This time I’d like to tell you a bit more about the mechanical aspects of the robot. The most interesting parts of each quadruped are undoubtedly the actuators. As I mentioned earlier the idea came from a project by Paul Gould. I got interested in cycloidal drives as I could easily prototype them on my 3d printer. I decided to integrate the actuators completely in the robot’s structure in order to save some space. The leg construction assumes it can be milled from a 7mm plate of material (this dimension is dictated by the width of a single bearing). Moreover I decided to put all the motors (which are quite heavy in comparison to the rest of leg) as far as possible form the leg tip. In that way it is possible to minimize the leg inertia and thus make the leg more agile. Many quadruped designers also claim that when the leg inertias are low enough, it is possible to neglect them in the dynamical model of the robot.
The shape of the cycloids was generated in the SolidWorks program using formulas found on the internet. Hole placement, the eccentricity of the cam (crankshaft?), outer pin count and diameter was computed using one of the online calculators. Knowing all the needed parameters the gearbox was 3d-printed and put together. Below You can see the 3d model of the leg.
Figure 1. 3D model of a single leg
Each actuator has two cycloids which are 180* rotated relatively to each other. This is to minimize the vibrations coming from non-axial movements. Each cycloid has a main bearing in the center and seven small bearings around it. The outer pins are just pins – no bearings there.
The project looked fine on the computer, however, when I started to put it together I noticed a few problems:
-The cycloids must be perfectly manufactured. When too large they tend to latch, when too small the gearbox runs smoothly but has a lot of backlash. Building first prototypes I used to grind the cycloids with a file in order to make them fit the desired space. This was quite time-consuming, however it worked. The gearbox ran smoothly and it would show only about 1-2 degrees of backlash.
-Outer pin holder was very susceptible for any imperfections coming from the 3d printing process. A bit of extra material could twist the outer pins relatively to each other making the whole pattern irregular. Even when the cycloids were fine, this part would make the whole actuator latch at the same spot every rotation. -The last problem, which is still unresolved, is the connection between the thigh motor case and the rest of the leg. Currently, it relies mostly on one bearing and partially on the rods connecting the motor holder to the internal part of the thigh actuator. This can cause problems when the leg is being strained from the side.
Figure 2. Section view of the leg. Red circle shows one of the screws holding the bearing to the motor case, whereas blue shows the dangerous spot where the leg rests mostly on one bearing.
Thankfully my university owns a milling machine able to mill in aluminum. I decided to try milling a few cycloids and check how they do in this design. At first, the cycloids needed a few attempts in order to make them fit well. When adjusted the gearbox was operating without any latches but was a bit louder than the 3dprinted one.
Figure 3. milled cycloids
In the end, all the cycloids were milled form PA4 aluminum. Rings for outer pins, previously 3d-printed, were now also milled form a material called metaplex. It is similar to plexi, but is more shock resistant and do not break that easily.
Figure 4. cycloidal gear inside
The torso parts were also 3d printed. Connection between two leg holders is made using PVC tubes and a 2mm laminate plate mounted on the bottom of the trunk. This makes the whole structure quite stiff. Actuators responsible for abducting and adducting the legs are placed in the trunk. The motion is transferred through pulley rods connected on the top and bottom of the thigh motor case.
Figure 5. 3D model of the trunk
Figure 6. debugging
Figure 7. some more debugging
Figure 8. Milled outer pin cases
Figure 9. Assembled leg
Figure 10. Fully assembled robot
The next step is to prepare the dynamic model, formulate it as a QP program, and then solve it :) Hopefully the drawbacks of cycloidal actuators will not influence the model too much.
Next time I'll write something about programming part as well as simulation.