IGVC - Encoders

IGVC, or the Intelligent Ground Vehicle Competition is a yearly competition in which autonomous vehicles developed at various universities drive about a course to prove their self-driving capabilities. This competition is the origin of the autonomous golf cart project at Cal Poly. The project has gone through many revisions, but never actually competed in the competition. In fact, the golf cart no longer meets the size constraints specified for the competition. While the aims of the project have diverged from competition to focus on small self-driving tasks around campus, the name stuck, and the project is still called IGVC.

Six-magnet disk

The encoder from one of our other “Dr. Robots”

            Recently, the team focused on making a major sensor upgrade to the golf cart, new encoders. To understand the significance of this upgrade, it must be understood how poor the previous encoders were. Initially we used two hall-effect sensors with six-magnet discs attached to the front wheels of the golf cart. The encoders were handmade and thus, were not perfectly aligned on the discs. This issue required a continual average of the speed to remove the error from the calculated speed. Additionally, due to the small number of magnets per revolution, the minimum speed we could measure was approximately .125 m/s. This meant that the golf cart wouldn’t know the difference between 0.124 m/s and stopped, allowing significant room for error when approximating distance from velocity. Another issue was the inability to determine negative velocity. With the hall-effect sensors, there was no effective way of calculating if the golf cart was moving in reverse, or it was moving forward. Again, this created additional error in the calculations, especially when a slight negative velocity is caused from stopping.

Encoder Disk #1: Acrylic painted white and laser etched

Thus, we set out to engineer a solution that would increase our minimum speed measurement, improve the alignment of ticks received by the sensor, and add the ability to measure negative velocity. After searching the robotics room to find a potential part that could help us solve these issues, we ran across a US Digital encoder on a couple of old robots. After taking the encoder apart, we discovered it was an optical encoder that could measure up to 300 ticks per revolution. The next step was to find a way to integrate the encoder with the golf cart.

Encoder Disk #2: DVD laser etched

Our initial attempt to integrate the optical encoder with the golf cart was to make disks with small slits that would allow for light to reflect back on certain areas of the disc better than others. These disks would then be mounted directly to the interior of the front wheels. We went through many iterations of this idea, each one better than the other, but we never were able to get the same consistency as the 300 tick disk included in the encoder. After running many tests, it seemed that the optical sensor worked best on reflective material and was made to trigger specifically off of aluminum and a black strip. We attempted to mimic this with our third attempt by using aluminum tape and vinyl covering which was eventually etched by a laser cutter.

Encoder Disk #3: Acrylic wrapped in aluminum tape and vinyl laser etched

When actually testing our third disk, we noticed that there were many missed ticks even with precise placement of the optical sensor. This led us to decide that the handmade encoder disk was not going to work, and we needed to find another way to integrate the encoder into our golf cart.

            Our next attempt at integrating the optical sensor with the golf cart was to couple the entire US Digital encoder directly to one of the shafts of the drive motor. After looking closely on our motor, we found a small shaft that was driven by one of the gears inside of the motor, but accessible from the exterior. In order to couple to this shaft, we took the entire motor apart and removed the shaft. We then milled a hole inside the shaft to which we press fit and additional shaft onto that would allow us to directly connect our encoder to the shaft. After reassembling the motor, we attached our encoder to the new shaft, and updated the software to match the new encoder output. Due to the gear ratio of the motor and the 300 tick disk in the encoder, our system receives approximately 1500 tick per revolution of our back tires. This corresponds to a minimum measurement speed of 0.001 m/s, which is a significant improvement from the previous encoder.

Encoder coupled directly to our motor

Close up of final encoder attachment

            Overall, our goal of improving our speed measurement was extremely successful. One of the largest lessons learned from this task was to try to incorporate premade products into our solution before attempting to manufacture home solutions on our own. Doing so for this project would have saved us the many hours we spent manufacturing encoder disks only to find out that they did not work effectively. Nonetheless, this task was a lot of fun to work on and a great learning experience.

UROV - Initial Lighthouse Trip

The original lighthouse building, well maintained and still standing.

My name is Spencer Shaw and I am a third year computer engineering student. This is my third year being a part of the Cal Poly Robotics Club UROV team. I have contributed to hardware and software on the robot. Last year I was the project lead and this year I am the software lead.

Jared Peter-Contesse, this year’s project lead, and I were invited to visit the Point San Luis Lighthouse on February 16th in order to survey two cisterns that require repair. The lighthouse keepers (A wonderful group of mostly volunteers) contacted us through the Robotics Club and asked if we would be interested in using our robot to survey the cisterns. Surveying the cisterns lines up almost exactly with the mission tasks mandated by the MATE competition for which our robot is designed. While our timeline for completing the robot does not fit theirs for completing the repairs, both of us are still interested in collaborating.

Waves crashing on the shore below the lighthouse.

       Jared and I really enjoyed the excursion. The views from the location were wonderful! The lighthouse and surrounding buildings are kept in great shape by the keepers. Their constant dedication to this project shows in every detail of the grounds. Our contact and tour guide gave us a brief tour of the area and  the lighthouse’s history.

The two cisterns surrounded by safety fence with the lighthouse in the background.

While not sore thumbs, the cisterns themselves were not in as good of shape the rest of the facility. This is precisely why they are the next candidates on the restoration by the keepers.  Measuring 30 feet in diameter, the lids were clearly rotting. Approximately 13 feet deep, each cistern is meant hold on the order of 50,000 gallons of water. Originally they held water for the residents at the lighthouse for drinking and domestic use. After restoration, they will not be used because contaminants in the ground make the water unsafe and another water source is available.

The cisterns can each be accessed by a rectangular hole in the lid. The hole is over two feet wide which fits the specification of our robot.

The cover for the access hole of a cistern, tape measure for scale.

Our mission in this case would include checking the interior of each cistern for cracks or other damage. Main challenges for this include tether management and personnel safety. A jig will have to be made to ensure that the tether does not forcefully contact the lid of the cistern as the robot moves around inside. This will also have to be monitored by a team member. The team member will need to be very careful to not fall into the cistern and should be doubly cautious of the rotting portions of the lid. We may consider having them wear a harness in order to be anchored because our team’s safety is our priority.

We are extremely excited about this collaboration for two reasons. First, it will allow us to exercise our robot in a manner very similar to the specifications of the MATE competition. Second, it is an opportunity to not only give back to our community but also to connect with other members of the community outside Cal Poly and the history of the region.