MicroFOBO / Spring 2020

Servo Trade Off Study

Author/s: Dakarai Scott, Fabio Mendoza 

Introduction

To ensure we choose the correct servo, the team investigated the SG90, MG90 and HS82MG servos. When choosing the correct servo, we took cost and the amount of torque/power the servo can output to make sure Fobo has a fluid walking motion

Dimensions and Torque from Data Sheet

The table above shows that the MG90 and SG90 are almost similar in all dimensions. Because we are using generation 3 MicroFobo this will allow us to swap the servos and make tests to confirm whether our calculations were correct.

Figure 1: Trade off chart for servos

  

SG90/MG90

MicroFOBO requires at least 8 servo motors to walk. The servos are placed at the hip, upper leg, lower leg and ankle. 

First we met with the previous MicroFobo team and asked which servo they used and what results they achieved. The servo they used was the plastic gear SG90 micro servo. Sadly, they were unable to make MicroFobo walk. However, when Fobo was lifted and did not have the load of all the parts, Fobo would appear to make a walking motion. They also added that the team had to constantly replace their SG90s because the plastic gear would strip when placed under load. Our team proceeded to investigate the MG90 which is similar to the SG90 in size but instead of a plastic gear it has a metal gear. Looking further into the specifications and with some unit manipulation we determined the rpm and power outputted by the MG ranged from 100-125 rpm and 2.2-3.3 watts. When compared to the SG90 it would only provide 67-84 rpm and 1.1-1.5 watts. Since Fobo expected to be able to traverse terrain similar to Mars a wider variety of power should be desired. Looking further into the MG vs SG servos the team was able to find a video on YouTube which showed how the servos worked when operated at the same time. From the video we can see the SG has more of a loose/weak motion when moving which could be from the plastic gear inside. When looking at the movement of the MG it moves with more precision and has no signs of loose movement. Combining the precise movement with the amount of power being outputted we could assume fobos movement would be most optimal with the MG.  

HS82MG

With regards to the HS82MG, it was quickly dismissed due to the high price of $20.99 per servo.

Power/Torque Estimations

Estimated Torque Expected at Servo Location

The following contains best estimations for the three servo motors under consideration. As seen here, only the Mg90s and HS-82MG are capable of providing the maximum torque required to keep MicroFOBO standing.

MG90 Over SG90

In conclusion the MG90 might be a bit more expensive but given that the previous group had to buy multiple SGs due to them stripping.In the end, the team would save money and have a more efficient walking since the MG90 has a metal gear. 

References/Resources

  1. https://engineering.tamu.edu/media/4247823/ds-servo-mg90s.pdf
  2. http://www.ee.ic.ac.uk/pcheung/teaching/DE1_EE/stores/sg90_datasheet.pdf
  3. https://hitecrcd.com/products/servos/micro-and-mini-servos/analog-micro-and-mini-servos/hs-82mg/product
  4. MG90 vs SG90
  5. MG90 Datasheet
  6. SG90 Datasheet
  7. HS82 DataSheet
  8. HS82MG Price