Mojo5 - Two Legs with Symmetric Control
Introduction to Mirrored Servo Control (Symmetric Control)
One of the primary challenges in developing Mojo5 was ensuring synchronized movements between the servos on opposite sides of the robot. To achieve this, we employed a straightforward yet effective approach: mirroring the servo movements by reflecting the target angles.
Technical Implementation
To implement the mirroring effect, we introduced a boolean parameter in our Servo structure to indicate whether a servo should be mirrored. The adjustment is applied directly in the servo control function.
void moveServo(const Servo& servo, int pos) {pos = max(servo.minPos, min(pos, servo.maxPos));int pulseWidth = map(pos, servo.minPos, servo.maxPos, servo.minPWM, servo.maxPWM);pca9685.setPWM(servo.num, 0, pulseWidth);if (servo.mirror) {pos = 180 - pos; // Adjust the position if mirroring is needed}pos = max(servo.minPos, min(pos, servo.maxPos));int pulseWidth = map(pos, servo.minPos, servo.maxPos, servo.minPWM, servo.maxPWM);pca9685.setPWM(servo.num, 0, pulseWidth);}
the following structure is used to define all the elements of the servo
struct Servo {uint8_t num;int minPos;int maxPos;int minPWM;int maxPWM;int minRange;int maxRange;int minRangePWM;int maxRangePWM;bool mirror; //true => adjust IK if opposite side};
The results of this code are directly observed in the output for the Pulse Width Modulation (pwm) values that are sent to the servos. Here you can see the servo values are 'reflected' or 'mirrored', symmetrical to one another:
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| Mojo5 - Symmetric control - pwm values over a rectangle gait |
Practical Application and Results
In our setup, the mirror effect (symmetric control) is particularly useful for maintaining symmetry in the leg movements. This approach simplifies the inverse kinematics (IK) calculations, as the same code can be used for both sides of the robot with the mirrored adjustment applied where necessary only at the servo control.
To illustrate this concept, we've embedded a short video demonstrating the addition of a leg from the opposite side of Mojo5. The IK calculations are identical, but the servo angles are adjusted by reflecting the target angles, resulting in a mirrored motion that maintains the robot's symmetry.
Insights on Mirrored Movements
Creating a mirrored movement (symmetric control) for servos is crucial for several reasons:
- Symmetry and Balance: Ensuring that both sides of the robot move in a symmetrical manner is essential for maintaining balance, especially in quadruped robots. Asymmetrical movements can lead to instability and erratic behavior.
- Simplified Coding: By mirroring movements, the same IK code can be reused for both sides, reducing complexity and the potential for errors. This makes the development process more efficient and the codebase easier to maintain. The servo angles are simply adjusted because all servos turn counter-clockwise and have a 0 to 180° range based on their orientation. Servos on the opposite side face differently, so this mirroring adjustment is necessary.
- Consistent Gait Patterns: Symmetrical leg movements are vital for creating smooth and natural-looking gait patterns. Mirroring helps in achieving uniform step lengths and timings, which are important for the robot's locomotion.
Summary
The mirroring technique (symmetric control) we've implemented in Mojo5 represents a significant simplification in controlling symmetrical movements in quadruped robots. By introducing a boolean flag in the servo structure and adjusting the servo angles accordingly, we achieve mirrored movements without duplicating the IK code. This not only enhances the efficiency of our development process but also ensures more consistent and predictable robotic movements.
