Spring 2017 Prosthetic Arm: Servo Load Test

The Robot Company | CEO Professor Gary Hill

Blog Post created by Project Manager | Bianca Esquivel

Project Test Executed by Mission, Systems, and Test Engineer | Phuong Tran and Electronics and Control Engineer | Mikael Movsisyan

Table of Contents

Preliminary Information

Test Objective

During Con-ops of the prosthetic arm operation, we will rotate the wrist when the prosthetic arm is at the vertical orientation. The concern is if the servo can hold the weight AND rotate in the vertical orientation. This test is performed to clear up the uncertainty.

This experiment is performed in response to requirements

L1.2 – Together, the hand and arm shall be able to perform 3 tasks: pick up a cup of water, pick up a Chips Ahoy cookie, and operate a computer mouse

L2.2.1 – The prosthetic hand shall provide a 90 degree of rotation at the wrist. (forward and backward)

L2.2.4 – The motor at the wrist of the prosthetic arm shall have the torque rating of at least 0.03156 kg.cm

Problem: How much current will be drawn from the servo? Will the servo provide enough torque for rotation with load exerting on its shaft?

Materials

1-liter empty water bottle

Liquid water

Scale (increment at least 1g)

Fishing wires

Servo MG996R

Arduino Uno

Digital Multimeter

Test Set Up

Step 1: Connect circuit to let the Arduino control the servo rotation with the multimeter measuring the current drawn

Step 2: Pour 100 mL of water into the bottle and weigh [Fig. 1]

Step 3: Tie the water bottle to the shaft of the servo with fishing wires [Fig. 2]

Step 4: Command the Arduino to rotate the servo

Step 5: Record the time the servo takes to complete one rotation and current drawn [Fig. 3]

Step 6: Repeat step 2 to 5 for 200 mL, 300 mL, 400 mL, 500 mL, 600 mL, 700 mL, 800 mL, 900 mL, and 1 L of water

Water bottle and the servo

Testing the Servo by having it hold the weight of the water bottle

Monitoring the current the servo is outputting as it rotates the weight of the water bottle

Results

  • Mass Tied to Servo Shaft (g)
  • 0
  • 104
  • 201
  • 301
  • 399
  • 500
  • 604
  • 702
  • 805
  • 903
  • 1002
  • Time (s)
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • Current (A)
  • 0.2
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32
  • 0.32

Conclusion

The servo MG996R can be incorporated in the prosthetic arm design because the servo can handle the weight AND rotate in the vertical orientation (point toward the ground). In addition, knowing how much current is drawn reduces uncertainty during power allocation.