Linear motor applications: Ironcore versus Ironless Solutions Herve Stampfli

Transcription

1 Linear motor applications: Ironcore versus Ironless Solutions Herve Stampfli Abstract: Linear servo motors have now established themselves as the drive technology of choice for high performance motion control applications. From automation to semiconductor and electronics industries, the constantly increasing requirements for throughput improvement, flexibility and dynamics in the machines has led the manufacturers to opt for the direct drive approach. In the marketplace today, there are two predominant types of brushless linear motors that are widely utilized: the ironless, or U-channel motor, and the iron core or single sided magnet type. While they are both linear servo-motors, they have vastly different performance characteristics. That makes the different types suitable for certain types of applications and unsuitable for others. While the general knowledge of engineers regarding the advantages of linear motors is advancing rapidly, the differences between the main motor types are not well understood. Further to the point, there are many misconceptions as to the advantages and drawbacks of each type. Thus, the wrong type is sometimes misapplied to an application, resulting in inferior performance. This article will present an engineering based comparison of the two predominant linear motor types, with the objective of providing the reader with the knowledge needed to properly specify the correct type of motor for a given application. Real world examples of both motor types will be presented. Specific topics will be discussed, including: Thermal considerations Stiffness, dynamic and static Efficiency Real definition of important, but often misused parameters like cogging, force ripple, commutation ripple, and other terminology, and their effect on motor performance versus the type of motor Force density Accuracy, velocity, stability, settling time Use of Hall effect sensors for initialization and current commutation; effect on performances Influence of the servo controller Magnetic attraction Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 1

2 The different linear motor types As a first step, a linear motor can be considered as the unrolled version of a brushless DC rotary motor. The rotor with permanent magnets would changed into a flat linear magnetic way, also called secondary, which is generally used as the fixed part of the motor. The stator is changed into a flat linear coiled part, also called forcer, glider or primary, which is generally used as the moving part. The design of the magnetic ways and the material that the forcer is composed of will determine the nature of the linear motor, ironless or ironcore. Ironless motor As its name implies, an ironless linear motor, also called air-core motor has no iron inside. The glider is basically a plate made of epoxy where the copper coils are inserted. The forcer slides in between two rows of magnets that are facing each other. They are linked on one side by a spacer. This is also called a U-channel magnetic way. By design ironless motors peak power range is limited to a few thousand Newtons. The peak to continuous force ratio is generally high (typical factor of 4 or more) and these motors are typically used in a high dynamic demanding application where payload is light. In application where very high smoothness of motion is required, ironless motors are preferred due to the absence of detent force. Ironcore motor The ironcore motor is composed of a slotted lamination stack made of steel. Laminations are insulated from each other in the same way as they are in a rotary motor, which reduces Eddy currents that would result in too high iron losses. By design ironcore motors peak force range can go up to several tens of thousands of Newton. By design there is a wide range of sizes available for the ironcore linear motors. The motor choice will be based upon the overall dimensions affordable in the application. The peak to continuous force ratio is usually in the range of 2.5 for a motor used with forced air cooling. Motor width is typically from 30 to 300 mm whereas length can vary from 50 mm up to 800 mm or more. High duty cycles and low heat dissipation From automation to semiconductor and electronics industries, the constantly increasing requirements for throughput improvement, flexibility and dynamics in the machines has led the manufacturers to opt for the direct drive approach. As with any other kind of motor, heat is generated during operation. Since the motor is directly linked to the payload in the middle of the mechanics, this heat has to be efficiently removed to avoid any thermal drift in the machine. If ones wants to evaluate the thermal behavior of a given motor, a key parameter has to be looked at, the motor constant or Km. This parameter is defined as the ratio between the force that a motor is capable of producing and the square-root of its power dissipation at this force level. Typical S.I. units of Km are N/ W. In other terms, this is a picture of the efficiency of the motor. The higher the motor constant, the more efficient the motor would be. When comparing different motors, ones should make sure of comparing the different Km calculated at identical conditions. The motor constant is proportional to mainly these factors. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 2

3 K m l a B δ. S enc l. K m cu where : l a Bδ Senc Kcu lm Motor width represents the magnet field represents the section of the slot represents the copper filling factor is average length of one turn of the coil The rare earth materials that the magnets are made of will fix the magnetic field level. The most commonly chosen compromise between power capabilities and price is for the Neodymium-Iron-Bore material. Open slots design The magnetic design of the motor will determine the section of the slot (S enc ), whereas the last two parameters (K cu, l m )are more linked to both magnetic design and coil manufacturing process. Etel has patented an opened slot design that allows to manufacture the coils as a separate part of the motor in a very compact and dense way. It is then possible to get a very high copper filling factor (up to 60% ) and thus a very high km depending on the motor size. One of the most important considerations to use a linear motor in machine design is heat dissipation. Here is where there are significant differences between ironcore and ironless motors. The heat generated by the motor can be dissipated thanks to the inherent conduction, convection and radiation properties. The design of each motor intrinsically characterizes the most efficient method of heat transfer. In the thermal conduction process, the heat transfer is directly dependent on the surface of attachment of the motor to the mechanics and on the thermal conductivity of the structural material of the motor. Typical thermal conductivity of epoxy is 1.02 W/(m.K) whereas iron s is 50 W/(m.K). In the ironcore motor, the lamination stack acts as a natural heat sink, whereas the epoxy structure of the ironless motor acts as a barrier to heat transfer. This conduction ends up as convection in free or forced air. Aluminium Steel Copper Epoxy Thermal conductivity W/(m.K) Young Modulus GPa Convection represents 1/3 of the total heat transfer in a free mode. The impact of convection on motor cooling can be important especially in applications where the movement amplitudes are large and the speed is high. Radiation contributes in major way to the heating up of the magnets whose power is strongly dependent on their temperature, and then to the heating up of the structure. Radiation contributes in a 2/3 ratio to the total heat transfer. The higher the temperature, the lower the magnets power. In an ironless based system, the motor is radiating on both sides to the two rows of magnets, which can lead to a fast increase of their temperature. In an ironcore solution, the motor radiates on one row of magnets only. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 3

4 Force density The first parameter that will certainly help make a decision whether to go with ironless or ironcore is the force density available with each of them. In the attached graph, you can see that for a given level of continuous force, the ratio force per active surface unit can be up to 2 times greater for the ironcore motor technology than for an ironless one. In other Force density (N/cm2) Comparable continuous force (coils@80 C) are assumed for the motors Areas calculated correspond to the active surfaces of the motors LMD LMD LMD LMD ILF ILF ILF ILF LMD ILM Continuous force (coils@80 C) with free air cooling Ironcore Motors Ironless Motors terms, this means that for a given level of continuous force, an ironcore motor can be up to 2 times smaller than an ironless one. This can be a key parameter in applications where compactness is needed. LMA ILM LMA ILM LMA ILM How to make your machine even cooler! One can bet you want your machine to be very cool! Let s see how motors options can help make it even cooler. Since the linear motor is generally buried into the mechanics, it has to be kept as cool a possible to avoid too much power dissipation and then mechanical distortion. By design, ironcore motors are easy to equip with a cooling option. To help remove the heat dissipated by the motor, different coolants can be envisaged. The water cooling of ironcore motors is the preferred solution in heavy duty applications where payloads are high. This is typically the case in machine tool applications. A water cooling allows continuous ratings that are 200 to 300% greater compared to non water-cooled motors. Until recently, air cooling of linear moors was not very effective, since most manufacturers simply blew forced air into the cooling channels instead of water. Etel s innovation was to design a motor specifically (and exclusively) for air cooling. This method has lately proven to also be very efficient when blown at the right location in the motor. This technique allows to increase the continuous ratings of the motors by almost 20% compared to the non-cooled version. This is an unprecedented improvement in the effectiveness of air cooling. Ironless motors due to their inherent low efficiency can benefit more from air-cooling. Some manufacturers claim more than 50% increase of the continuous ratings. An alternative to cooling down the motor is to let it heat up. In that case, the motor has to be well insulated from the mechanics. This solution leads to a decrease of the motor performances since the continuous ratings are generally described based upon a given amount of heat dissipation through the mechanics. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 4

5 Heavy-duty application with ironcore motors: a PCB drilling machine (Courtesy of Posalux) This PCB drilling machine is integrating mainly two kinds of tools namely drilling and milling cutters. The Y axis is moving a table holding several PCB ready to be drilled. The Y motor is doing a back and forth movement as shown on the attached picture. The moving mass is of about 500 kg. Acceleration in the range of 1 g and the speed of about 1 m/s. The X axis is doing a movement from left to right on the attached picture and supports up to 10 different working stations. The moving mass on the X axis is in the range of 350 kg depending on the configuration. Acceleration and speed are in the same range as for the Y axis. Each working station is equipped with a Z axis moving from top to bottom on the attached picture. The moving mass is in the range of 8 kg and the acceleration can be up to 4 g s. Z Y X High smoothness of motion and non mechanical bearing applications Apart from the thrust force itself, additional forces can be generated in a linear motor depending on its type. In applications where a perfect smoothness of motion is required, the system generally integrates air bearings. Perturbating forces are thus not desirable. They can be easily overcome by an appropriate magnetic design or servo controller choice. Force ripple is the result of two effects namely the cogging (or detent force) and the commutation effects. The cogging is always present in an ironcore motor. Indeed, the force between magnets and lamination stack causes not only an attraction force but a force in the direction of motion as well. This force depends on the relative position of the laminated teeth with regards to the magnetic poles. The cogging is independent of the current flowing in the motor. Nevertheless using techniques like skewing lamination stack or magnets can drastically reduce this effect. Choosing an appropriate and optimized combination of teeth and magnetic pitches will provide the same effect as skewing one or the other part. The choice of the right angle of clearance at the both ends of the linear motors will help reduce the cogging induced by the end effect. Ironless motors do have zero cogging since the moving part has no iron at all. Ironless motors will therefore be the right solution when an extremely high smoothness of motion is required. The commutation effect can be generated by both motor and electronics. The motor can generate force oscillations if the back emf voltage is not a perfect sinus. Electronics will induce force oscillations as well due to current ripple. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 5

6 Reducing the ripple force is possible. As a stage mapping can help improve the accuracy of a mechanical slide by learning the mechanical defaults and compensating for them, a ripple compensation can be envisaged in the electronics to reduce force ripple. In that case, the actual force ripple will be measured at different positions and stored in a table. The current loop will then integrate a corrective current value associated with a given position of the motor along the magnetic period to balance the ripple force. A typical electronic compensation can easily reduce the force ripple by a factor of 5 and even up to a factor greater than 10. If iron is needed to concentrate the magnetic flux in an ironcore motor, one of the major penalty of having iron in front of magnets is that a very high attraction force is generated. Depending on the motor size, this attraction force can be up to 6 times greater than the motor peak force ratings. Depending on the type of guiding system that is used, mechanical, aerostatic or hydrostatic, this attraction force can be a good help or a bad problem to overcome. All the mechanical ball bearings available on the market today have huge load capacities which will overcome the attraction force level without any problem. It is then a matter of choosing the right size of rails depending on the harshness of the application to ensure a reasonable lifetime. As opposed to mechanical bearings which stiffness is very high, air bearings have no or very low stiffness. In order to generate the film of air that is required between both moving and static parts, a preload is required. The attraction force of the ironcore motor can be used as an efficient way of preloading the bearing. Etel is typically providing ironcore motors to manufacturers of grinders for machines using hydrostatic bearings. The main problem is that the attraction force is not of constant intensity depending on the current that flows in the motor. If the attraction force is defined at zero current, its value at peak current can vary within ±10%. This particular point makes the choice of an ironcore motor for a system based on non-mechanical bearings difficult for application where very high smoothness of motion is required. For these applications, an ironless solution would be preferred because of the absence of attraction force and related fluctuation. A semiconductor application: Die bonding machine (courtesy of Muhlbauer) Optimized for high speed eutectic and epoxy processes, this die bonder is designed to deliver high throughput, high reliability and very high yield. The machine is composed basically of an X and a Z axes. Four linear ironless motors are used to perform the very stringent specifications characterizing the machine. The peak force is 400 N per axis whereas peak acceleration is in the range of 15 g s and maximum speed is up to 4 m/s. Coupled to very high performances drives, this machine is the fastest one on the semiconductor market with its 0.2 second cycle time per component (5 components per second). The maximum authorized overshoot is 3 microns. To respect the die s positioning accuracy on the lead frame, the repeatability of the movement must be within less than 1 micron. When the movement starts, the final position is not yet known. A correction is performed on the fly in real time thanks to a vision feedback to the position controller and thanks to a very high communication rate between the PC and the servo controller during the whole Ironless motors fits perfectly this application where compactness and very high dynamics are required and light moving mass is involved. trajectory. Clean room, vacuum and magnetic field sensitive applications Often referred as to non contact motors, linear motors are therefore perfectly suited to work in a clean room environment. On standard both ironless and ironcore motors can work in an ISO6 class (former Class Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 6

7 1000). An ISO class5 (Class 100) or below will require special cares with regards to the materials that are used such as cables, magnets coating etc Vacuum applications in the range of 10-3 mbars (750 Torr) do not require any special care with regards to the materials that the motor is composed of. When this vacuum level drop to a lower value, cables, magnet coating, and epoxy resins have to be chosen as non-outgasing materials. Magnetic field sensitivity is often an issue in some applications where electron beams are used. This is typically the case in e-beam metrology tools, SEM (Scanning Electron Microscope) or ion implanters devices. Y=95 The magnetic field around the motor has 100 to remain as small as 70 possible in order not to make the e-beam deviate from its trajectory. By construction, the U Magnet Plate shape of the ironless motor magnetic ways make the magnetic field self contained in the magnetic way. As shown on figure X, the magnetic field measured around a magnetic way of ironless motor is of a very small amplitude as opposed to the measurements taken on an ironcore motor magnetic way that constitutes an open structure. Position and speed stability applications Increasing the stiffness Short settling times associated with high dynamics or high position stability requires stiff mechanics. Stiffness related to both type of ironless and ironcore motors is composed of three components: the motor built-in stiffness, the motor mounting stiffness and the servo-loop stiffness. 4 8 Motor built-in stiffness - The epoxy structure of an ironless motor has a low inherent stiffness. The motor rigidity is given by the copper coils that are inserted and is dependent on how the coils are physically put together: separated or overlapped. An overlapping configuration would lead to a higher bending stiffness compared to a construction with independent coils whereas the lateral stiffness will be almost identical in both overlapping and separated configuration. The steel structure of an ironcore motor make it obviously much stiffer than an ironless solution. Figure : Mounting of an ironless motor Small contact area, cantilevered support Mounting stiffness - By design, the mounting surface of an ironless motor corresponds generally from 10% to 25% of the active surface of the motor. The attachment surface is located on one side of the motor. Besides allowing almost no thermal conduction, this kind of mounting can be problematic when high position stability or very tight settling times are required. The main reason is that the current that is flowing in the motor phases can excite the transversal natural frequency of the motor, leading to oscillations once in position. In an ironcore motor, the mounting surface is generally 100% of the active surface of the motor and centered above the motor. The stiffness of this mounting is Figure : Mounting of an ironlcore motor Large contact area, uniform over motor Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 7

8 therefore much higher. As long as the carriage is stiff enough to withstand the attraction forces, no vibration problem should be foreseen. Servo-stiffness - Ironcore or ironless, a linear motor provides a zero stiffness in the direction of the movement when the power is off. Once the power is on, the stiffness is given mainly by three factors: The stiffness of the mechanics The encoder resolution The servo amplifier sampling rates on both current and position loops Figure x shows a typical graph of the stiffness of a linear stage versus the perturbating frequency. This test has been performed with an Etel s linear ironcore motor moving 250 kg and an Etel s amplifier. One can see on this graph that the servo-stiffness is decreasing with the increase of the frequency up to a frequency corresponding to the natural frequency of the load. The stiffness then increases again. The stiffness never recede 400N/µm Stiffness in N/um Measured stiffness of the system in closed loop Natural frequency given by a 250 kg moving mass. (Calculated) Frequency in Hz Figure x: Servo Stiffness with a linear motor moving a mass of 250 kg Influence of servo controller Whether an ironcore or an ironless solution is integrated in a given application, the final specifications are directly linked to the servo controller characteristics. Special care should be taken to choose a digital servo controller as well as a digital servo amplifier with encoder commutation. High position and current loop sampling times and a high interpolation factor on the feedback signals will allow to get the required servo stiffness and final resolution on the stage. Printing application with an ironcore motor (ARRI) To create special effect on movies, each image has to be digitalized to be modified on computers. After adding the special effects, the image has to be re-printed on the tape, This can be done by using special scanners and lasers. This last operation usually takes 25 seconds. Etel has developed a linear stage equipped with an ironcore motor capable of decreasing this time from 25 to 5 seconds. With 24 images per seconds, the time saving is dramatic. To avoid any deformation on the screen, no default are allowed on the image itself on the tape. Thus the speed stability during the scanning movement is crucial. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 8

9 The laser beam and the stage are controlled and synchronized with an Etel s electronics. The linear stage has a stroke of 29 mm and is moving at 6mm/s at a speed stability of 0.2% all along the stroke (measurement frequency is 1 khz). These performances are achieved thanks to ETEL s state of the art DSB2 drive electronic, a linear encoder from Heidenhain and special linear bearings in order to minimize any mechanical perturbation. ETEL s ironcore linear motor is implemented in this stage. With this stage, the measured tracking error is 70 nanometer and the position stability has been measured at 3 nanometers. Courtesy of Arri Start position : 16.5 mm Tracking at constant low speed Tracking error (M2) Positive tolerance Negative tolerance Standard deviation at 6 mm/s : 6.44 Tracking error without filter : Time in ms The costs of linear motor solutions There is often a misconception about the cost of a linear motor solution. How compare prices of the two solutions? To be compared to alternative solutions like brushless servo motor and ballscrew or belt, ones will take care of taking all the relevant costs into account. A linear motor is a contact-free motor which means that the maintenance costs associated are drastically reduced. The number of parts in the mechanical assembly will be reduced as well. The ironless solution is almost never cheaper than an ironcore solution. The main reason for that is that the cost of a linear motor is primarily related to the price of the magnets. For a given level of force required, when an ironless magnetic way requires two rows of magnets, an ironcore motor requires only one. The longer the stroke in the application the more obvious the price difference is. Hereby is a graph that shows typical price difference between an ironcore and an ironless motor solutions for a defined application. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 9

10 Relative Price (USD) ILF Comparable peak forces are assumed for the motors Usable stroke of 300 mm ILF LMD LMD ILF ILF LMA LMD ILF LMD ILF IILM LMA LMA ILM LMA ILM LMA Peak force level (N) ILM LMA ILM LMA Ironcore Solution Ironless Solution Figure: Relative price comparison for Ironless and Ironcore solution for a given application The power-on issue Initialization and current commutation As any synchronous motor, the current injected in the motor has to be synchronized with the back emf voltage generated by the motor. This is done through the initialization process that takes place at the switch on of the amplifier. Three main methods are commonly used to perform this synchronization, depending on the motor type as well as on the electronics capabilities. Initialization by constant current A constant current is generated in one motor phase. The motor moves to a stable position where current and back emf are 90 shifted. Initialization with Hall effect sensors Hall effect signals sent to the servo-amplifier input allow it to estimate the commutation phase within ±30 upon power up. The phase resolution is good enough to drive the motor. Once a transition point of any of the Hall effects is passed, the servo-amplifier detects it and adjust the phase position accordingly. Initialization by pulse This type of initialization is not common but available with more sophisticated digital amplifiers. It allows a very accurate initialization without any movement of the motor. This is typically required to prevent tooling or part damages after a power failure on the main supply. At the next power on, without any movement on the motor, the amplifier has to be able to properly commutate the current. Linear motors can be commuted trapezoidally using Hall effect sensors or sinusoidally using linear encoders in conjunction with the appropriate motion controller or sinusoidal amplifier. Obviously a sinusoidal commutation allows very high interpolation factor that will let reach high resolution on the amplifier and then on the final stage. Why always move the motor? In applications where small strokes are needed, it is very useful to keep the motor static and have the magnetic way moving. This rids the designer of the cable management problem and helps reduce the Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 10

11 moving mass. The reasonable stroke limits for such a solution should not exceed mm. This particular application case cannot be easily envisaged with the use of an ironless motor due to the U shape of the magnetic way and its associated weight. Ironcore motors would be preferred in that case. 6 myths about linear motors 1. Linear motors are too expensive Wrong. Direct driven solution can compete with non direct driven ones thanks to benefits such as the following ones: reduction of maintenance costs increase of the machine throughput smaller number of mechanical parts simplicity of integration lower cost: ironcore motors can be used in many applications 2. Ironcore motors can t be used in ultra-precision applications 3. Linear motors get too hot to be used in high-precision application 4. A linear motor has to jump to initialize (uncontrolled initial movement) 5. High dynamic can not be achieved with ironcore motors 6. Air bearings require ironless motors that previously required ironless motors Wrong. Ironcore motors applications can provide results like: Nanometer position stability (semiconductor applications) Tracking error at low speed in the sub-micron level (printing applications) Very high Smoothness of motion (scanning applications) Wrong. As any motor, linear motors heat up during operation but different solutions are available to limit or remove the heat generated or to stop it from going into the mechanics: Ironcore motors are very efficient (low level of losses for a given level of force) Insulation of the motor Air or water cooling available Wrong. Linear motors can initialize witthout any movement thanks to Hall effect sensors. Better, ironcore motors can initialize without movement and without Hall effect sensors Wrong. There is no physical reason why an ironcore motor would achieve lower dynamic than an ironless motor. Thanks to a very good mechanics and an appropriate servo-controller, ironcore motors can achieve 25 g s and more. Wrong. Ironcore motors can be used with air bearings as well. The attraction force is then used as a preload. It depends on the complete application to determine the best approach. Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 11

12 Application requiring or implying: Which motor to choose? Should integrate Ironless motor Ironcore motor Sensitivity to magnetic field Recommended Not Recommended Continuous force > 500 N Possible with several motors Recommended electrically mounted in parallel Peak force > 2,500N Possible with several motors Recommended electrically mounted in parallel Air bearings with perfect smoothness of Recommended Impossible motion Air bearings with relative smoothness of Possible Possible motion Hydrostatic bearings with perfect Recommended Not recommended smoothness of motion Hydrostatic bearings with relative Possible Possible smoothness of motion Down to nanometer position stability equivalent Equivalent (but cheaper) Speed stability Equivalent (up to 0.1% at 1 Khz) Recommended (up to 0.1% at 1 Khz) Several motors on the same axis Possible Recommended Thermal dissipation kept at the lowest Not recommended Highly recommended level Tight dimensional constraint in the Possible Recommended overall length Tight dimensional constraint in the Recommended Possible overall height Very high acceleration (>10 g s) Possible Possible Very high speed (>10 m/s) Possible Possible Stroke <150 mm Possible Recommended (moving magnet) Vertical axis Possible Possible Very long stroke (>1000 mm) Possible Recommended Lowest possible price Not recommended Recommended Vaccum application recommended possible Clean room application Possible Possible No move for initialization at power on Possible Recommended Safety margin on continuous force Possible Recommended Ironless versus ironcore motor solutions HST-Etel Inc. 09/17/03 12