A parallel continuum robot redesigned to move fast
A parallel continuum robot has very little moving mass: just thin rods and a light platform. In principle that lets it move very fast. The lab's rod-driven platform did not take advantage of this, because its actuators were slow. This project redesigned them. With the new motors, each actuator moves the rod at more than 400 mm/s under load, and the assembled robot moves in its three degrees of freedom. Camera-based object tracking, which was the goal, was not reached.
Context
In this type of robot, each rod is pushed or retracted from the base by a mechanism similar to the extruder of a 3D printer: a motorized roller presses it and drives it forward. Robot speed depends on motor speed, and precision depends on the rod not slipping under the roller.
The aim was to move a camera at the tip of the robot to follow a moving object, a task that requires fast changes of orientation. The project was carried out as a double degree in mechanical and electronic engineering.
Redesign
The main changes from the original prototype were:
- Three actuators instead of six, enough for one segment with three degrees of freedom.
- Brushless motors with integrated drivers, commanded over a CAN bus from an ESP32 microcontroller. Unloaded they reach about 5200 °/s and a linear speed of 1.58 m/s at the rod.
- A new extruder with adjustable pressure springs and a knob for moving the rod by hand.
Model of the general assembly, with the three actuators under the base
Power and communication connections between the control station and the robot
For vision, a color-based image filter was implemented that locates the object in the camera frame.
Characterization
Each motor-extruder set was evaluated separately. A 0.8 mm rod with a marker at its tip was extruded cyclically in both directions, and the motion was tracked on video.
Test of one motor-extruder system with a marker at the rod tip
Linear position, speed and acceleration of the rod over one cycle
| Actuator | Maximum linear speed with rod | Mean slip |
|---|---|---|
| 1 | 608 mm/s | 5.6 mm |
| 2 | 428 mm/s | 1.7 mm |
| 3 | 528 mm/s | 0.7 mm |
Slip was measured as the difference between commanded and reached position in each move.
Distribution of slip per move for each actuator, for two amplitudes
Results
All three actuators far exceed the speed of the original prototype, though with notable differences between them. Mean slip is small, but its spread is not: it reaches about 10 mm at three standard deviations. Without a sensor measuring actual rod length, that error accumulates.
The assembled robot moves in its three degrees of freedom.
What is missing
Because of schedule, no dynamic tests of the complete robot were run and the loop with the camera was not closed. The one-meter range of motion was not verified. Each extruder has more than 30 parts, and its size forces the rods farther apart than desirable, which changes the robot's behavior. The author describes the objectives as partially met.
How it fits in Robiolab
This is the second project on the lab's rod-driven continuum robot. It took the original platform and solved its speed limitation, and left the hardware on which the next project added cameras and neural-network-based control. The spread in slip measured here explains why that later work needed to measure end-effector position with vision rather than trust the commanded length.