Introduction

The objective of this trade off study is to select an electrical device that can detect RPMs for our motors. One way RPM detection is important for our project would be our wheel slip detection.

Subsystem Relation

On the subsystem level, there are many things that we hope to accomplish with rotary encoders. We would like to detect wheel slip, wheel has lost traction or free spinning in the air, for each motor. We would also like to use the rotary encoders within differential turning and the climbing algorithm.

In order to fulfill this requirement, we researched optical shaft encoders, magnetic shaft encoders, and even considered designing a custom shaft encoder PCB.

Optical Shaft Encoder

Pololu’s optical encoders are sensor boards and reflective wheels added to micro metal gear motors with extended back shafts. They have the option of a 3-tooth and a 5-tooth encoder that provides a 12 counts per revolution or 20 counts per revolution. These encoders output are direct photo-transistor outputs. [1]

Figure 1. Optical Encoder

Magnetic Shaft Encoder

Similar to the optical encoder, Pololu’s magnetic encoders are attached to the extended shaft of an geared motor. These boards use 6-pole magnetic discs that can be used to add quadrature encoding. This board senses the rotation of the magnetic disc and provides a resolution of 12 counts per revolution of the motor shaft when counting the ends of both channels. [2]

Figure 2. Magnetic Encoder

Custom Shaft Encoder PCB

The custom shaft encoder was inspired by the magnetic shaft encoder just mentioned. The architecture is exactly like the shaft encoder. Although, after detailed research to find the exact EagleCad symbol for our hall effect sensor TLE4946-2K that the magnetic sensor uses. We discovered that the TLE4946-2K hall effect sensor can be with in TO-236-3, SC-59, SOT-23-3 packages. Digikey will send the package in SC-59. Fortunately, there are MOSFETs and Transistors that share similar packages. I choose PNP transistor that has a package of SC-59-BEC, SC59 (SOT23) Motorola, to be place holders for the hall effect sensor that we will purchase.

Figure 3. Magnetic Encoder Schematic and Layout

We were considering creating our own because we would have the ability to produce four from one square inch of PCB. The decision factor revolved around cost and time to have them implemented in our design.

Conclusion

In conclusion, we realized too late that we did not purchase the extended metal gear motors that would fit perfectly with the optical or magnetic shaft encoders. We decided to decline creating custom shaft encoders because they would be too time costly and we wanted to ensure that we had working encoders. We chose to purchase the magnetic shaft encoders instead. Although, we did not place the encoder behind the motor but inside a slit that held the motor attached to the suspension. We placed magnets on the inner wheel. As the wheel rotates the sensor should catch the change in poles from the magnet.

Resources

[1]https://www.pololu.com/product/2590

[2]https://www.pololu.com/product/3081

Spring 2017 Mini Pathfinder Rotary Encoder Tradeoff Study