# Design and Control of a Open-Source, Low Cost, 3D Printed Dynamic Quadruped Robot
## 2 Hardware Design
### Modular Actuators  
Since the introduction of the MIT Mini Cheetah and its modular actuator, several commercial actuators with similar design principle have been released to the market. They all share relatively large diameter stator for high torque output, single stage planetary reduction gear at the center cavity, and integrated encoder and driver board. We summarize the commercially available QDD actuators we considered for the robot and their properties in Table 1. They all share several similarities to the MIT Cheetah actuator, including roughly the same dimension, daisy-chainable bus interface with 1 M bps baudrate, and position and torque controllability using built-in motor controller and encoder. Among the four commercial QDD actuator brands, which includes Steadywin, T-motor, Gyems and Unitree, we chose the Gyems RMD-X8 series actuators as they were widely available in large quantities, relatively well documented and supported, and most affordable at the time of writing this paper. RMD motor shares the same 6:1 gear reduction ratio and CAN bus connectivity with the MIT mini cheetah actuator, and has additional functionalities inherited from their previous actuators, which includes hardware DIP switch that can be used to set motor ID and CAN bus termination resistor manually, and a dedicated UART port for motor diagnosis and setup. In terms of peak torque and weight, the RMD-X8 actuator has 23 percent higher peak torque (21 Nm), yet 29 percent heavier, and slightly thicker and heavier RMD-X8 Pro actuator has 105 percent higher peak torque (35 Nm) while 56 percent heavier. We think the most of the weight penalty comes from the fully  
Figure 1: The PADWQ robot. enclosed outer shell of RMD actuators, which on the hand allows the actuators to have IP54 ingress protection rating against dust and water splash, which should greatly help the robot operating outdoors. Overall, both of the Gyems actuators compare very favorably to the Mini Cheetah actuator on paper, especially considering their retail prices are only slightly higher than the bill of material (BOM) of the Mini Cheetah actuator. To check the actual capability of the RMD-X8 series actuators, we ran through several tests using the RMD-X8 and RMD-X8 Pro actuators. Although the actuator officially supports \"torque current control\" according to the provided control protocol sheet, it is not clear whether the control input is target current or target torque value, and its coefficient is not provided either. Thus, we first measured the relationship between control value, input current and output torque of the actuator using a torque sensor and current meter, as shown in Figure 2. Output torque is measured by a BLUETEC BEM-60S3 torque wrench which has maximum operation range of 60 Nm and resolution of 0.01 Nm, and input current is measured by a HTRC inline power analyzer with 0.01 A resolution. For each measurement, the actuator control value is gradually increased to the target value for 2 s and the output torque and input current are measured after they converges to stationary values. 10 tests are performed for each control values to find the average and standard deviation of the measured values. The test results with the RMD-X8 and RMD-X8 Pro motors with an input voltage of 24 V are shown in Figures 3 and 4, which clearly shows that the control signal of \"torque current control\" is not proportional to the input current, but to the output torque. Surprisingly, two different actuators show different slopes for control value--output torque graphs. We used least square regression to find two different slopes values, which are 52.0639 for RMD-X8 actuator and 43.721 for RMD-X8 Pro actuator. The input current is fairly consistent over multiple trials, showing very low variations over multiple trials. On the other hand, the torque output has small but measurable standard deviation, which we think is mainly due to the cogging of the motor, which are presented as vertical bars in Figures 3b and 4b.