ULV Drives

← 2.2.2 BLDC - Closed Loop - Speed v 1.0
← 2.2.3 BLDC - Speed Mode - Acceleration Feedforward control v1.0 

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. Configuration Steps
4. Closed Loop Position Relative Tuning
5. Velocity Feedforward Tuning
6. 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 operates using a cascaded control structure. the Position Loop drives the Speed Loop, which in turn drives the Torque Loop, ensuring stable and efficient motor control. 
• Proper configuration and tuning of all internal modes is essential for optimal performance. 

2. Theory: Mode Operation

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 time there is a change in the trajectory and thus a motor movement. 
• The controller 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 to the position mode PID.

3. Configuration Steps

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

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

2. Set the command mode

If pulse or analog inputs are used, they should be configured accordingly. Set the input's minimum, center, and maximum values to correspond to -1000, 0, and +1000 for the motor command. Depending on the input mode, the range values will use different units. For example, in analog mode, the input values are expressed in millivolts (mV), while in pulse width mode, they are measured in microseconds (µs). For more details, refer to the Roborun+ manual and respective tutorials.

3. Pay attention to the command priorities. 

If a pulse or analog input is used, the Roborun+ slider should be muted to avoid conflicting commands. If the drive receives commands from multiple sources, the command priorities must be configured appropriately. For more details, refer to the Roborun+ manual and related guides.

4. Configure the feedback sensor

Position Tracking mode for DC drives supports pulse, analog, encoders, and SSI sensors as feedback. The sensors' minimum and maximum values must be set to scale their output to ±1000. The configuration process is similar to that of analog and pulse inputs. 

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

5. 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 looks for the size of the tracking error and the duration the error is present. Both loop error limit and time parameters are configurable.

6. Set the desired motion profile

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. Configure the acceleration, deceleration, and position mode velocity. Note that higher acceleration and deceleration values will require more current for the movements.

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

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.

Warning: 

Before testing closed-loop Position Relative, 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 motor can be instructed with !G cc nn command, where cc is the channel used and nn is the desired position scaled to -1000 … +1000 command

For example, !G 1 100 command will instruct the motor to reach a target position of 2000 counts, assuming the sensor's min/max limits are set to ±20000. In other words, the target position will be 10% of the sensor's configured maximum value.

4. Closed Loop Position Relative Tuning

5. Velocity Feedforward Tuning

8. Add some Velocity Feedforward Gain.

The Velocity Feedforward control can be enabled by adding a non-zero value to the Velocity Feedforward gain.

An easy way to determine the responsiveness of the position mode is to monitor the Loop Error. Increase the Velocity Feedforward gain, until the loop Error is minimized.   

The negative Loop Error denotes an overshoot, so the Velocity Feedforward gain value that we will keep in this example is 1, which gives a small overshoot, accelerating the speed response. 

• Once the feedback precisely follows the track command by adjusting the P and feedforward gains, the system can be considered tuned!
• The position response should resemble that of the figure below:

6. Troubleshooting

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

Move the input and the feedback sensor to both edges and verify that they correspond to the ±1000 motor command and feedback.

Script and Fieldbus always have the highest priority. The priority of other command sources can be configured through the command priorities parameter. The command mode will change from the highest to the lowest priority if the command source remains inactive for longer than the watchdog timer’s time.

The motor drive uses a cascaded control operation, where control modes are interdependent. Poor configuration of Torque or Speed modes can affect Position mode.

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.

This could be due to misconfiguration of the sensor (feedback changes in the opposite direction). Additionally, if the motor exceeds the sensor’s minimum or maximum limits, the feedback value will jump from one limit to the other (e.g., from +1000 to -1000). This may cause the motor to behave in an uncontrolled manner.

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.

← 2.2.2 BLDC - Closed Loop - Speed v 1.0

← 2.2.3 BLDC - Speed Mode - Acceleration Feedforward control v1.0