3Dot Biped barbEE

Motor trade-off Study between GM-7 and GM-9 DC motors

Author/s: Donovan Minto
Verification: Cynthia Lopez
Approval: Cynthia Lopez


This blog post will outline the difference in current consumed between two motors that were considered for this project. The two motors that were considered for this project were the GM-7 and GM-9 motors. These are very common motors used in toy applications, and were heavily considered for this project.


The I-V characteristic curve shows the current draw versus the voltage. This can be useful in terms of deciding how much power can be allocated to the motor at a certain point. The speed is also directly proportional to the voltage provided. As voltage increases, so does the rpm of the motor. The speed of the motor is important because if they rotate too fast the Biped risks getting off balance. The right amount of current and rpm needed to be selected to pick the right motor for the project. We knew that the 3Dot could provide our robot with 5V and up to 1A, so these specs were taken into account when making the decision. We decided to use the GM-7 motor because it was smaller, used less power, and produced greater torque provided compared to the GM-9.  Another deciding factor to choose the GM-7 motor was the fact that they have models that are manufactured with an extended shaft. The extended shaft can allow for the addition of a shaft encoder which then can be used to help with the motor code and the implementation of a PID controller. The characteristic curves are shown below and are a good indicator of how much power could be allocated.

Figure 1: I-V Curve of GM-7 and GM-9 motors

Figure 2: IV Curve Data


The results of this trade off study was important because this lead is in a certain way that changed our body design. This is very key in the beginning of the design process because mostly everything is decided based off of the motors. it is very important that the electronics or systems engineer make this decision early on and stick with it because it is a key element to the design.