action, if needed, at each successive stage of the process.
Step 2 : Avoid $10 mistakes
Understanding the user's needs-- This step is considered the most important interface between user and vendor and should be allocated as much time as possible. Typical issues include: Machining process of interest, existing positioning and CNC solutions, required positioning specifications, environmental conditions, project schedule, cost objectives, growth potential, time to the market, and in-house engineering analysis capabilities. To correct a 'mistake' at this point has an equivalent cost of $10.
Step 3 : Avoid $100 mistakes
Proposal and price quotation-- Rockwell Automation provides a complete solution proposal. It includes system configuration, cost/performance analysis of linear motor/amplifier sizing, and a detailed list of all required system components with prices for each. The proposal is faxed or presented at the user site. The cost of correcting a mistake at this point is now equivalent to $100. A purchase is expected at this time to proceed to the next step.
Step 4 : Avoid $1,000 mistakes
System analysis and design review-- A Finite Element Analysis must be conducted as a basis for the detailed machine design. It can be done by the user's engineering team, a third-party consultant, or by our organization. The FEA is intended to assure an optimal structural design to support the required performance. Our proprietary simulation tools will conduct a complete servo simulation of the proposed solution. The tools will be used as a basis for design review in which our expertise in linear motor machine design is shared to propose any change we feel will improve the design. An original wrongful decision caught at this point will cost $1,000 to correct.
Step 5 : Avoid $10,000 mistakes
Prototype development- In this project phase, the customer builds its machine prototype or tests its design on a test bench. When the user is ready for the servo system installation, our long range customer support team will be available to assist in conducting a preliminary system check; interface all components; review our documentation; provide limited on-site training; power up the system; and do initial parameter setting, phasing, tuning, testing, and performance optimization. The objective is to support the customer to qualify the machine and move into production. A correction of the same original error at this time will cost $10,000.
Step 6 : Avoid $100,000 mistakes
Ramp-up with preproduction machines - - A limited number of machines are typically built and sold as Beta Sites to a well-controlled customer base. We will follow-up very closely on feedback, and will be sensitive to any last minute changes needed to accommodate end-users. We recognize that this is the machine builder's most critical phase of the whole project. At this stage, the machine tool design is being tested in the real environment. One hour downtime in an automotive assembly plant machining cell may costs $100,000, which is the estimated cost to correct the same old bug if not previously caught.
Step 7 : Avoid million dollar mistakes
Production and after-sales support-- Reaping the rewards of a successful project begin here. As strategic partners we are committed to product reliability, on time delivery, and competitive cost/performance products. Rockwell Automation will process orders and shipping. Further services provided include frequent visits by our regional support team to keep close touch with ongoing needs, periodic training on new upgrades as soon as they are available, and invitation to train at our facilities for more proficiency in wider motion control products. As appropriate, we will use our strategic partnership for mutual benefits such as selecting the customer facility as a Beta Site, providing an option for a head start in the market with our own new product development in a new process.
Catching bugs at this stage of the process may end the project with millions of dollars of losses. That's why we like to catch all bugs and mistakes at the start of the process where the cost to correct is low. Expertise becomes so important. This is why Anorad's 20-plus years of linear motor expertise and Rockwell Automation's worldwide supportive arm are the choice in reducing risks and lowering costs on the way to the market, says Dr. Eidelberg.
Overview comments on linear motors and controls
Baumüller (Nürnberg, Germany; Bloomfield, Conn.) - Linear drives enjoy advantage where space, dynamic requirements, and compliance between load and motor can't be tolerated or compensated. Today, linear drive systems are more costly than rotary systems, but pricing is expected to be competitive in the near future, says Baumuller.
A specialty at Baumuller is long-stroke linear motion for transporting heavy loads. Its systems are based on the synchronous motor with a so-called 'modified long stator' principle. The moving element is a ferromagnetic bar to which permanent magnets are attached. In this type of drive, feedback devices trigger the amplifier system such that only specific stator sections-- located along the track-- are powered as needed. This also minimizes power usage. Wider air gap tolerances (up to 15 mm) are tolerated, yet positioning accuracy of
Versatility of these linear motion systems includes curved tracks (50 mm minimum radius), switches and cross-over points for changing direction of loads, and transporting of web materials such as paper (with up to 3 webs in parallel).
GE Fanuc Automation (Charlottesville, Va.) - Linear motors and controls are viewed as one integral system at GE Fanuc. 'Performance will be optimal if the control and motor are designed to complement each other,' explains Chuck Weidner, Servo Product manager. GE Fanuc designs and manufactures its own line of linear motors, albeit via a technology license, according to Mr. Weidner. 'However, we add our unique design for the motor wiring, impedance, and force/speed curves.'
Among notable features of GE Fanuc linear systems are support for multiple feedback options. This includes serial communication between the absolute scale and controller, providing higher bandwidth (hence faster, more accurate motion) than the standard 'a quad b' method. Software functions are used to advantage, for example, to align the feedback read head to detect magnetic lull. A related function-- useful during setup and maintenance-- indicates whether the control is in fine or coarse commutation, also through software. A half PI loop is said to provide higher performance current loop and extremely stiff response.
Rexroth Indramat (Hoffman Estates, Ill. ) - Linear motor prices have come down from a few years ago when these motors were in their infancy and more of a curiosity for the machine tool industry. Larger linear motors, such as those Indramat provides, were initially accepted in the machine tool industry. However, in other industries, such as packaging and processing, the limits of performance of ballscrews, timing belts, rack and pinion, etc., are being reached as the quest for better machine performance continues. 'As more companies incorporate linear motors into their systems, prices will continue to come down because of competition, optimized production processes and higher volumes,' says Karl Rapp, mnachine tools applications manager.
Sulzer Electronics AG (Zürich, Switzerland ) - LinMot is a division of Sulzer Electronics Inc. (a subsidiary of the $4 billion Sulzer Corp.). LinMot has just appointed 13 regional distributors in the U.S. and Canada. A new range of linear motor drives is now being launched in the North American market, according to Dr. Ing. Ronald Rohner, Business Unit manager in Zürich.
LinMot-P linear actuators are often a relatively low-cost replacement for standard motion technologies, such as servo motors, stepper motors, mechanical cams and levers, and pneumatic cylinders. Input power requirements are in the 24-72 V dc range, depending on unit size. See product table in CE's April 1999 article. A full range of hardware, software, and accessory products is offered.
THK America Inc. (Schaumburg, Ill.) - The planned April 1999 release of THK's M1A2 OEM Series linear motor slide supports the trend toward packaged linear actuators. Chris Miyazaki, manager of THK America's Mechatronics Division, calls the M1A2 package a complete servo-motor-driven, single-axis linear positioning system. It includes the linear slide, actuator, pre-engineered drivers, and controller. 'Our intent is to cause explosive growth in linear-motor-driven slide applications via this productline's attractive features,' he states. M1A2 is said to be highly reliable; easy to install, power up, and run; and comes with support documentation and software. 'It is designed and priced to re-educate engineers about linear motor slides as a true alternative to ballscrew, belt, and rack-driven positioning systems.'
Trilogy Systems (Webster, Tex.) - The company's linear motor/LEM (Linear Encoder Module) product is seen as the linear equivalent to the rotary servo motor/encoder combination widely used today. 'As actuators become more compact, they will have the look of more traditional ballscrew or belt-driven actuators,' says Bruce Beakley, president of Trilogy. Plug-and-play products will become more evident-- not limited to motors, but including as well motor/encoder combinations (e.g., LEM), low-cost linear motor actuators, and high-powered drive/controllers using digital signal processors.
Commenting on Anorad's recently introduced moving magnet linear motors, Mr. Beakley thinks they're 'useful in some applications,' since the stationary cable eliminates any chance for cable breakage. However, he adds, 'Advances in high-flex cable design have largely eliminated cable failure from flexure as a problem area. High-flex cable has come far over the last 5 years with a number of manufacturers offering competitive products.'
Linear motors in rotary application
One novel usage of a linear motor cited by Anorad Corp. actually produces a rotary output in a laser cutting application. The rotating element is an 800-mm diameter circular magnetic assembly-- developing 250-Nm torque, acceleration of 5.5 rad/sec2, and velocity of 3.0 rad/sec (all maximum values)-- while the linear motor coils are held stationary.
This approach provides a lower cost alternative to a rotary torque motor-based solution, says Anorad. An optimum magnet track radius exists for a given cost and torque target. Therefore, design tradeoffs must be carefully examined. With only discrete radii available, the design is a compromise between the number and size of magnets and coils. For example, a smaller magnet track radius has fewer magnets, but requires larger coils. One, two, or multiple coils can be placed around the magnet assembly's periphery.
Positioning performance with 0.6 arc-sec resolution and