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National Instruments NI-Motion Kd (Derivative Gain), Kv (Velocity Feedback), Vff (Velocity Feedforward)

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Chapter 3 Tuning Servo Systems

© National Instruments Corporation 3-5 NI-Motion User Manual

Kd (Derivative Gain)

The derivative gain (Kd) determines the contribution of restoring force

proportional to the rate of change (derivative) of position error. This force

acts much like viscous damping in a damped spring and mass mechanical

system. A shock absorber is an example of this effect.

The PID loop computes the derivative of position error every derivative

sample period. A non-zero value of Kd is required for all systems that use

torque block amplifiers, where the command output is proportional to

motor torque, for the servo loop operation to be stable. Too small a Kd

value results in servo loop instability.

With velocity block amplifiers, where the command output is proportional

to motor velocity, it is typical to set Kd to zero or a very small positive

value.

Kv (Velocity Feedback)

You can use a primary or secondary feedback encoder for velocity

feedback. Setting the velocity feedback gain (Kv) to a value other than

zero (0) enables velocity feedback using the secondary encoder, if

configured, or the primary encoder if a secondary encoder is not

configured.

Kv is used to scale this velocity feedback before it is added to the other

components in the 16-bit DAC command output. Kv is similar to derivative

gain (Kd) except that it scales the velocity estimated from encoder

resources only. The derivative gain scales the derivative of the position

error, which is the difference between the instantaneous trajectory position

and the primary feedback position. Like the Kd term, the velocity feedback

derivative is calculated every derivative sample period and the contribution

is updated every PID sample period.

Velocity feedback is estimated through a combination of speed-dependent

algorithms. Velocity is measured based on the time elapsed between each

encoder count.

Vff (Velocity Feedforward)

The velocity feedforward gain (Vff) determines the contribution in the

16-bit DAC command output that is directly proportional to the

instantaneous trajectory velocity. This value is used to minimize following

error during the constant velocity portion of a move and can be changed at

any time to tune the PID loop.

Contents

Main Page Important Information Warranty Copyright Trademarks Patents Contents PART III Programming with NI-Motion Chapter 4 What You Need to Know about Moves Chapter 5 Straight-Line Moves Page Chapter 10 Electronic Gearing and Camming Chapter 11 Acquiring Time-Sampled Position and Velocity Data Chapter 12 Synchronization Chapter 13 Torque Control Chapter 14 Onboard Programs PART IV Creating Applications Using NI-Motion Chapter 15 Scanning Page About This Manual Conventions Documentation and Examples Page Part I Introduction Introduction to NI-Motion About NI-Motion NI-Motion Architecture Software and Hardware Interaction NI Motion Controller Architecture NI 73xx Architecture Page NI Motion Controller Functional Architecture Figure1 -3 shows the components of the NI 73xx motion controllers. Figure 1-3. Typical NI 73xx Motion Controller Functional Architecture Figure 1-4. NI SoftMotion Controller Functional Architecture Figure1-4 shows the components of the NI SoftMotion contro ller. Typical NI 73xx Motion Controller Architecture Page NI SoftMotion Controller Architecture NI SoftMotion Controller for Ormec NI SoftMotion Controller for CANopen NI SoftMotion Controller Communication Watchdog Page Figure 2-1. Generic Steps for Designing a Motion Application Adding Measurements to an NI-Motion Application Figure 2-2. Input/Output with Data and Image Acquisition Page Page Tuning Servo Systems NI SoftMotion Controller Considerations NI SoftMotion Controller for CANopen NI SoftMotion Controller for Ormec Using Control Loops to Tune Servo Motors Control Loop Page PID Loop Descriptions Kp (Proportional Gain) Ki (Integral Gain) Kd (Derivative Gain) Kv (Velocity Feedback) Vff (Velocity Feedforward) Aff (Acceleration Feedforward) Kdac Ga Kt 1/J Dual Loop Feedback Page Velocity Feedback NI Motion Controllers with Velocity Amplifiers Page Part III Programming with NI-Motion Page What You Need to Know about Moves Move Profiles Trapezoidal S-Curve Basic Moves Coordinate Space Multi-Starts versus Coordinate Spaces Trajectory Parameters NI 73xx Floating-Point versus Fixed-Point NI 73xx Time Base Page Page Page Page Page Page NI 73 xx NI 73xx Arc Move Limitations Timing Loops Status Display Graphing Data Event Polling Straight-Line Moves Position-Based Straight-Line Moves Straight-Line Move Algorithm Page Figure 5-2. 1D Straight-Line Move in LabVIEW Figure 5-3. 2D Straight-Line Move in LabVIEW 1D Straight-Line Move Code Page 2D Straight-Line Move Code Page Page Velocity-Based Straight-Line Moves Page Page Page Page Page Page Velocity Profiling Using Velocity Override Page Figure 5-8. Velocity-Based Move Using Velocity Override in LabVIEW Page Page Page Page Arc Moves Circular Arcs Page Figure 6-3. Positive and Negative Travel Angles Arc Move Algorithm Figure 6-4. Circular Arc Move Algorithm Page Page Page Spherical Arcs Page Figure 6-8. Spherical Arc Algorithm Page Page Page Helical Arcs Figure 6-11. Helical Arc Algorithm Page Page Page Page Contoured Moves Overview Arbitrary Contoured Moves Contoured Move Algorithm Figure 7-2. Contoured Move Algorithm Absolute versus Relative Contouring Figure 7-5. Contoured Move in LabVIEW Page Page Page Page Page Reference Moves Find Reference Move Reference Move Algorithm Figure 8-1. Find Reference Move Algorithm Page Page Page Page Blending Moves Blending Superimpose Two Moves Blend after First Move Is Complete Blend after Delay Blending Algorithm Figure9-5 illustrates a generic algorithm for blending moves. Figure 9-5. Blending Algorithm n Figure 9-6. Blended Straight-Line Move and Arc Move in LabVIEW // Main Function Page Page Page Electronic Gearing and Camming Gearing Page Page Gear Master Page Page Page Camming Page Page Figure 10-8. Camming Algorithm Camming Table Page Page Slave Offset Page Master Offset Figure 10-17. Camming Profile Starts when First Material Passes Figure 10-18. Camming Profiles with and without Master Offset Page Page Page Page Acquiring Time-Sampled Position and Velocity Data Algorithm Page LabVIEW Code C/C++ Code Page Page Synchronization Absolute Breakpoints Buffered Breakpoints (NI 7350 only) Buffered Breakpoint Algorithm Figure 12-1 shows the basic algorithm for implementing buffered breakpoints. Figure 12-1. Buffered Breakpoint Algorithm Page Page Page Single Position Breakpoints Single Position Breakpoint Algorithm Figure 12-4. Single Position Breakpoint in LabVIEW Page Page Relative Position Breakpoints Relative Position Breakpoints Algorithm Page Page Periodically Occurring Breakpoints Periodic Breakpoints (NI 7350 only) Periodic Breakpoint Algorithm Page Page Page Modulo Breakpoints (NI 7330, NI 7340 and NI 7390 only) Page Modulo Breakpoints Algorithm Figure 12-13 shows the basic algorithm for modulo breakpoints. Figure 12-13. Modulo Breakpoints Algorithm Figure 12-14. Modulo Breakpoint Using LabVIEW Figure 12-15. Modulo Breakpoint with RTSI Using LabVIEW Page Page High-Speed Capture Buffered High-Speed Capture (NI 7350 only) Buffered High-Speed Capture Algorithm Figure 12-16. Buffered High-Speed Capture Algorithm Page Page Page Non-Buffered High-Speed Capture High-Speed Capture Algorithm Figure 12-18. High-Speed Capture Algorithm Page Page Page Real-Time System Integration Bus (RTSI) RTSI Implementation on the Motion Controller Position Breakpoints Using RTSI Encoder Pulses Using RTSI Software Trigger Using RTSI High-Speed Capture Input Using RTSI Torque Control Analog Feedback Page Torque Control Using Analog Feedback Algorithm Figure 13-2. Torque Control Using Analog Feedback Algorithm Figure 13-3. Torque Control Using Analog Feedback Using LabVIEW 86 754321 Page Page Page Monitoring Force Torque Control Using Monitoring Force Algorithm Figure 13-5. Torque Control Using Monitoring Force Algorithm Figure 13-6. Torque Control Using Monitoring Force in LabVIEW 1 98765432 Page Page Page Speed Control Based on Analog Value Speed Control Based on Analog Feedback Algorithm Figure 13-8. Speed Control Based on Analog Feedback Using LabVIEW 10987653 421 Page Page Page Onboard Programs Using Onboard Programs with the NI SoftMotion Controller Using Onboard Programs with NI 73xx Motion Controllers Writing Onboard Programs Figure 14-2. Writing Onboard Programs Figure 14-3. Basic Onboard Program Algorithm Figure 14-4. Onboard Program in LabVIEW Page Page Running, Stopping, and Pausing Onboard Programs Running an Onboard Program Stopping an Onboard Program Pausing/Resuming an Onboard Program Automatic Pausing Conditionally Executing Onboard Programs Figure 14-5. Executing Onboard Programs Onboard Program Conditional Execution Algorithm Figure 14-6. Onboard Program Conditional Execution Algorithm Page Page Using Onboard Memory and Data Figure 14-8. Updating Velocity Based on ADC Channel Algorithm Before you execute this program, set the operation mode of the axis to velocity mode. Figure 14-9. Updating Velocity Based on ADC Channel in LabVIEW Page Page Branching Onboard Programs Onboard Program Algorithm Figure 14-10 shows an onboard program waiting for an I/O line to go active before starting a move. Figure 14-10. Using Labels with Onboard Programs Figure 14-11. Continuously Executing Onboard Program in LabVIEW Page Page Math Operations Indirect Variables Figure 14-12. Reading an Indirect Variable Figure 14-13. Onboard Buffer Data Flow Onboard Buffers Synchronizing Host Applications with Onboard Programs Page Figure 14-17. Synchronization Onboard Code in LabVIEW Page Page Page Page Page Onboard Subroutines Figure 14-20 shows the main onboard program used to determine the subroutine call. Figure 14-20. Onboard Subroutine Call Using LabVIEW Figure 14-21 shows the subroutine that causes the motor to rotate clockwise. Figure 14-21. Clockwise Subroutine Using LabVIEW Figure 14-22 shows the subroutine that causes the motor to rotate counter clockwise. Figure 14-22. CounterClockwise Subroutine Using LabVIEW Page Page Page Page Automatically Starting Onboard Programs Changing a Time Slice Part IV Creating Applications Using NI-Motion Scanning Connecting Straight-Line Move Segments Raster Scanning Using Straight Lines Algorithm Figure 15-2. Raster Scanning Using Straight Lines Algorithm Figure 15-3. Scanning Using LabVIEW Page Page Page Blending Straight-Line Move Segments Raster Scanning Using Blended Straight Lines Algorithm Figure 15-5. Raster Scanning Using Blended Straight Lines Algorithm Figure 15-6. Scanning Using Blending Page Page Page User-Defined Scanning Path Page User-Defined Scanning Path Algorithm Figure 15-8. User-Defined Scanning Path Algorithm Figure 15-9. Scanning Using Contouring Page Page Page Page Rotating Knife Solution Page Figure 16-2. Rotating Knife Application Algorithm Figure 16-3. Rotating Knife Application Using LabVIEW Figures 16-4 and 16-5 show the remaining cases for the block diagram in Figure 16-3. Figure 16-4. Figure16-3 Sequence Structure 1 Figure 16-5. Figure16-3 Sequence Structure 2 Page Page Page A Sinusoidal Commutation for Brushless Servo Motion Control Phase Initialization Hall Effect Sensors Shake and Wake Direct Set Determining the Counts per Electrical Cycle of the Motor Commutation Frequency Troubleshooting Hall Effect Sensor Connections B Initializing the Controller Programmatically Page C Using the Motion Controller with the LabVIEW Real-Time Module Page Page D Technical Support and Professional Services Glossary A B C D E F G H I K L M N O P Q R S T Page Index A B C D E F G H I J K L M N O P R S T U V W