Linear Shaft Motors in Parallel Applications

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1 Linear Shaft Motors in Parallel Applications Nippon Pulse s Linear Shaft Motor (LSM) has been successfully used in parallel motor applications. Parallel applications are ones in which there are two or more Linear Shaft Motors in a parallel arrangement. The advantage that the Nippon Pulse LSMs have over other systems is the capability to run all parallel motors with only one amplifier and one encoder. Utilizing a single amplifier and encoder in the system has the stability and power of multiple motors while keeping down the costs of the servo loop. There are three main motivations to utilizing the Linear Shaft Motor in parallel. One of the first reasons, and the motivation for development of the parallel system, is for use in ultra-high precision, high accuracy applications that are required to operate in the sub-nanometer range. A second motivation is the use of multiple motors for the generation of forces greater than that achievable with a single motor. The third motivation is in the use of gantry or X-Y systems, where the parallel motors on one axis add significant stability. Nippon Pulse Linear Shaft Motors have successfully been used in applications with up to four motors in parallel. Parallel drive systems are most commonly thought of as being used in Cartesian/Gantry robots, though parallel drive systems also include other major areas of motion control. Application Groupings of Parallel Motors High and ultra-high precision High force Gantry robots Optics Material tensile testing Pick and place operations Microscope focusing Material punches Sheet goods cutters Sample scanning tables Die presses Automated MIG welding Machining equipment Semiconductor processing equipment Assembly operations Complex surface modification/engraving Incorporation of the Linear Shaft Motor into a parallel application is as simple as the design of the motor itself. The construction of the motor is simple in that it has only two components: a magnetic shaft and an electromagnetic coil forcer. With this motor design, several other design concerns can be eliminated or reduced in importance. One advantage of the Linear Shaft Motor over other non-contact linear motors is that the design of the LSM with the magnetic shaft in the center makes the air gap spacing non-critical. The coil completely surrounds the magnet and this symmetry does away with any force variations that would have been caused by air gap differences between the circumference of the shaft and the inside of the forcer. This can be caused by such factors as alignment or machining variances. Alignment and installation of the motor are very simple because of this non-critical air gap. Other types of non-tubular linear motor have a critical distance that is required to be able to generate the commanded force, as the force due to interacting magnetic fields is proportional to the distance between the magnetic track and the electromagnetic coils. In addition to the non-critical air gap, force reduction due to sine error is significantly reduced, due to the large magnetic pole distances in the Linear Shaft Motor. This feature allows for a larger tolerance in the motor s alignment, thereby decreasing in-the-field servicing time while increasing the capability of service capability. Aligning of the motor shafts can be easily done with a tape measure.

2 In designing a system that will utilize a parallel motor configuration, there are design criteria that should be followed to ensure smooth and accurate performance of the system. The areas that can cause a loss of accuracy are the parallelism of the motors, the alignment of the motor shafts, and induced sine error. The way that the LSM is constructed and the manner in which it operates helps to reduce these losses over other systems. Design criteria and the approaches to minimize error and force reduction are covered in the Parallel Application System Design Considerations section of this note. Examples of the Linear Shaft Motor in Parallel Applications Nippon Pulse s LSMs have been used in multiple types of parallel applications to meet customer application requirements and to solve problems encountered with other linear motion mechanisms. A common application is to use the motors in X-Y gantry systems. The advantage of using a parallel motor arrangement in an X-Y gantry system is the reduction of motor size and the ease of creating an orthogonal arrangement between the X and Y axes. It is easier to make a square than it is to make a right triangle through measuring the legs of that geometric shape. An X-Y gantry or Cartesian robot has been used for many types of applications. In providing motion on two perpendicular axes, the entire plane defined by these axes is available to be manipulated or inspected. Using a parallel motor in this application type eliminated torque due to a moment arm created by having a load suspended some distance from the motor. Moment arms can cause twisting or binding of the system. Some of the applications where the LSM has been used in a parallel arrangement are shown in images below. As can be seen, the Y or second axis is supported by the parallel arrangement of motors on the other axis.

3 Advantages Offered by the Linear Shaft Motor Linear Shaft Motors are highly responsive motors, and connecting them into a parallel system is easy. As with all parallel drive systems, the Linear Shaft Motors must be mechanically and rigidly coupled with a mechanism that, when applied, allows the axis to realize only 1 degree of freedom of movement. Since the dynamic motion generated by two identical Linear Shaft Motors is the same when they are given the same control signal, the asynchronous motion of the above-described parallel system is inevitable. This, in effect, makes it possible to operate the system with a single encoder and single servo driver. The Linear Shaft Motor is a non-contact system; when installed properly, it is impossible for it to introduce any mechanical binding into the system, due to the air gap. Parallel Application System Design Considerations The major issue with all parallel drive systems (e.g., gantries) is orthogonal alignment (the ability to keep the parallel axis square). In mechanically driven systems (screw drive, rack and pinion, belt and chain drive, to name a few) the main issue that arises is binding of the system due to misalignment or stacked up tolerances of the mechanical system. In direct drive systems there is the added issue of sine error that is introduced due to installation errors and variances in the linear motors themselves. To overcome these issues, the common practice is to drive and control each side of the parallel system and electronically sync them. However, the cost of such a system is higher since it requires twice the electronics (drivers and feedback, etc.) when compared to a single-axis system. This type of tracking control system can also add synchronization and tracking errors, which adversely affects the performance of the system. What is Sine Error? In a parallel motor application, a misalignment of the magnetic fields between the motors will cause a reduction in the force generated versus what is anticipated based on the current and voltage conditions supplied to the motors. The reason that this misalignment error exists is because the motors are rigidly connected. This constraint causes the moving part of the motors to go to the position that is being commanded, and due to the misalignment, each motor will be displaced from its magnetic equilibrium

4 position. This can be seen in how the magnetic field of the forcer coil and the magnetic field of the shaft motor interacts. Let s look at an approximation of the magnetic field in a coil. The forcers in a set of three coil windings are alternated, typically denoted U, V, and W. The coils are connected in series (denoted U-U, V- V, W-W ) but with the next coil in the series having the opposite winding direction. The last three coils at the end of the forcer are all connected together so that the current that is fed into the initial coil series has a return path through the other two coil sets. Return current is split between the other two coils, i.e. if U is the input coil, then V and W are the return path coils. This is done to create a duplicate magnetic pole spacing to that of the shaft being used. A schematic representation of the magnetic field distribution of a forcer is shown below. As one set of windings is energized and current flows, a magnetic field is created. Coils that are longer than the radius of the coil windings and that are closely spaced will have a magnetic field distribution similar to that of a bar magnet, with one end having a North field orientation and one a South field orientation. The field outside of these coils will be weak and divergent. This lets us know that that in the inside of the coil near the end we must reach a peak intensity of a North field and a South field while also transitioning from North to South within the coil. As the field transitions from North to South, it will have to pass through a region of zero field intensity within the coil. As there are three sets of windings in the forcer, passing current through one combination (i.e. U with V-W returning) is then followed by energizing another coil combination (V with U-W returning) that changes the field distribution within the coil. It is the changing field distribution and its interaction with the shaft magnetic field that produces the motion. The figure below shows an approximation of the magnetic field distribution within a forcer for each of the three sets of phase paths for the current input coils, U, V, or W. Forcer Coil Magnetic Field Intensity Distribution Schematic

5 The North and South fields of the forcer coils will interact with the North and South fields of the shaft. Since the magnetic fields from the shaft s permanent magnets cut the magnetic field of the forcer coil at 90, the force generated will be parallel to the shaft s axis. When current is applied, the coil will generate a magnetic field and the peak intensities of the coil will attempt to line up with the opposite component peak intensity such that the lowest energy state is obtained. The field distribution of the shaft is shown below. As can be seen, the magnetic intensity can be large and spatial very discrete. These magnetic fields emanate radially out from the shaft, producing maximal force for a given coil field intensity. The magnetic field distribution within the coil can be altered through exciting the coils in an infinite arrangement of percentages across the coils producing a field describe by the superposition of the three U-V/W, V-U/W, and W-U/V input coil and returning coil pairs. Motor Shaft Magnetic Field Distribution Schematic Energizing the motor coils produces a magnetic field distribution that moves the motor from its current to the desired position. When more than one motor is connected in parallel, introducing error becomes a possibility, due to misalignment of the motor shafts or the mechanically coupled forcers. This is illustrated in the figure below, where you can see the misalignment of the coil peak intensities between the two motor forcers. The servo loop will be attempting to move the magnetic peak intensity position within the forcer coils so that it will move to a specific position on the shaft. In so doing, it will create the same field intensity distribution profile in both forcers. The misalignment of the shafts or forcers will create a condition where neither coil will be able to occupy its equilibrium position of N-S interacting magnetic pole positions. The forcers will instead move to position in between the equilibrium positions of both forcers as a magnetic restoring force will be pulling each forcer in the opposite direction in an attempt to reach the individual equilibrium position. This restoring force is counter to the commanded force. The subtractive effect of the restoring force to the commanded force results in a reduction of the overall force generated by the system. This can also produce a position error, but positioning is typically compensated by the encoder and servo loop.

6 Sine error can be calculated by the following formula: Where: F dif Force difference between the two coils F gen Force generated Δx Length of misalignment MP n-n North-to-North magnetic pitch Illustration of the Cause of Force Sine Error F diff = F gen Sin (2π x MP N N ) Most linear motors are designed with a north-to-north magnetic pitch in the range of 25 to 60mm long with the intention of trying to reduce IR losses and the electrical time constant. So, for example, a misalignment of just 1mm in a linear motor with a 30mm N-N pitch will cause a loss of about 21 percent of its power. On the other hand, the Linear Shaft Motor working in parallel uses a much longer North-to-North magnetic pitch in order to reduce the effect of sine error due to accidental misalignment. Therefore, the same misalignment of 1mm in a Linear Shaft Motor with a 90mm N-N pitch will only result in a 7 percent loss of power. A comparison of the force/ power lost due to misalignment is graphically shown in the figure below.

7 Percent Force Reduction Versus Shaft Misalignment (ΔX) Positioning Error- Encoder and Motor Placement The most accurate positioning is when the feedback is directly in the center of mass of the work point, and ideally force generation from the motor will also act on the center of the mass of the work point. By putting an encoder in the center of mass and using parallel Linear Shaft Motors equally spaced off the center of mass it is possible to get the desired feedback and force generation at the center of mass. This is impossible for other types of parallel drive systems, which require two sets of encoders and servo drives to provide parallel functionality. It is possible to connect any number of Linear Shaft Motors, thus allowing their forces to be added together. Regardless of the number of parallel axes, you will only ever need one driver, one controller and one encoder. Design Considerations 1. Sizing of the amplifier Choosing the amplifier needs to take into account that the amplifier will be driving two or more motors, instead of the usual single motor in a typical application. In sizing the amplifier, care must be taken to ensure that the amplifier is capable of supplying the peak current of both motors during operation. 2. Physically connecting the motors All of the motors that are used in parallel need to be physically connected, with shafts connected together and forcers connected together. Connection of the motors needs to be done in such a manner as to leave only one degree-of-freedom in the motor movement axis. This ensures that when the control loop commands a positon or force, it generates the commanded quantity. When more than one degree of freedom is present, the force can be applied along the other possible free directions of movement.

8 3. Motor Installation and Orientation Both forcers must be oriented in the same direction on their shafts. It is suggested that the end of the forcer that has the serial number be pointing toward the end of the shaft that is marked with yellow paint. If the orientation of the coils isn t properly aligned, it s possible to have a totally inoperable or a runaway system, or it will cause significant loss of thrust. The standard for parallel drive system is mirrored cable exit locations. See drawing below. 4. Deviation in mounting the motors To minimize loss of thrust due to sine error, it is recommended that the mounting position difference between the Linear Shaft Motors [Δx] be less than the values shown in the table below.

9 Maximum Misalignment Permissible for Optimal Performance Model Magnetic Pole Pitch (N-N) Max. ΔX (mm) S S S S S S S S S S S S Motor spacing It is recommended that the minimum spacing (P) between the parallel shafts be maintained as shown in the table to the right. If the shafts are installed closer than what is shown in the table to the right, it is possible to warp the shafts due to magnetic interference. Minimum Spacing Between Motor Shafts Model Magnetic Pole Pitch (N-N) Spacing -P (mm) S S S S S S S S S S S S