← 3.2.2 DC- Closed Loop - Speed v 1.0         

 

Note: The Guide can be downloaded in PDF format at the end of the article.

 

Disclaimer and Safety Information

 

Disclaimer

This quick startup guide is provided as a complementary resource to the official motor drive manual and datasheets. It is not intended to be the sole source of information for proper motor drive configuration and operation. Incorrect configuration or software bugs may cause unintended behavior, including uncontrolled motor operation or runaway. Users must always conduct tests cautiously and ensure they have a reliable method to safely stop the system in such scenarios. Roboteq, the author, and related parties are not liable for any hardware damage, personal injury, or other consequences arising from the use or misuse of the information in this guide.

 

Safety Symbols Explanation

 

 

Table of Contents

 

  1. How to Use This Guide
  2. Theory
  3. Required Parameters List
  4. Configuration Steps
  5. Closed Loop Speed Position Tuning
  6. Velocity Feedforward Tuning
  7. Troubleshooting

 

1. How to Use This Guide

  • This guide is part of a series of documents that must be followed sequentially to configure and test a Brushless DC motor. The process begins with the Open Loop configuration and continues through the sequence of documents until reaching the final target operating mode.
  • For example, configuring a higher-level operating mode, such as Count Position with Hall sensors, requires completing the configuration and testing of lower-level modes, including Open, Torque and Speed modes.

 

2. Theory: Cascaded Operation

  • The motor drive uses a cascaded control structure: the Position Loop controls the Speed Loop, which controls the Torque Loop, ensuring stable motor control. 
  • Tuning Position mode requires both Speed and Torque loops to be tuned, while Speed mode needs only the Torque loop. Torque and Open Loop modes operate independently. 

 

2. Theory: Mode Operation

  • Position Relative mode is a position control mode where both the input command set by the user and the sensor feedback are scaled to ±1000. This allows the use of potentiometers and analog sensors for motor command and feedback.
  • If the motor command is sent through a communication parameter (e.g., serial command, CAN object, etc.), it will be internally limited to ±1000. 
  • To allow scaling of both the input and feedback to ±1000, the input and sensor minimum/maximum values must be configured.
  • The motor drive will drive the motor to the desired position using a closed PID loop

 

  • Upon receiving the speed command, a trajectory is generated based on the acceleration and deceleration values. The target position is continuously updated according to the user command and motion profile, ensuring the motor follows the desired speed.

 

2. Theory: Velocity Feedforward control

  • Velocity Feedforward control is an additional feedforward loop taking part in Position mode. 
  • In contrast to the Acceleration Feedforward control that acts in Speed Loop only during the acceleration of the motor, the Velocity Feedforward control acts every whenever there is a change in the trajectory and a resulting motor movement. 
  • The motor drive operates by multiplying the motor's desired velocity by the velocity feedforward gain. The higher the velocity, the more feedforward power will be applied to the motor. The velocity feedforward power is added to the power output of the Position mode PID, so this mode operates in parallel with the position mode PID.

 

3. Required Parameters List

To complete the configuration sequence, ensure you have the following specifications readily available:

 

4. Configuration steps

1. Set the operating mode to ‘Closed Loop Speed Position’

Closed-Loop Speed Position mode uses a predefined acceleration, constant velocity, and deceleration so it can move the motor a precise number of counts

 

2. Configure the feedback sensor

The feedback sensor should have already been configured during Speed mode tuning. The encoder's or SSI sensor's resolution, channel, and usage should have been set, and the sensor readings should have been evaluated in Open Loop and Closed Loop Speed.

 

Warning

Failure to configure the feedback sensor according to the instructions provided in the previous guides may result in unintended and potentially hazardous motor behavior, such as uncontrolled movement.

Note

For quadrature encoders, which do not provide an absolute position, a homing process must be performed each time the system is powered on.

 

 

3. Configure the Closed Loop Error Detection 

This parameter is used to detect large tracking errors due to mechanical or sensor failures and shut down the motor in case of problem in closed loop speed or position modes. The detection mechanism checks the size of the tracking error and the time the error is present. Both loop error limit and time parameters are configurable.

Be aware that in Speed Position mode, the Loop Error will be measured in counts and not RPM.

Note

The Loop Error Detection may need to be temporarily disabled during the tuning of the motor, where the optimal response may not be yet achieved.

 

4. Set the desired motion profile by configuring the acceleration, deceleration and position mode velocity parameters. 

The motor will move to the desired position following a trapezoidal acceleration, speed, and deceleration profile. How closely this profile is followed is determined by the PID gains. Note that higher acceleration and deceleration values will require more current for the movements.

 

5. Closed Loop Speed Position Tuning

5. Set a starting value for the position proportional gain and start the tuning. 

Speed Position mode uses the Position PID. Similar to Position mode, Speed Position mode, is primarily tuned using proportional gain. A small value, such as 0.03, can be tested initially. Increase the gain until the feedback matches the track command. Integral gain may reduce the final position error but can also cause overshoots. Test with a small amount of integral gain.

 

The Speed command can be sent either from the slider, or with the !S cc nn command, where cc is the channel and nn the requested speed in RPM.

For example, the !s 1 100 will instruct the motor to reach target speed of 100 rpm

By using serial commands and enabling the watchdog timer, the drive can safely stop the motor when the watchdog timer has expired. Be aware that the watchdog timer will not expire if the command is sent repeatedly e.g. by using the slider.

 

Warning: 

Before testing closed-loop Position, Open Loop and Speed modes must be configured, and the sensor must be set correctly so that the reported speed is positive when a positive command is given. If the sensor is not set properly, the motor may enter a dangerous runaway condition. Ensure you have followed the respective guides.

 

  • The following screenshots illustrate the motor response by using different sets of PI gains.

 

6. Velocity Feedforward Tuning

6. Add Velocity Feedforward Gain.

  • The Velocity Feedforward control can be enabled by adding a non-zero value to the Velocity Feedforward gain.
  • Start by setting a small value, such as 0.1
  • Increase the gain until the desired response is achieved
  • An easy way to determine the responsiveness of the position mode is by monitoring the Loop Error.
  • Increase the Velocity Feedforward gain, until the loop Error is minimized.  
  • A negative Loop Error indicates that the motor position is being updated faster than the desired trajectory
  • Select a value that results in the minimum positive or negative loop error.
  • In this example, the value of feedforward gain that will be retained is 1

 

  • By plotting the Track and Feedback data that can be recorded from the Run tab, we can assess the position response. In a well-tuned system, the feedback values should closely follow the track command, as illustrated in the following figure:

 

 

7. Troubleshooting

Speed Position mode operation should be an easy and straightforward process. If the motor does not behave as expected, consider checking the following general points:

  • Test the speed , Track command and Feedback
    1. Send !s command along with the desired position to the Run Tab and verify that the feedback follows the track without delays and performs as expected.
  • Ensure that Torque and Speed modes are properly configured and tuned. 
    1. The motor drive uses a cascaded control operation, where control modes are interdependent. Poor configuration of Torque or Speed modes can affect Position mode.
  • PI Tuning. 
    1. Large amounts of P gain can cause vibrations. If the motor cannot reach the setpoint using only P gain, a small amount of I gain can be tested.
  • Motor Runaway
    1. This could be due to misconfiguration of the sensor (feedback changes in the opposite direction). Refer to the configuration guides of Open Loop, Torque and Speed modes to ensure that everything is set up properly.
  • Slow response
    1. The motor will follow the trajectory based on the PID settings. Achieving a faster response will require more current for the movement. However, if the current reaches the amp limit parameter, the response time cannot be further improved.
  • Plotting advice
    1. In Speed Position mode, both the input and feedback are measured in counts. As the motor rotates, the count values continuously increase, making it difficult to evaluate the position response when plotted. Instead, logging and plotting the Velocity Demand Value (VDV) and motor speed provide a clearer representation of the motor's performance.

 

 

← 3.2.2 DC- Closed Loop - Speed v 1.0