Spring 2017 Velociraptor: Servo Torque Test

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Authors

By: Mohammar Mairena, Electronics & Control Engineer

Approved by: Jesus Enriquez, Project Manager

Table of Contents

Introduction

The torque needed to move the different parts of the robot are specified by the Design and Manufacturing engineer. By testing the torque required at a specific location of the robot, one can prove the servo chosen will handle the stress placed at a certain location.

Analysis

Based on the measurements given by the Design and Manufacturing engineer, the servo placed at each hip will need to support 400 g at a horizontal position.

The experiment was done using a water bottle, HXT900 Micro Servo provided by Professor Hill, and a piece of string attached to the water bottle. The water bottle weighed approximately 408 grams, mimicking the weight that the servo (at the hip) will handle. The servo must rotate completely, without a stall under the 408 g load. The torque requirement at the hip was successful and rotated without a stall. Power came from the 3.3 V pin on the Arduino Uno and the current reading at 3.3 V came from the digital multimeter from the lab. At 3.3 V and under a 408 gram load, the HXT900 Micro Servo drew 180 mA of current.

Since the shaft radius of the servo is 2 mm and the weight of the water bottle is 408 grams, we can multiply them to get the Torque required at the hip. 2 mm converts to .2 cm, 408 g converts to .408 kg. Together, the torque required is .0816 kg*cm, which is equal to 1.13 oz-in.  

 

HXT900 Servo Placement Voltage Current Drawn Weight Torque Needed Shaft Radius
Hip 3.3 V 180 mA 408 g 1.13 oz-in 2 mm

 

 

Conclusion

It is important to note that at each hip, the servo will need to provide 1.13 oz-in of torque. As a result, the current drawn at each hip will be around 180 mA. Since the servos at the hips will only be used to make turns, the current drawn does not place much stress on the 3Dot battery. Max current output is 500 mA for the 3Dot Battery. The servos will not exceed the 500 mA limit.

Resources

  1. https://hobbyking.com/en_us/hxt900-micro-servo-1-6kg-0-12sec-9g.html
  2. http://www.rcuniverse.com/forum/giant-scale-aircraft-3d-aerobatic-110/2197950-how-do-you-test-servos-torque.html