Brushed DC Motor Feedforward Control for Load Torque Rejection
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Solution Overview
Problem
Brushed DC motors in power steering systems face challenges in maintaining constant speed profiles due to signal disturbances from friction forces and mass inertia, leading to potential controller malfunctions and instability.
Innovation Solution
A robust control method using a feedforward approach is implemented, which compensates for load and friction torques by adding a feedforward block to the DC motor model, minimizing disturbances through proper profiling and applying compensation torques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a DC motor speed controller is used to maintain constant speed profile, then speed control is achieved, but the system becomes vulnerable to signal disturbances from friction forces and mass inertia causing controller malfunction and instability
Solution Approach 1:
The controller proactively compensates for disturbance torque by modifying the voltage command or speed difference signal before disturbances can cause significant deviations. The system determines disturbance torque acting on the motor and applies compensation torque through signal modification, preventing controller malfunction and instability before they occur.
Solution Approach 2:
The system continuously monitors motor speed, determines speed difference signals, and uses this feedback to adjust the voltage command. Additionally, disturbance torque is determined and fed back into the control loop to modify the speed difference signal or voltage command, creating a closed-loop system that maintains stability despite friction and inertia disturbances.
2Ease of operation
If friction forces and mass inertia are present in the system, then the motor can perform work in power steering applications, but these factors create signal disturbances that lead to output deviations from setpoints
Solution Approach 1:
The system converts the harmful effect of friction forces and mass inertia into useful information by determining the disturbance torque caused by these factors. This disturbance torque information is then used to modify the control signals, transforming what was previously a source of error into a compensatory mechanism that maintains precise speed control despite the presence of friction and inertia.
3Device complexity
If disturbance torque is not compensated, then the control system remains simple, but output deviations from setpoints occur and undesirable vibrations and noise increase
Solution Approach 1:
The system performs preliminary compensation by determining disturbance torque and modifying control signals before vibrations and noise can develop. By proactively addressing disturbances through signal modification, the system prevents the generation of harmful vibrations and noise rather than reacting to them after they occur.
Data Source
AI summary
A method for controlling a brushed DC motor includes: determining a speed difference signal based on a difference between a speed command signal and a motor speed; determining an initial voltage command based on the speed difference signal; determining a final voltage command based on the initial voltage command; applying a DC voltage based on the final voltage command to cause the brushed DC motor to turn a lead screw and to move a load along a path; determining a disturbance torque; and offsetting the disturbance torque by at least one of: determining a feedforward voltage command based on a position of the load along the path and determining the final voltage command as a sum of the initial voltage command and the feedforward voltage command; or determining a speed compensation signal based on the disturbance torque and determining the speed difference signal further based on the speed compensation signal.