Kubo: Velocity Actuation System - olinrobotics/gravl GitHub Wiki

Velocity Actuation System

Overview

This system serves the purpose of controlling the tractor's velocity by setting the heights of the velocity and reverse pedals. Remember that there is a pedal that controls the tractor's velocity rather than acceleration and a separate pedal that puts you in reverse. The two pedals are connected and rotate around the same axle. However, the radius each turns around is different. The radius of the reverse pedal is less than that of the velocity pedal. Besides that obstacle, this system needs to be designed so that one can remove the system with some ease and drive the tractor like a normal tractor.

Past System

The previous design was a pulley cable connected to a linear actuator and the velocity pedal. There were a few problems with this design. Since it was a cable, it could only pull the pedal, but not push. This meant we could not actuate the reverse pedal. Additionally, it was quite bulky, so much so that the tire would bump into the box that held the linear actuator while turning. This meant that the right turning radius suffered. Adding another linear actuator and pulley cable on the reverse pedal would overconstrain the system. It would also add more unnecessary bulk to the area.

Current System

The current system is a chain actuated system. There is a sprocket on a motor and another free-spinning sprocket to reach the reverse pedal. When the sprocket would spin clockwise, it would pull the velocity pedal down and the reverse pedal up, and when it would spin counter-clockwise it would pull the reverse pedal down and velocity pedal up.

  1. Motor: I specced a motor by determining the force required to push the pedal down using a scale. I also wanted the motor to have an encoder attached to it to keep track of how many turns it has made (necessary for programming the system). The current motor is a 118 RPM HD Premium Planetary Gear Motor w/Encoder from ServoCity.
  2. Chains and sprockets: The chain and sprockets had a pitch of .25. Here is a link to the chain used. One sprocket attaches to the motor shaft and the other sprocket attaches to the tensioner. The sprocket was rotationally attached to the motor shaft with a 6mm clamping hub. So, 4 holes were drilled on the mill into the sprocket. The other sprocket is supposed to freely rotate on the tensioner so that it can follow the other sprocket’s rotation. A .25 pitch was a reasonable size for the chain given the interference issues under the tractor and the amount of load on the chain. 2 attachment links are necessary to have at both ends of the chain to bolt the chain to a milled aluminum piece.
  3. Mounting motor to the tractor: To mount the motor and sprocket, the mounting block that the previous actuation box mounted to was used to avoid drilling more holes in the tractor. A hollow extruded aluminum cylinder with an ID equal to the motor's OD holds the motor and is TIG welded to a ¼ inch aluminum plate. The plate mounted to the block. The plate had to be long enough that the cylinder would not interfere with the bottom of the tractor. Holes were milled in the plate for bolting in the motor such that the motor shaft would stick out the other side of the plate. The cylinder was also milled (drilled and tapped to allow a 3D printed motor cap to attach for waterproofing.
  4. Tensioner: When speccing a tensioner, mounting holes upwards with the sprocket axis sideways was ideal. McMaster has an Adjustable-Arm Roller Chain and Belt Tensioner. This would allow mounting to an existing horizontal plate used as a pedal stopper under the tractor. This plate has a bolt in it set to a certain height that stops the reverse pedal from going passed a certain point (the pedals have physical stops). This plate is in a fairly good plane in relation to the other sprocket and allows a turning point for the chain to reach the reverse pedal. A ¼ aluminum mounting plate attaches to the reverse pedal stopper plate. This aluminum plate is larger than the one on the tractor to allow the tensioner to attach to. One hole was borrowed from the reverse pedal stop and the other hole had to be drilled. Drilling into the tractor upside down takes a long time, so avoiding drilling more than one was a huge plus. The hope of the tensioner was to give the chain a shorter path when pulling the reverse pedal since it was a greater distance for the forward pedal to move upwards. The tensioner didn’t quite do the job because it would not decrease the distance between the reverse pedal and the motor enough. It also tensioned the chain in an arc, which made adjustments difficult. Instead, the spring in the tensioner was removed and a linear spring in the middle of the chain was attached. This works a lot more effectively to remove the slack when the forward pedal moves down and allowing more chain length for when the forward pedal moves up.
  5. Attaching chain to pedals: The hope with all attachments on the tractor is that they can be easily removed and the tractor can be driven as a normal tractor. Milled aluminum clevises attach to each of the pedals. Milled pieces slide onto the clevis pins and have attachment threaded holes for the chain attachment links to bolt onto.
  6. Disengaging and engaging the system: To disengage the system, press down on the forward velocity pedal and remove the forward pedal's clevis pin. To engage the system, press down on the forward velocity pedal (you may have to pull on the chain a bit since we want the spring tensioned at all times) and slide the clevis pin in.