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LD & LM SERIES Linear Motor Operator’s Manual P/N PCW-5050 Revision 1.1 12/98 This manual covers the following IDC Products: LM Series- Linear Servo Modules LD Series- Linear Servo Motors LM Series Linear Modules LD Series Linear Motors INDUSTRIAL DEVICES CORPORATION LD/LM Operator’s Manual Introduction CHAPTER 1: INTRODUCTION • • • • In This Section: Page Product Overview................................................................. 1-1 LD & LM Features............................................................... 1-1 Hardware Overview and Function ........................................ 1-2 Product Storage and Shipment ............................................. 1-4 This manual provides the basic information required to install and maintain the LD and LM range of tubular, linear servo motors, and modules from IDC. After a brief introduction to the products, the manual gives details of electrical and mechanical installation. The manual contains full technical drawings, specifications for the motor parts, and a quick guide for trouble shooting. The unique range of linear motors from IDC can be supplied in component part form (the LD), or as complete “ready to run” linear modules (the LM). This allows the system designer to have the flexibility to fit the motors into their system in the optimal arrangement. 1.1. PRODUCT OVERVIEW The LD/LM range of brushless linear servo motor components from IDC is designed to interface with IDC’s B8000 series servo control products, and to most commercial 3 phase brushless amplifiers. The LM module provides the user with a complete packaged linear motor system. The motor components of an LD consist of the thrust rod, and the thrust block. The thrust rod is comprised of a thin walled stainless steel tube housing high-energy permanent magnets. The thrust block is made from an epoxy filled aluminum housing, and contains cylindrical coils arranged in a 3-phase star pattern. Energizing the motor coils produces an electromagnetic field. By controlling the phase and magnitude of this magnetic field, the motor is able to produce precise values of force by its interaction with the permanent magnetic field of the thrust rod. For near perfect linear force along the whole length of travel, sinusoidal motor commutation is typically achieved via encoder feedback with IDC’s B8000 series drive products. Alternatively, a trapezoidal commutation option can also be provided to a 3rd party servo drive by utilizing the digital hall effect devices present within the motor. Pre-wired motor and encoder cables minimize installation errors, and greatly reduce installation time. 1.2. LD & LM FEATURES IDC’s linear motors have been designed to comply with the EMC and Machine Safety directives. However, proper installation is required. A single, high-flex robotic cable carries both motor power and hall-effect signals. This cable achieves noise immunity through double shielding. The Hall effect signal wires are separately shielded from the motor power leads. Options include protective bellows, and end of travel limit switches. A variety of encoder options can be selected depending on the user application and environment. 1-1 LD/LM Operator’s Manual 1.3. Hardware Overview and Function 1.3.1 The Motor Components Introduction The LD Series of linear motor components consist of the thrust block, thrust rod, and robotic motor cable. R obotic M otor Cable Thru st Block Th rust R od Thrust Block The aluminum thrust block contains a series of cylindrical coils wired in a 3 phase star configuration forming the stator of the motor. Connection to the motor coils is made via screw terminals. The screw terminals are located in the termination pocket mounted on the side of the motor, and are accessed through a watertight cover. The termination pocket also houses Hall Effect Devices (HEDs) which can be used for commutation, as well as the motor thermistor connections. Mechanical attachment of your load to the thrust block is typically achieved via T- slots on the top of the motor. The LD motors are typically mounted to linear bearing blocks via screw holes on the bottom of the motor. The central bore of the thrust block contains felt wipers at each end to minimize dirt ingress to the inner bore. The inner bore is coated with an insulating sleeve that provides electrical insulation between the motor coils and the stainless steel thrust rod. For moving block systems, a cable track can be fitted to the thrust block via standard brackets. Thrust Rod The thrust rod contains high-energy permanent magnet pieces within a stainless steel tube. The thrust rod should not be used as load bearing surface. Correct mounting of the thrust rod with respect to the central bore of the thrust block is essential for safe, reliable operation of the motor. Motor Cable The motor cable is a high flex robotic cable. The cable consists of two parts: an inner section, and outer section. The inner section contains the Hall Effect and motor thermistor signals, and is independently screened from the motor power conductors in the outer section. The outer section is also screened to minimize EMI radiation. 1-2 LD/LM Operator’s Manual Introduction 1.3.2 The LM Series Motor Module The LM series motor module is a complete, ready-to-run mechanical motion platform requiring only a servo amplifier/controller to achieve motion. In a LM series motor module, the thrust block is mounted on a precision linear bearing guidance system, which in turn is mounted to an extruded aluminum base. A high flex robotic cable is attached to the motor housing. A linear encoder is fitted within the extrusion base. The thrust rod is supported at either end by end supports fitted to the base extrusion. Cable management of the motor power cable, and encoder feedback cable, is achieved by a cable track. Optional end of travel limit switches can be fitted to ensure system protection. Profile Rail Bearing System Thrust Block Magnetic Thrust Rod (Not Load Bearing) M10 Thrust Rod Clamp Bolt End Support Bearing Lubrication Port Rubber Buffer Customer Adjustable Limit Switches (Optional) Encoder Cable (Ø5mm) System Clamps M4 Fixing Bolts on Cable Access Cover Energy Chain for Cable Routing (Larger Size Optional) Field Replacable S021 Twin Shielded Ø10mm Robotic Cable 1-3 LD/LM Operator’s Manual Introduction 1.3.3. Encoder Types IDC offers two distinct types of non-contact, incremental linear encoders for position feedback: optical and magnetic. These linear encoders are housed within the motor module to insure robust operation. Each encoder type has its own operating limits and performance characteristics. Selection of an appropriate encoder type for the application is an important issue, and should be discussed in detail with your regional sales and applications engineer before the system specification is finalized. The optical encoders offer high accuracy, and high resolution based on the reflected light principle. The scanning read head measures reflected light from a linear scale tape. Although designed to minimize errors, optical encoders are susceptible to contamination of the scale and sensor head, and hence are restricted to relatively clean operating environments. Optical encoders also tend to be more sensitive to EMI, and require more precautions to insure high noise immunity. Magnetic type linear encoders come standard with the LM series, and use a readhead sensor to measure the field of a magnetic strip. Magnetic encoders are suitable for systems that operate in harsh environments such as applications with dust, or high humidity. Magnetic linear encoders typically have coarser resolution, and lower accuracy. 1.4 STORAGE AND SHIPMENT IDC’s linear motors may be stored or shipped in environments within the following limits: Temperature………………………………….-40°C to +75°C Humidity…………………………………….up to 90% RH at 60°C Altitude………………………………………7600m IDC’s LM and LD series linear motors should be protected from temperature extremes that can cause condensation within the equipment. The original shipping containers and packing materials should be saved in case the product ever needs to be returned to the factory for repair. If the equipment is being returned, attach a label indicating return address, RMA number, and the model number. Additional Notes: 1-4 • Contact your distributor or IDC for an RMA number prior to shipment. • Use a strong shipping container (use the original shipping container if available). • Use anti-static packing materials only. • Ensure that the motor thrust block is not able to move during shipment. • Ensure the thrust rod is rigidly fixed and is at least 75mm from the side of the packing crate. For shipment of more than one thrust rod in the same container, ensure at least 100mm between rods, and insert rigid packing material to prevent the magnetic thrust rods attracting each other. • Seal shipping container securely. • Mark container FRAGILE to insure careful handling. • Mark container to indicate which side is UP to insure proper handling • Include detailed reason for return. LD/LM Operator's Manual Installation CHAPTER 2: ELECTRICAL INSTALLATION • • • • • • In This Section: Page Introduction ........................................................................2-1 Electrical Installation ..........................................................2-1 Motor Commutation............................................................2-3 Servo Integration ................................................................2-7 Electromagnetic Compatibility............................................2-21 Wiring Diagrams ................................................................2-22 2.1 INTRODUCTION This section covers the electrical installation of the LD series tubular linear motors, and the LM series linear modules. Topics covered are wiring, grounding, and shielding techniques. Motor commutation methods and servo integration are also discussed. Finally, advice on electromagnetic compatibility is given. 2.2 ELECTRICAL INSTALLATION All connections to the motor are made through the termination pocket mounted on the side of the motor. Access is gained by removing the four M4 screws holding the cover plate. High voltages can be present within the termination pocket. Ensure that all power is removed from the motor before the termination cover plate is removed. The cable enters the termination pocket via a watertight gland. The gland also contains a metal sleeve for bonding the outer cable screen to the thrust block. 2.2.1 Motor Power Motor power is applied via the screw terminals in the termination pocket marked U, V and W. For correct operation, the flying leads on the end of your motor cable should be connected as detailed in your servo amplifier instructions. These wiring connections may be indicated on your servo drive connector as R, S and T; or U, V, and W; or A, B and C; or simply 1, 2 and 3. When using the motor cable supplied by IDC, the following color coding applies when connecting an IDC supplied LD/LM motor cable to a B8000 series servo amplifier. Each phase has two conductors to achieve the correct power rating. The B8000 series drive connection R corresponds to the U connection in the motor termination pocket, S corresponds to V, and T corresponds to W. B8000 Series Function Color (Before 1/1/99) Color (After 1/1/99) R Brown + Brown/White Yellow S Red + Red/White Red T Blue + Blue/White Blue Motor Ground Green Green Temp N/A (Hall Cable) Black/White Temp Ref N/A (Hall Cable) Black 2-1 LD/LM Operator's Manual 2.2.2. Installation Safety Earth and Shielding The motor ground wire must be connected at both the servo amplifier’s earth ground terminal and the solder tag within the motor termination pocket. When using an IDC supplied motor cable, the motor ground is pre-connected to the solder tag within the motor termination pocket. The cable outer screen should be connected to earth ground at the servo amplifier. The screen is connected to the motor via a metal sleeve within the cable entry gland in the termination pocket. The cable inner screen should also be connected to ground at the servo amplifier. Always keep the connection between screens and the earth point as short as possible. For best results, use a heavy gauge, multi-strand earth strap. 2.2.3. Motor Over-Temperature Sensor The motor over-temperature sensor is embedded in the motor thrust block, and detects when the motor phases exceed their maximum operating temperature. The sensor is a special type of positive temperature coefficient (PTC) thermistor that exhibits a sharp rise in its resistance as the maximum operating temperature is reached. This increase in resistance can be detected and used to shut down , or disable the drive amplifier. The connections for the PTC are as shown in the table below. Typical resistance values for the PTC are 900 ohms at 20oC ambient, rising to greater than 3000 ohms at the motor phases’ maximum operating temperature. See Chapter 4, page 4-7 for more details. 2.2.4. Hall Effect Devices The Hall Effect Devices, and their associated circuitry, are mounted on a PCB located in the termination pocket. There are two types of Hall Effect Devices (HEDs) used on the LD and LM series depending on the commutation requirements of the servo amplifier. The two types of HEDs available are: Analog (-AH option) and Digital (which is standard). The pitch of the HED’s depends on the motor type. In addition, the polarity of the digital HED’s can be adjusted to suit your servo amplifier’s requirements. Contact your supplier if you require more detailed information. All external supply and signal connections are made via a 10-way, fine pitch, surface mount connector. The mating half of this connector is a Molex Part Number 51021-1000 (housing) requiring 9 off 50058-8100 (connection crimps suitable for 28-32 AWG wire). When using an IDC supplied cable the following color code applies: Pin number 1 2 3 4 5 6 7 8 9 10 2-2 Analog +Hall 1 -Hall 1 -Hall 2 +Hall 2 +V 0V -V Over temperature thermistor Over temperature thermistor No Connection Digital Hall 1 Hall 0 No Connection Hall 2 +V 0V No Connection Over temperature thermistor Over temperature thermistor No Connection Color Color (Before 1/1/99) (After 1/1/99) Blue Yellow White Violet Red Green Black Pink Grey Brown Blue Yellow White Violet Red Green Black N/A (Motor Cable) N/A (Motor Cable) Brown LD/LM Operator's Manual Installation A strain relief point for assisting with the cable routing is provided on the pocket side wall. Thrust Block Gland (& Scre en Termination Olive) Termination Pocket Earth Screw Commutation Board Connector Coil Connection Screw Terminals Motor Cable 2.3. MOTOR COMMUTATION Different servo amplifiers have different commutation arrangements. The LD and LM series linear motors have built-in flexibility to cater to most servo amplifiers. Analog Hall Effect Devices (the –AH option) can be used where commutation is achieved through analog multiplication of the current demand from the servo controller and the HED signal. Alternatively, digital HED’s can be used where trapezoidal commutation is required, or where sinusoidal commutation is achieved through encoder feedback, and the location of the thrust block relative to the magnetic fields of the thrust rod is read on power-up. If the servo amplifier you are using does not look at the hall signals on power up, your motor is likely to jump when power is applied to the system as the commutation sequence is typically initiated by energizing one of the motor phases. IDC’s B8000 series servo drives utilize sinusoidal motor commutation for optimum smoothness, and look at the digital halls to initialize the commutation sequence (which means the motor will not jump on power up). 2.3.1 Analog Hall Effect Devices Analog Hall Effect Devices (HEDs) provide commutation by sensing the field of the thrust rod directly. The analogue HED’s are at a different pitch for the LD38 and LD25 series of motors. The relationship between the motors’ back EMF, and the HED signals are shown in the diagrams on the next page (2-4) for a motor moving in the positive direction. It should be noted that these diagrams are correct for one motor direction only, and that when the motor direction is reversed, the back EMF from each motor phase is inverted in relation to its respective HED signal. It should also be noted that the motor’s Back EMF phase sequence is reversed: i.e. instead of phase W leading phase V, and phase V leading phase U as shown in the diagrams, phase U would lead phase V, and phase V would lead phase W. 2-3 LD/LM Operator's Manual Installation 15 Volts 10 5 Back EMF phase U to star point 0 + HALL 1 -5 -10 -15 0 120 240 360 Degrees 15 Volts 10 5 Back EMF phase V to star point 0 + HALL 2 -5 -10 -15 0 120 240 360 Degrees 15 10 Volts 5 0 Back EMF phase W to star point -5 -10 -15 0 120 240 Degrees 2-4 360 LD/LM Operator's Manual 2.3.2 Installation Analog Hall Effect Devices Set Up When using Analog Hall Effect Devices (HEDs), it is essential for smooth “cog free” motion of the motor that that the analog HEDs are adjusted correctly. The analog HED signals are factory set, and will not normally require adjustment. If, however, the signals do need adjustment the following equipment is required: • A 2 channel Oscilloscope and X1 probes • Potentiometer trimming tool • M4 Allen key • Servo Amplifier that is compatible with the analog halls The adjustment procedure is as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Undo the four M4 fixing screws using the Allen key, and remove the pocket cover plate on the motor block. Remove the thrust rod as described in Chapter 3. Connect channel A oscilloscope probe to pin 1 of the 10-way Molex connector (see section 2.2.4 for pinout). Connect channel B oscilloscope probe to pin 4 of the 10-way Molex connector. Connect the oscilloscope ground to the motor block (green wire on solder tag). Zero the oscilloscope traces to the center of the display. Plug the motor connector into the servo amplifier. Using the potentiometer trimming tool, turn RV1 and RV3 fully clockwise. Set the oscilloscope to a sensitivity of 20mV per division for both channels, and to a time base of 1ms. Apply power to the servo amplifier. Using the potentiometer trimming tool, adjust RV2 for 0V output on channel A of the oscilloscope. Using the potentiometer trimming tool, adjust RV4 for 0V output on channel B of the oscilloscope. Remove power from the servo amplifier. Replace the thrust rod as described in Chapter 3. Set the oscilloscope to a sensitivity of 2V per division for both channels, and a time base of 20ms. Apply power to the servo amplifier. While moving the motor block back and forth by hand, using the potentiometer trimming tool adjust RV1 for 10V peak to peak output on channel A of the oscilloscope. While moving the motor block back and forth by hand, using the potentiometer trimming tool, adjust RV3 for 10V peak to peak output on channel B of the oscilloscope. Remove power from the servo amplifier. Replace the pocket cover-plate, and secure with the four M4 fixing screws. If it is difficult, or undesirable, to remove the thrust rod, it is possible to carry out the set up procedure with the thrust rod installed. 2-5 LD/LM Operator's Manual 2.3.3 Installation Digital Hall Effect Devices The Digital Hall Effect Devices (HEDs) are at a different pitch for the LD38 and LD25 series of motors, and are in different locations in the motors relative to the position used for the analogue HEDs. The polarity of the outputs can be changed to suit the servo amplifier’s requirements. The relationships between motor Back EMF and HED signals are shown in the diagrams below. It should be noted that these diagrams are correct for one motor direction only, and that when the direction is reversed, the Back EMF from each motor phase is inverted in relation to its respective HED signal. It should also be noted that the Back EMF sequence is reversed: i.e. instead of phase V-U leading phase U-W leading phase W-V as shown in the diagrams, phase W-V would lead phase U-W which would lead phase V-U. Volts Back EMF phase V to phase U HALL 0 0 120 240 360 Degrees Volts Back EMF phase W to phase V HALL 2 0 120 240 360 Degrees Volts Back EMF phase U to phase W HALL 1 0 120 240 Degrees 2-6 360 LD/LM Operator's Manual Installation 2.3.4 Digital Hall Effect Devices Set Up There is no set up required when using digital HED’s. 2.3.5 Encoder When sinusoidal encoder commutation is used, the electrical cycle of the motor is a required setting within the amplifier. The electrical cycle is normally defined in terms of encoder counts per pole pair (the distance between consecutive like poles). For example, for a LM25 (where the pole pair pitch is 51.2 mm) with a 10 micron encoder, the number of encoder counts per pole pair is 51.2/0.01=5120. Similarly, for the LM38 (pole pair pitch =71.2 mm) this value would be 7120. 2.4 SERVO INTEGRATION This section describes the integration of the motor components or module to a servo control system. Topics covered include encoders types, servo tuning a 3rd party servo drive, tuning an IDC B8000 series amplifier for a linear motor, and error checking. 2.4.1 Encoder Types Suitable for use with Linear Motors One of the advantages of linear motors is that there is no inherent backlash in the motor. It is therefore possible to produce systems that can be moved to the same position from either direction without errors due to mechanical backlash. It is always desirable to use encoder systems that do not suffer from backlash (i.e. the use of rotary encoders with conversion systems is not advisable). Basically, any type of system that can produce a measurable signal based upon distance moved can be used. The actual choice is often dependant on a number of variables, such as repeatability required, operating environment, and signal type. The most commonly used linear encoders available consist of an encoded strip (attached to a parallel surface to the motor), and a sensor read head mounted to the moving part (motor). These are normally either optical, magnetic, or inductance based systems. If your encoder is supplied by IDC, it is either magnetic or optical, and is contained within the motor module extrusion. For very high accuracy systems it is also possible to use a laser interferometer. It is always important to ensure that the encoder type selected is compatible with the controller that you are intending to use. It is also imperative that you insure the controller is capable of counting the frequency of encoder pulses produced at your application’s maximum speed. The higher the resolution of the encoder the higher the frequency of the pulses that are produced at any given speed. The higher the resolution of the encoder, the easier it also is to maintain a particular repeatability, and reduce settling time. Please Note: Since the linear motor is capable of very high speeds, the frequency of encoder pulses can be higher than might normally be expected from a conventional rotary to linear motion system. A table with the IDC’s LM series linear encoder specs is shown on the next page. 2-7 LD/LM Operator's Manual Installation IDC Linear Encoder Specs (Standard Cataloged Encoders)1 Encoder Resolution 10 micron magnetic* 5 micron optical 1 micron optical 0.5 micron optical 1 Minimum Edge Seperation 1 µs 600 ns 500 ns 500 ns Minimum Pulse Width 2 µs 1.2 µs 1 µs 1 µs Z Channel Pulse Distance 5 mm 50 mm 50 mm 50 mm Max Speed with IDC B8000 controls 5 m/sec (196 in/sec) 5 m/sec (196 in/sec) 0.63 m/sec (25 in/sec) 0.31 m/sec (12.5 in/sec) High Speed, high clock frequency encoders are also available from IDC as specials. Please contact IDC Applications Engineering if you have questions, or require information on these encoders. *Please Note: IDC 10 micron magnetic encoders utilize a ‘burst mode’ to send encoder positional information in pulse packets to the servo controller for speeds greater than 0.1 m/sec (4 in/sec). The pulse packets are sent out every 100 µs. While in burst mode, the minimum edge seperation and minimum pulse width specs given in the table above are maintained. Encoder Error Signal IDC recommends the use of the magnetic, or optical encoder, Error Signal when using IDC’s LM systems with a 3rd party servo drive utilizing hall commutation. Using the encoder’s error signal will allow the servo controller to detect when the system is missing pulses (drifting), or when the encoder signal is lost. Many servo drives using hall commutation may try to apply full power to the motor when the encoder signal is lost, which will cause a highly undesirable system hard stop. To prevent this, the servo drive should be disabled by the servo controller, or commanded to stop in a controlled manner when the encoder signal is lost. Error Signal Characteristics Encoder Type 10 micron Magnetic Encoder Optical Encoders Wire Color Black Purple Encoder Feedback OK 5 VDC (High) 5 VDC (High) Encoder Error (Faulted) 0 VDC (Low) 0 VDC (Low) In general, encoder errors are normally due to either : • • • • • Incorrect sensor read head alignment with the encoder scale Incorrect gap between sensor read head and the encoder scale Damaged or dirty encoder scale, particularly optical scales Damaged signal wires Noise on the encoder signals Complete technical specifications on the encoders used with the LM series are supplied with the system (see the encoder manual). 2-8 LD/LM Operator's Manual Installation Optical Encoders and EMI Linear motors inherently emit electrical noise. This noise can be picked up on the encoder feedback. If not filtered properly, this noise will result in cumulative errors in the position feedback (e.g., drifting). In order to reduce the susceptibility of optical encoders to EMI, IDC recommends the following procedures be performed: • Attach the motor shield wire to the control’s chassis earth ground. For the B8001, B8961 and B8962 the chassis earth ground is the double spade terminal located next to the motor cable input on the control. • Connect the factory installed ground wire on the linear motor frame to the control chassis earth ground. 2.4.2 Beware of Magnetic Encoders and Thrust Rods Magnetic encoder strips can be affected by the high magnetic fields produced by the thrust rod. It is possible for the magnetic field of the rod to interfere with the field of the strip, or affect the read head directly; it is therefore necessary to ensure that there is sufficient distance between the components to ensure that this does not occur. If the rod and strip come into contact, or are in very close proximity with one another, then the magnetic profile in the strip will be permanently damaged. 2.4.3 Encoder Commutated Systems An increasing number of amplifiers and drives are using the feedback from the encoder to commutate the motors, as well as provide feedback for position and velocity control. For optimal control, the servo amplifier should provide sinusoidal commutation (see section 2.3.5). 2.4.4 Encoder Count Direction The direction of count of a two channel (Quadrature decoded) incremental encoder is defined such that a signal denoted as channel A should lead channel B when the motor is moving in the forward direction (as specified in section 2.3.1 and 2.3.3). It is sometimes not possible to mount the encoder systems so that the counts will conform to this convention. Under these circumstances, it becomes necessary to reverse the direction of count as seen by the controller. There are two possible methods of reversing the direction of the count from an incremental encoder which are described below. • If a channel is inverted (i.e. A wired to A- and vice versa) then the signal from channel A will then lag behind channel B. This will cause the controller to reverse the count as perceived from the encoder. • If the signals from channel A and channel B are swapped completely with one another (i.e. A+ wired to B+, A- wired to B-, and vice versa), this will result in channel B leading channel A, and reverses the count. 2.4.5 Servo Amplifier Types and Control Considerations In order to control the position of the motor, it is necessary to employ a servo controller and amplifier combination. There are many different makes and models of amplifiers available, but they tend to fall into one of three possible categories: 1) Intelligent amplifiers that have built in servo controllers 2) Velocity amplifiers capable of controlling only the velocity of the motor 3) Current/Torque amplifiers that control only the force of a linear motor (torque in a rotary motor) 2-9 LD/LM Operator's Manual Installation Intelligent amplifiers do not require external control signals in order to position the motor. Depending on the unit, they can perform very simple point to point moves up to very sophisticated moves with external synchronization and I/O handling. Generally, they can operate in either position/velocity, or force control modes. Velocity amplifiers are used to move the motor at a velocity determined by an analog command. The unit requires an external servo controller to determine the move profiles. In addition, some are available where the command can be input to the drive through a serial link. Units of this nature can sometimes be given a position set point that can be used to move the motor to a defined position. The motor will move towards the required position at a predefined velocity and acceleration. Encoder feedback is required to calculate the motor’s velocity. The advantages of using such a system is that the processing by the main controller is reduced, and the update time within the amp for the velocity loop can usually be much higher than the servo controller. Current/Torque amplifiers produce a force proportional to the command signal. The speed with which the motor will move is therefore controlled entirely by the external servo controller. The most common type of programmable digital servo controller used with current amplifiers employs a PIDF (see next section) algorithm to control the position of the motor. 2.4.6 3rd Party PIDF Servo Controllers PIDF controllers use the error between the desired position of the motor and its current position to control the force that the motor will produce. PID refers to proportional, integral and derivative terms applied to this error (referred to as the following error) that are used in this type of control system. Many of these controllers will also have feed-forward terms (F) to help reduce the response times of the system. In order for the controller to move the system to the desired position it is necessary to set values to these terms. The process of selecting the value to which these parameters should be set is called tuning. In order to tune a system it is necessary to understand the effect of each of the terms. Refer to your servo tuning guide for detailed information. Proportional gain. The proportional gain in a system causes the motor to produce a force directly proportional to the following error. So, the further away from the desired position the motor , the greater the following error, and the greater the amount of correcting force produced. As this value is increased the position error is reduced. It is possible to use too large a value of proportional gain, as the system can become unstable. This parameter also provides stiffness when in position. Derivative/Velocity feedback gain. One method of stabilizing a system requiring a high proportional gain is to introduce a velocity feedback factor into the loop. This parameter reduces the force that is available to the motor as the speed of the motor increases. Although this allows higher gains to be used, there is still a limit to the maximum value, as the system will still become unstable if very large values of velocity feedback are used. Integral gain. When the above two terms have been set there may still be an unacceptable following error in the system. This integral term is combined with the following error in a continuously incrementing accumulation to produce a force to drive the motor. Because of the time dependency of this term, it tends to have a much slower response rate when compared to the above two terms. For most systems, a quick response is required, and so a high value for this gain is tried. Unfortunately, even at fairly low values this term can cause the system to become unstable. For linear systems this term is generally very small, or set to zero. 2-10 LD/LM Operator's Manual Installation Feed-forward gains. There are several different types of feed-forward gains that can be available, depending on the controller type. Velocity, acceleration, deceleration and friction feed-forward compensation are a few of the more common ones. During a move, feed-forward terms allow the controller to produce a force based upon the commanded move rather than on the following error. An example would be to consider the acceleration feed-forward term. Using Newton’s law of motion, F = MA, it is possible to assume that if an acceleration is required, then a certain current needs to flow in the motor windings (force is directly proportional to current). An acceleration feed-forward term would produce a command signal that could be expected to achieve this acceleration. This does, however, mean that the feed-forward terms are open-loop in nature. Just as with all the other gains, if any feed-forward terms are too large the system will be unstable. In general, the feed-forward terms are used to minimize following errors and improve system response time. Unfortunately, there is no universal method of tuning, or predetermined gain values, that can be used on all servo controllers available commercially. Each servo controller has its own control algorithms and scaling. Before attempting any tuning of your system, read the servo controller manual, and understand what each tuning term is used for. If possible, use an oscilloscope or tuning software to assist you. When using an IDC B8000 series servo drive, default gain values are preset for each motor (the motor type is selected from a menu), and serve as a good starting point for tuning in your application. If you are using a non-IDC servo drive or controller , do the following steps: 2 Remove any sensitive equipment mounted to the linear motor, and fit a dummy payload if at all possible. Fit soft sponges, or dampers at the ends of travel of the linear motor in case of runaway conditions. Runaway conditions are fairly common during initial set-up of the system. 3 Set the RMS continuous current limit of your amplifier to a low value to prevent damage to the motor by over-current. 4 Set all gain terms to zero. 5 Enable the amplifier. 6 Increase the proportional gain by small increments until the motor just starts to become unstable when pushed out of position and released. 7 Now increase the derivative (velocity feedback) gain until the motor is completely stable when pushed out of position, and the motor does not overshoot when returning to position. 8 Now increase the proportional again, while increasing the derivative gain to maintain stability and minimize overshoot, until you have either reached required positioning accuracy, or the system is unstable. If the system is unstable reduce the gain values appropriately. 9 Do some short moves to check overshoot at higher velocities, and increase the derivative term (or reduce proportional gain) if necessary. These should be recorded as your starting gain values. 10 You can now tune the system more accurately for your particular application, including the integral and feed-forward terms as required. It may be necessary to reduce the proportional and derivative starting gains to achieve the final desired motion profile. 2-11 LD/LM Operator's Manual 2.4.7 Installation IDC B8000 Series Tuning Every application has different requirements. Some applications demand absolute smoothness, but can trade off settling time and disturbance rejection. Others need minimum settling time, but can trade off some end of move oscillations provided that they are under a certain limit. For contouring applications, tracking error below a certain limit over the entire path may be the key parameter. The possible permutations of response requirements are infinite. In general, the tuning procedures described below lead to a good compromise response, however, you may want to fine tune the gains to your needs. In order to do this you need to use your indexer/ motion controller as the test command signal, and not the internal tuning stimulus in the drive which only tests the step response. Although step response tests can be illuminating, they are not representative of your real-world demands (which are almost always less stressful than step response tests). For this reason there is usually a good amount of latitude to both increase and decrease the gains from the values found in the procedures below. Some examples: Increasing the gains makes the system "stiffer" and you might elect to do this if you are designing a machine for a contouring application where you expect only small, low frequency disturbances that will not excite an otherwise "hot" (under damped) axis. Conversely, reducing gains makes the system motion smoother and you might do this for an image scanning machine, but this would reduce the disturbance rejection and point to point settling time. Tuning the Velocity Loop Although unlikely, this procedure may overheat and damage a motor if a large inertial load is being driven. If this is the case, keep the test short, and closely monitor the motor’s temperature. It is recommended that your system be fitted with rubber bumpers or soft end stops prior to beginning this tuning procedure. 1. Open the Performance Tuning window in Servo Tuner, and depress the 1-Kv Tune button in the Setup section. This will zero Kv, Kp and Ki gains, and set up the monitor port and the tuning stimulus. If a 25 series linear motor is used, the axis should be free to move ± 275 mm without hitting an obstruction in order to run this test; if a 38 series linear motor is used, the axis should be free to move +/- 375 mm without hitting an obstruction in order to run this test. [The monitor port will be set to Velocity Error with a range of ±1,000 RPM (250 RPM/division) and the tuning stimulus will be set to inject a ±750 RPM command signal into the velocity loop with a period if 500 ms.] 2. Adjust the Horizontal Time Base on your oscilloscope to 50 ms/division. 3. Enable the drive. Start the tuning stimulus by clicking on the On/Off button. The motor should not move at this point. The indicator above the On/Off button should come on and stay on (please see the diagram on the next page). If it does not stay on, the drive is not enabled, enable the drive and try again. Your oscilloscope should display a single cycle of the square wave stimulus with an amplitude of ±3 divisions. 2-12 LD/LM Operator's Manual Installation 4. Begin to increase Kv: the motor should start to move. Keep increasing Kv until you notice overshoot on your oscilloscope as shown in the figure below. The field entry accelerator will allow you to quickly adjust this value in steps of 1, 10, 100 or 1000. Values for Kv usually fall between 2,000 and 500,000. So you should start around 500 and increase geometrically, i.e. 1,000, 2,000, 4,000, etc. until you bracket the correct response. Then go back and iterate in smaller linear steps until you see 2% to 5% overshoot. If, during this procedure, a drive shutdown occurs due to an Output-Time alarm the motor may have been about to overheat. If this occurs, the toggle period could be increased (this will require that the motor to move farther during this procedure). Be sure to check that your system will be able to move the distance you are commanding, before initializing the new tuning period! Distance commanded will equal the velocity amplitude * ½ the toggle period. 5. Begin to increase Kfv, the velocity Feedforward gain, until the "steady state" following error (just before the next toggle transition) is at a minimum. You may want to reduce the range of the monitor port to ±200 RPM to get a better view of this. Values for Kfv usually fall between 200 and 10,000. So you should start around 100 and increase geometrically, i.e. 200, 400, 800, etc. until you bracket the correct response. Then go back and iterate in smaller linear steps until you see minimum steady state following error as shown in the diagram above. Don't be surprised if your system requires little of no Kfv gain, this is often the case if the friction in your system is very low. 2-13 LD/LM Operator's Manual Installation 6. Stop the test by clicking on the On/Off button in the Toggle Generator section of the Performance Tuning window in Servo Tuner. Because you are viewing velocity error and not actual velocity, you will see response spikes and not square waves on your oscilloscope after you are done with this procedure. This is because you are viewing the difference between the square wave stimulus and the response of your drive as it attempts to follow the abrupt changes in commanded velocity. With the Kv gain at zero, the motor will not move and you should see the step (square wave) stimulus on your oscilloscope. As you increase the Kv gain, the square wave will change shape as shown above, at some point only spikes will remain. It is the "overshoot" of these spikes that you are trying to adjust. Tuning for a small overshoot usually works quite well. Sometimes, however, the system will exhibit extended ringing even at this low level of overshoot. If you detect more than a few cycles of ringing you should scale back the Kv gain until this is corrected. Tuning the Position Loop Although unlikely, this procedure may overheat and damage a motor if a large inertial load is being driven. If this is the case, keep the test short, and closely monitor the motor’s temperature. It is recommended that your system be fitted with rubber bumpers or soft end stops prior to beginning this tuning procedure. 1. Disable the drive, and move the linear motor to its mechanical center. 2. Click on the 2-Kp Tune button. This will zero out the Kp and Ki gains, set up the monitor and tuning stimulus. The monitor port will be set to tracking error with a range of ±40 degrees (10 degrees/division), and the tuning stimulus will be set to inject a ±30 degree command signal into the position loop with a period if 500 ms. 3. Enable the drive. 4. Start the Tuning stimulus by clicking on the On/Off button. The motor should not move at this point. The indicator above the On/Off button should come on and stay on. If it does not stay on, the drive is not enabled. Enable the drive and try again. 5. Begin to increase Kp: the motor should start to move. Keep increasing Kp until you notice overshoot on your oscilloscope as shown in the figure on page 2-15. 2-14 LD/LM Operator's Manual Installation Values for Kp usually fall between 1,000 and 20,000. You should start around 500 and increase geometrically, i.e. 500, 1000, 2000, etc., until you bracket the correct response. Then go back and iterate in smaller linear steps until the desired response is achieved. Adjust Kp for approximately 5% overshoot. As you increase Kp, the linear motor may react quite violently. If you think that your mechanism may be damaged, reduce the excitation amplitude. If you adjust Kp too high, the linear motor may oscillate on its own even when the toggle reference is turned off. If this occurs, be ready to do one of the following: reduce Kv immediately to zero, disconnect the enable line, or remove power from the system. If during this procedure a shutdown occurs due to an Output-Time alarm the linear motor may have been about to overheat. If this occurs the toggle period should be increased (this will require that the linear motor move farther during this procedure). If a shutdown occurs due to a tracking error it is probably because the Kp gain was set too high, and very large overshoot and/or ringing occurred. If this occurs reduce Kp. In either case make the appropriate adjustment, disable and enable the drive, start the tuning stimulus and continue tuning. 6. Stop the test by clicking on the On/Off button in the Toggle Generator section of the Performance Tuning window. 2-15
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