Technical Application Notes

Transcription

1 Technical Application Notes Applying the Autotune Feature to SERCOS Drives Applications Purpose of this Document: This document serves as a guide to (a) manually adjusting system gains after a successful, albeit, sub-optimal auto-tune, or (b) manually tuning an axis that cannot be auto-tuned when coupled to the load. The following are general recommendations. Since each machine application is unique, special consideration should be made on a per machine basis. Application Description: SERCOS configuration using RS Logix 5000 has been simplified, because when setting up a program, we know drive and motor parameters. Armed with that information a default out of the box set of gains can be established for the combination of drive amplifier, and motor selected. These gains assume a 1:1 inertia match, and add no integral gain into the system, providing a stable set of gains for commissioning a system. These gains, while adequate, may not provide the positioning accuracy required for the application. What is not known about the application is the inertia of the mechanical system connected to the servo shaft, and the stiffness of any couplings connected to the shaft. Nor do we know the performance requirements of the system. These are the big questions that must be answered to set up a stable system that performs as required. It would be an atypical system where the user has calculated all of the mechanical torques of the system (and calculated them precisely). But most mechanical designers know their positioning accuracy (requirements) and general system inertia. Using this information, we will demonstrate how auto tuning takes these values into consideration and how we can use the auto tuned values to serve as a baseline tune or possibly used as the final values. Most systems can be auto-tuned with the load attached. However, some applications require manual adjustment of multiple parameters to achieve optimal performance while other applications cannot be autotuned with the load attached. Control System Description the following block diagram depicts the control hardware used in this application, and shows the interaction between the CLX controller and the servo (loop).

2 Logix Controller Motion Planner ControlBus Coarse Update: 1-32 msec 1756M08SE 2nd-order Fine Interpolation SERCOS SERCOS Update: 1-2 msec SERCOS Drive Servo Loop Kinetix Drive Servo Update: 250 usec Control challenges and decisions that must be made before proceeding. Tight follower (Gearing/Camming) Smooth motion Zero Overshoot. (Point to Point ing) After a successful auto-tune procedure is run, you must decide which performance is desirable: (a) decreasing position error or (b) one that moves smartly to a position without jerking the load. If you decide in favor of low position error, then your system is subject to fast Maximum Acceleration and Deceleration, and will posses high jerk rates. If you opt for smoother moves, then you have lower Maximum Acceleration and Deceleration values and less jerk, but increased position error. Just as several new model cars permit the driver to modify transmission shift points to run in Sport Mode for optimal driving conditions, Winter Mode for harsh conditions, or Normal Mode for balanced operation, you should strive for a balance between all-out performance and smoothness in tuning your system. Tuning a digital system (Sercos) is a little different than an analog system ( M02AE) because the servo loops are closed at the drive with Sercos. On the Analog module(m02ae) the loop is closed at the M02AE module and a +-10v analog signal is generated at the M02AE module. Also with sercos there are a number of loop types that are available. The M02AE supports servo drive interfacing with a torque servo drive, in which case both the velocity and position loops are closed within the M02AE. When interfacing with a velocity servo drive, only the position loop is closed in the M02AE. The velocity loop is closed in the drive With SERCOS you have 7 different loop types to consider. Yikes! Let s explore each one and determine which is the right choice for your application. 2

3 Figure 2: Less positioning accuracy, but smoother motion. The red pen in the trend is, notice it has much greater magnitude if smoother motion is required. Control Solution. Servo is the most common and the default configuration. This is similar to the servo drive configuration for the M02AE module. Unlike the M02AE, the M08SE does none of the fine interpolation. It is essentially a communications card to the drive, with the drive performing the fine interpolation. If you look at the diagram below, you can see that the motion group and coarse update performs the same as always, i.e. the Processor does the coarse loop calculations and then sends that information out at a regular schedule, which the Coarse Update Rate. The amount of time varies by the processor type, number of axes, and the types of moves each axis is performing, because it takes time to calculate the move and update the position and velocity command for each axis, then send it out as a command to the M08SE modules. The next layer is the M08SE to the drive SERCOS Interface. This is either 1ms or 2ms based on axis count (1ms <=4 axes). This is also referred to as the Ring Rate. The drive also sends back axis and drive information each ring cycle. As seen in this diagram you will have 8 ring cycles per coarse update on the M08SE 1 network, and 4 ring cycles on the M08SE 2 network, with this information being less than 1ms old when the coarse update comes through to pick up the new data. Servo The default loop configuration, smoothest and most stable because it uses motor mounted feedback for position and velocity feedback. iliary Servo (Not applicable to Ultra 3000 drives) Tight position control, uses auxiliary(load) mounted feedback for position and velocity feedback. Could prove troublesome if mechanics are not tight. Dual Servo (Not applicable to Ultra 3000 drives) Tight position control, uses the auxiliary (load) mounted feedback device for position feedback and the motor mounted feedback device to provide velocity feedback. Could prove troublesome if mechanics are not tight. 3

4 Logix 5563 Processo MOTION GROUP Coarse Update 4ms12 Axes M08SE 1 M16SE SERCOS RING 1 1 MS UPDATE 4 Axes SERCOS RING 2 2 MS UPDATE 8 Axes usec. Servo Update 250usec. Servo Update 250usec. Servo Update 250usec. Servo Update Dual Servo - Provides full position servo control using only the motor-mounted feedback device to provide position and velocity feedback. Unlike the Servo configuration, both command position and command velocity are applied to the loop. iliary Dual Servo - Uses only the auxiliary mounted feedback device to provide position and velocity feedback. Unlike the Servo configuration, both command position and command velocity are applied to the loop Servo Pure velocity control. Provides velocity servo control using the motor-mounted feedback device. Servo Pure torque control. Provides torque servo control using only the motor-mounted feedback device for commutation. This does not consider position at all, and can be adjusted with the value. 4

5 Servo The Servo configuration provides full position servo control using only the motor mounted feedback device to provide position and velocity feedback. This servo configuration is a good choice in applications where smoothness and stability are more important that positioning accuracy. ing accuracy is limited due to the fact that the controller has no way of compensating for nonlinearity in the mechanics external to the motor. Note that the motor mounted feedback device also provides motor position information necessary for commutation. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The value is derived directly from the output of the motion planner, while the and values are derived from the current value of the corresponding attributes. These offset attributes may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Servo d 2 /dt Acc d/dt Vel Pos/Neg Fine Interpolator Pos P Accel Vel P Scaling Frict. Comp Amplifier Pos I Vel I Polarity ulator 5

6 iliary Servo The iliary Servo configuration provides full position servo control using an auxiliary (i.e., external to the motor) feedback device to provide position and velocity feedback. This servo configuration is a good choice in applications where positioning accuracy is important. The smoothness and stability may be limited, however, due to the mechanical non-linearity external to the motor. Note, that the motor mounted feedback device is still required to provide motor position information necessary for commutation. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The value is derived directly from the output of the motion planner, while the and values are derived from the current value of the corresponding attributes. These offset attributes may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Servo d 2 /dt Acc d/dt Vel Pos/Neg Fine Interpolator Pos P Vel P Accel Scaling Frict. Comp Amplifier Pos I Vel I Polarity ulator 6

7 Dual Servo This configuration provides full position servo control using the auxiliary feedback device for position feedback and the motor mounted feedback device to provide velocity feedback. This servo configuration combines the advantages of accurate positioning associated with the auxiliary position servo with the smoothness and stability of the motor position servo configuration. Note that the motor mounted feedback device also provides motor position information necessary for commutation. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The value is derived directly from the output of the motion planner, while the and values are derived from the current value of the corresponding attributes. These offset attributes may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Dual Velocit d 2 /dt Acc d/dt Vel P Pos/Ne Positio Comman d(coarse ) Fine Interpolato Positio Comman d Positio k Velocit Comman Positio d Pos P Accu Pos I Positio Integrato Velocit k Accu Velocit Vel I Velocit Integrato Accel Comman d Vel P Scaling Frict. Comp kpolarit Comman Amplifie Hardwar kpositio kchanne k Positio k(coarse Positio ulator Hardwar kpositio kchanne l 7

8 Dual Servo The Dual Servo configuration provides full position servo control using only the motor mounted feedback device to provide position and velocity feedback. Unlike the Servo configuration, however, both command position and command velocity are applied to the loop to provide smoother feedforward behavior. This servo configuration is a good choice in applications where smoothness and stability are important. ing accuracy is limited due to the fact that the controller has no way of compensating for non-linearity in the mechanics external to the motor. Note that the motor mounted feedback device also provides motor position information necessary for commutation. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The and values are derived directly from the output of the motion planner, while the value is derived from the current value of the corresponding attributes. The velocity offset attribute may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Dual d/dt Acc Fine Interpolator Vel Pos/Neg Fine Interpolator Pos P Vel P Accel Scaling Frict. Comp Amplifier Pos I Vel I Polarity ulator 8

9 iliary Dual Servo The iliary Dual Servo configuration provides full position servo control using only the auxiliary mounted feedback device to provide position and velocity feedback. Unlike the Servo configuration, however, both command position and command velocity are applied to the loop to provide smoother feedforward behavior. This servo configuration is a good choice in applications where smoothness and stability are important as well as positioning accuracy. Note, that the motor mounted feedback device is still required to provide motor position information necessary for commutation. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The and values are derived directly from the output of the motion planner, while the value is derived from the current value of the corresponding attributes. The velocity offset attribute may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Dual d/dt Acc Fine Interpolator Vel Pos/Neg Fine Interpolator Pos P Vel P Accel Scaling Frict. Comp Amplifier Pos I Vel I Polarity ulator 9

10 Servo The Servo configuration provides velocity servo control using the motor mounted feedback device. Synchronous input data to the servo loop includes,, and. These values are updated at the coarse update rate of the associated motion group. The value is derived directly from the output of the motion planner, while the and values are derived from the current value of the corresponding attributes. These offset attributes may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Servo d/dt Acc Pos/Neg Vel Pos/Neg Accel Pos/Neg Fine Interpolator Vel Accel Accel Vel P Scaling Frict. Comp Amplifier Vel I Polarity ulator Servo The Servo configuration provides torque servo control using only the motor mounted feedback device for commutation. Synchronous input data to the servo loop includes only the. These values are updated at the coarse update rate of the associated motion group. The value is derived from the current value of the corresponding attribute. This offset attribute may be changed programmatically via SSV instructions or direct Tag access which, when used in conjunction with future Function Block programs, provides custom outer control loop capability. Servo Config = Servo Pos/Neg 0 Frict. Comp Amplifier Polarity ulator 10

11 Now that we have determined the correct loop type, the next major considerations are: Calculating the inertia of the system: Determine the peak motor torque required to accelerate the load. If the motor must accelerate within a specified time, determine the system inertia using the formula sheets for your specific power train configuration, otherwise go to step 5. Use the time (Time) to achieve peak rpm, change in rpm (Drpm), power train inertia (System Inertia) and load torque (Tl) in one of the two formulas that follow: Where: Peak = total motor torque required to accelerate the load in lb.-ft. System Inertia = total system inertia (including motor) in lb.-ft.2 Time = acceleration time (in seconds) Tl = load torque present at the motor shaft during accel in lb.-ft. Drpm = change in motor velocity during acceleration time. System Inertia in lb.-in.-s2 If the motor s total time to accelerate/decelerate (t1 + t3) exceeds 20% of the total cycle time (t1+t2+t3+t4), determine the motor s average torque with the formula shown. Where: Trms: The motor s RMS or average torque over the duty cycle. (Expressed in lb.-in. or lb.-ft. The same units must be used throughout the formula.) Tpa: peak torque to accelerate to maximum speed. (Expressed in lb.-in. or lb.-ft. The same units must be used throughout the formula.) Tss: torque present at the motor shaft during constant speed segment. (Expressed in lb.-in. or lb.-ft. The same units must be used throughout the formula.) Tpd: peak torque to decelerate to zero speed. (Expressed in lb.-in. or lb.-ft. The same units must be used throughout the formula.) Tr : when motor is at zero speed (typically is Tss). 11

12 This is a system with no load attached. Notice that the velocity feedback tightly follows the velocity command. This is a system with a large inertia mismatch attached. With high inertia load attached, notice how much the velocity feedback lags the command. 12

13 Issue Definition: Case 1: The axis can be auto-tuned coupled to the load, but the performance is not optimal. You can configure the M08SE to interface with any of the 7 loop types but remember that Servo is a good choice in most cases. With the SERCOS Interface, all of the loop types are closed in the drive, so there is no concern about tuning the drive separately. Therefore, in all cases the drive tuning method identical. In most cases you can connect the load and perform the auto-tune function, and the system should automatically set gains that result in acceptable servo operation. However, this is not always the case. For example, the motor could be coupled to a load that has a high inertia mismatch (motor sizing problem) or lots of compliance (poor choices in coupling to load). Such systems frequently require manual adjustment to achieve optimal performance. With newer motor technology, greater resolution is available, and this has been extremely beneficial in compensating for inertia mismatch. There are two cases to consider where the auto-tune is sub-optimal: Auto-tuned axis is too sluggish. Need to squeeze more bandwidth out of the system. Auto-tuned axis is tuned too hot. Need to back off the servo bandwidth to achieve a more damped response. The manual tuning procedure is the same for either case. In this procedure we use the Scaling gain as a convenient real-time handle to adjust the bandwidth of the servo loop. If your axis can be moved back and forth, a good method to determine the performance of the system is a battery box (Step Response type) program. Some use an oscilloscope to watch the velocity profiles, but we have found that the Trending function included in RSLogix 5000 works well to provide a visual reference. 13

14 This trend profile shows, and. This axis has a high inertia mismatch, and shows some overshoot and unsteadiness. Two of the more critical values to trend are: and. You may also wish to trend and. The Knobs and Adjustments (The fun stuff): By clicking Manual Adjust, you can tweak the gains wh the system is moving to view the results with RS Trend. Integral : The higher the I gain value, the faster the axis is driven to the zero condition. Unfortunately, I control is intrinsically unstable. Too much I results in axis oscillation and servo instability. You can add I gain to the loop or the loop. If you add I gain in both loops they tend to fight each other. Therefore, we typically recommend adding I gain in the loop only. The software allows you to add it into both, but you will most likely see that doing so will cause instability in positioning performance. Feedforward : This gain scales the current and adds it as an offset to the velocity servo command input. Hence, the Feedforward allows the following error of the servo system to be reduced to nearly zero when running at a constant speed 14

15 Acceleration Feedforward : This gain scales the current Acceleration and adds it as an offset to the Servo generated by the servo loop. Hence, the Acceleration Feedforward allows the following error of the servo system to be reduced to nearly zero when running at a constant acceleration. Servo Drive Tuning Method: a. Tune system coupled b. Note Scaling value after tuning. c. Note and P gains At this point if low position error is the goal, try adding Feedforward gain. This value is typically 100%, but you can manually crank it up or down to suit your application. If the system is a little sluggish getting off the line, add Acceleration Feedforward. This value is typically 100%, but you can manually crank it up or down to suit your application. Hint for changing overall gains performance: During the autotune function, one of the popup dialog boxes reports the position and velocity bandwidth. Change both of these by the same factor to maintain parity between the loops. Note: and I gains should be considered mutually exclusive. Do not add I gain if you are using I gain at the drive. Issue Definition: Case 2 - The axis cannot be auto-tuned coupled: This usually happens in an application that has high inertia. We will discuss a method of determining the percentage of measured inertia mismatch and attempt to apply that to the system. It must be noted that motor-to-load sizing should have been done beforehand to select a suitable motor and drive combination for the application. Method: Servo Drive method of determining needed bandwidth adjustment: Using auto tune in RSLogix 5000 tune the motor uncoupled. Take note of the () Scaling value after tuning. Observing all safety precautions, connect the load to the motor and enable the axis. Exercise the axis to determine overall system performance. Leaving the gains as they are, crank up (or down) the Scaling. Increasing this value has the effect of stiffening position performance. By increasing the Scaling value, the system bandwidth is increased, and conversely lowering the Scaling value decreases the bandwidth of the system. Note the Scaling value after cranking it. Use the formula: Initial Scaling / Cranked Scaling = Percent of Bandwidth Change. The next step is critical to proper scaling and system performance! Set the Scaling to the Initial Scaling value. With this in mind, applications using MP-Series s with high resolution encoders can push the envelope and go beyond the old inertia mismatch rule of thumb of 10:1. They have proven to be stable, in this type of high inertia mismatch application up to 30:1.Greater inertia mismatches are possible depending on the mechanical compliance of the application and the desired acceleration. At some point the servo system will eventually reach the drive or motor peak current limitation as the inertial load increases, and a larger drive and/or motor will be required. For more information on High Resolution see Appnote Pub# 2098-AT002A-EN-P February 2002 GMC-AT001A-EN-P October

16 Important User Information Solid state equipment has operational characteristics differing from those of electromechanical equipment. Safety Guidelines for the Application, Installation and Maintenance of Solid State Controls (Publication SGI-1.1 available from your local Rockwell Automation sales office or online at describes some important differences between solid state equipment and hard-wired electromechanical devices. Because of this difference, and also because of the wide variety of uses for solid state equipment, all persons responsible for applying this equipment must satisfy themselves that each intended application of this equipment is acceptable. In no event will Rockwell Automation, Inc. be responsible or liable for indirect or consequential damages resulting from the use or application of this equipment. The examples and diagrams in this document are included solely for illustrative purposes. Because of the many variables and requirements associated with any particular installation, Rockwell Automation, Inc. cannot assume responsibility or liability for actual use based on the examples and diagrams. No patent liability is assumed by Rockwell Automation, Inc. with respect to use of information, circuits, equipment, or software described in this document. Corporate Headquarters Rockwell Automation, 777 East Wisconsin Avenue, Suite 1400, Milwaukee, WI, USA, Tel: (1) , Fax: (1) Headquarters for Allen-Bradley Products, Rockwell Software Products and Global Manufacturing Solutions Americas: Rockwell Automation, 1201 South Second Street, Milwaukee, WI USA, Tel: (1) , Fax: (1) Europe/Middle East/Africa: Rockwell Automation SA/NV, Vorstlaan/Boulevard du Souverain 36, 1170 Brussels, Belgium, Tel: (32) , Fax: (32) Asia Pacific: Rockwell Automation, 27/F Citicorp Centre, 18 Whitfield Road, Causeway Bay, Hong Kong, Tel: (852) , Fax: (852) Headquarters for Dodge and Reliance Electric Products Americas: Rockwell Automation, 6040 Ponders Court, Greenville, SC USA, Tel: (1) , Fax: (1) Europe/Middle East/Africa: Rockwell Automation, Brühlstraße 22, D Elztal-Dallau, Germany, Tel: (49) , Fax: (49) Asia Pacific: Rockwell Automation, 55 Newton Road, #11-01/02 Revenue House, Singapore , Tel: (65) , Fax: (65) Publication RA-AP009A-EN-P September, 2003 Copyright 2003 Rockwell Automation, Inc. All rights reserved. Printed in USA. 16