4. linear motor basics
- 1. Linear Motor Basic What Is A Linear Motor? A Linear Induction Motor (LIM) is basically a rotating squirrel cage induction motor opened out flat. Instead of producing rotary torque from a cylindrical machine it produces linear force from a flat one. It is not a new technology but merely design in a different form. Only the shape and the way it produces motion is changed. But there are advantages: no moving parts, silent operation, reduced maintenance, compact size, ease of control and installation. LIM thrusts vary from just a few to thousands of Newtons, depending mainly on the size and rating. Speeds vary from zero to many meters per second and are determined by design and supply frequency. Speed can be controlled by either simple or complex systems. Stopping, starting, reversing, are all easy. Applications are many and varied, ranging from simple sliding doors to full control of a ten tonne vehicle. What Can LIMs Do? Horizontal travel is only limited by the length of the reaction plate or motor. Long conveyors use multi- motor systems. If the reaction plate is a disc then rotary motion is produced. Motion is not dependant on friction between wheels and rails so LIMs can be used in adverse conditions. Either LIM or reaction plate can move whilst the other is fixed. A non-magnetic stainless steel barrier can be used between LIM and reaction plate. What Are The Advantages? LIMs are simple to control and easy to use. Fast response, high acceleration and braking forces are developed. Speed is not dependant on contact friction. There are no gears or chains to cause backlash, no lubrication, no maintenance. Fewer moving parts mean simpler systems. Long term operating costs are reduced. What Are The Down Sides? Cost: Linear Motors are expensive. Higher Bandwidth Drives and Controls: Since there is no mechanical reduction between the motor and the load, servo response, bandwidth, must be faster. This includes higher encoder bandwidth and servo update rates. Force Per Package Size: Linear motors are not compact force generators compared to a rotary motor with a transmission offering mechanical advantage. For example to produce even 65 N (15 lb) of continuous force, a linear motor’s cross section is approximately 50 mm x 40 mm (2” x 1.5” ). Compare this to the cross section of a 10 mm (3/8” ) diameter ball screw which produces 100 lb. of thrust and one can see that linear motors are not brute force devices. Heating: In most linear motor applications, the forcer is attached to the load. Any I2 R losses are then directly coupled to the load. If an application is sensitive to heat, thermal management techniques need to be applied. Air and water cooling options are popular and common. No (minimal) Friction: This may not sound like a problem, but it certainly can be. For instance, a linear motor is traveling at 3 meters/second (120 ips) and loses power. Without some resistance in the system, it does not take long before the motor reaches the end of stroke and mechanical stops. Slip: Slip is relatively higher. Air gap: Large air gap. Magnetizing current: Large magnetizing current required (∵Xm is small). • Efficiency: Low efficiency (∵ loss in secondary is high). 1
- 2. How Does It Work? 1. Take a squirrel cage induction motor... 2. open it out flat... 3. smooth the rotor bars into a conductor sheet... 4. apply AC power and you have a LIM 5. With two stators you can remove the reaction plate iron As the diagrams show, the LIM is essentially a circular motor opened out flat. The magnetic field, instead of rotating, now sweeps across the flat motor face. The stator, usually known as the LIM, consists of a 3 phrase winding in a laminated iron core. When energised from an AC supply a travelling wave magnetic field is produced. Travel can be reversed by swapping two phases. The reaction plate is the equivalent of the rotor. This is usually a conductor sheet of aluminium or copper backed by steel, but any of these may be used alone. Currents induced in the reaction plate by the stator travelling field create a secondary field. Where Are LIMs Used? Wherever straight line motion or reciprocating forces are needed, or where unusual rotary drives are an advantage. Mechanical transmissions are often eliminated, increasing reliability. The LIM is ideal for applications where space is at a premium. Rugged, they can be used in hazardous environments. Power Supply A 3 phase AC supply is required for most LIMs but 1 phase can be used to 600N. All standard voltages are available; 220 / 380 / 400 / 415 / 460 , 50 / 60 Hz. Speed Control The LIM makes an ideal variable speed drive. It behaves like a magnetic clutch and gives soft-start action. Maximum speed is fixed by design and frequency but load speed can be controlled in several ways. From on / off switching to phase control with tacho-feedback. LIMs can also be operated at stall to produce static thrust. Which LIM To Use? 2
- 3. When selecting a LIM it is important to understand the interaction between LIM, reaction plate and airgap. The variety of designs suited to different speeds, airgaps and reaction plates are extensive. The correct choice is not always obvious. Let Force Engineering design the best motor and reaction plate combination for your particular application. Our extensive design and test facilities are available to you. Typical Applications Sliding Doors, Aluminium Can Propulsion, Mixer / Stirrer Drives, Wire Winding, Baggage Handling, Pallet Drives, Flexible Manufacturing Systems, Robotic Systems, Bogie Drives, Conveying Systems, Steel Tube Movement, Revolving Doors, Sheet Metal Movement, Linear Accelerators, Automated Postal Systems, Ship Test Tank Drive, Extrusion Pullers, Multi-Motor, In-Track Systems, Low Profile Drives Target Movement, Sewage Distributors, Slewing Drives, Crane Drives, Stage / Curtain Movement, Scrap Sorting / Movement, Research Machines, Turntable Drives, Automated Warehousing, Flat Circular Motors, Theme Park Rides, Personal, Rapid Transport Systems, etc. Diagrammed layout of LIM products Example of a Motoring Configuration Example of a Controller 3
- 4. The standard controller configuration consists of a standard feedback loop in combination with acceleration-, velocity and friction feedforward. In this controller structure, a simple model of the linear motor is used for the design of the feedback controller, realizing a stable system. A learning feedforward component is added to this feedback controller to compensate for model deficiencies. In this way, an inverse process model is created in the learning feedforward. When the inverse model is perfect, the feedback control signals are zero (unless disturbances arise). As long as the feedback signal is not zero, the inverse model apparently is not perfect. The feedback control signal can be interpreted as an error signal for the current inverse model output. So, this signal can be used to improve the inverse model, briefly to learn. The part of the feedback controller in the control signal will decrease, while the feedforward part will increase. In this direct adaptive controller structure, a neural network is used for the feedforward controller component. From a number of possibilities a single-layer network with second order B-spline functions has been selected and implemented, mainly because of its computational efficiency. Choosing a linear motor Choosing the right linear motor for an application is not a simple task. Selecting the right technology for the application, force calculations, thermal considerations, bearing loading, commutation methods, etc., must be considered. However, knowing the basic types and the associated advantages and disadvantages will assist in the end solution. Three technologies of brushless motors are discussed. They are; ironcore, aircore (ironless), and slotless. IronCore Linear Motor Construction: This motor takes its design straight from a brushless rotary motor. As shown in fig.1, the motor consists of a flat iron rail to which rare earth permanent magnets are bonded. The Forcer is constructed of laminations and coil’s wound around the “teeth” of the laminations. Thermal sensors are mounted internal to the windings, sensing temperature. Hall effect sensors are either mounted in the coil area or mounted on the edge of the motor. These sensors are activated by the magnets on the rail and used for commutation of the windings. 4
- 5. AirCore Linear Motor Construction: This design is referred to as AirCore or Ironless. Two magnet rails oppose each other, north and south, as shown in figure 2. A spacer bar between them keeps the two sides from closing together. The forcer is constructed of coils wound and held together with epoxy. This winding assembly is then topped off with an aluminum bar. This bar is used for mounting the load and also for heat removal. The Winding itself has no iron in it, thus the names “AirCore” or “Ironless." As with the IronCore, thermal sensors and hall effect sensors are mounted to the Forcer. 5
- 6. Slotless Linear Motor Construction: Designed by the Compumotor and Daedal Division of Parker Hannifin, the motor takes its operating principle from Parker’s rotary slotless motors that have grown popular over the past few years. The rail is the same as those used for the IronCore design. Simply a flat iron plate with magnets bonded to it. The Forcer is unique. It begins with a coil similar to those used in the AirCore. A “backiron” plate is placed behind the coil. This assembly is placed inside an aluminum housing with an open bottom. The housing is then filled with epoxy, securing the winding and back-iron into the housing. Thermal sensors are internal. Comparison 6
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