By Kevin Huynh, Project Manager / Computer Systems and Software

The purpose of this servo examination was to discern the reason for the servos malfunctioning. During the servo testing, it was discovered that three Power HD 1501MG servos were broken and the broken servos were closer to the feet of the robot, suggesting that the load was wearing out the servo by shear stress. We took apart the servos to figure out why the servos were actually breaking. Four servos were taken apart to examine the servo gears and shaft, three of the servos were the nonresponsive Power HD 1501MG servos mentioned in the servo functionality testing and one was a functional Power HD 1501MG servo as a base to compare the broken servos to. There seemed to be nothing physically wrong with the servos.

Next, we examined the circuit board on the other side of the servo. Two of the three broken servos had burned circuit boards, the third servo appeared to have no damage at all.

Servo Examination Conclusion:
Since there seemed to be nothing wrong with the gears or shafts of the servo, it is unlikely that the servos were directly damaged by carrying the weight of ROFI. Since the circuit boards were burned, it is likely that the servos drew too much current and overheated. This is likely the result of the servos stalling, but continuing to draw a large amount of current in an attempt to reach the position specified by the programming. The constant draw of current eventually overheated the servo and burned the circuit board. We will be looking into ways to prevent the servos from drawing too much current from the LiPo batteries even if the servos stall, starting with a foldback current limiter.

By Elaine Doan, Systems and Test Engineering

Servo Functionality Testing
The objectives of this test were to verify that ROFI’s servos were working correctly. There are two servos that the CSULB Biped group is concerned with: the Towerpro MG996R servo and the Power HD 1501MG servo. These two servos were used in both ROFI and ROFIA, with ROFI having a combination of both servos and ROFIA using exclusively Power HD 1501MG servos.

Servo Test Materials
1. Arduino MEGA 2560
2. Towerpro MG996R and Power HD 1501MG on ROFI
3. UBEC-5A-HV DC-DC regulator

Servo Wiring Procedure
1. Connect the red wire to the positive terminal of the UBEC and the brown wire to the negative terminal of the UBEC DC-DC regulator. Do not connect either the red wire or the brown wire of the servo to the Arduino, there is a possibility that the servo will draw a lot of current and destroy the Arduino.
2. Connect the orange wire to an Arduino output pin. The orange wire is the control signal terminal of the servo and will allow you to control the servo with the Arduino.

 

Testing Code
The following code will command a servo on Arduino PIN 8 to rotate 60° clockwise within two seconds, then rotate 60° counterclockwise within two seconds. This process is repeated until the Arduino is turned off. The servo is assumed to have been centered before the test, but can be centered by using myservo.write(90).

#include <SPI.h>
#include <Servo.h>            // Include servo library

Servo myservo;               // create servo object to control a servo

void setup()
{
 myservo.attach(32);           // attaches servo on pin 8 to servo object
}

void loop()
{
 myservo.write(1);             // Move servo to 1 degree angle
 delay(2000);                  // Delay 2 seconds
 myservo.write(120);           // Move servo to 120 degree angle
 delay(2000);                  // Delay 2 seconds
}

Servo Test Conclusions:
Three of the twelve servos on ROFI were found to be completely non-responsive. All of the nonresponsive  servos were Power HD 1501MG servos and tended to be close to the feet of the robot, suggesting that the load was responsible for breaking the servos. Following this is a diagram and table detailing the results of the servo test.

 

Servo#           Servo Type                       Result
    1                      Towerpro MG996RC        Functional, rotates 90°
    2                      Power HD 1501MG           Functional, rotates 120°
    3                      Towerpro MG996R           Functional, rotates 90°
    4                      Power HD 1501MG           Nonresponsive
    5                      Towerpro MG996R           Functional, rotates 90°
    6                      Towerpro MG996R           Functional, rotates 90°
    7                      Towerpro MG996R           Functional, rotates 90°
    8                      Towerpro MG996R           Functional, rotates 90°
    9                      Towerpro MG996R           Functional, rotates 90°
    10                    Power HD 1501MG           Nonresponsive
    11                    Power HD 1501MG           Nonresponsive
    12                    Towerpro MG996R           Functional, rotates 90°

Demonstration Video
http://youtu.be/hO09e36LdrU

Datasheets available at:
Power HD 1501 MG: http://www.pololu.com/file/0J729/HD-1501MG.pdf
TowerPro MG996R: http://www.towerpro.com.tw/driver/drivers/Towerpro%20servo%20spec.pdf