By: Kevin Armentrout, Electronics and Control Engineer

Introduction

For the head and tail servo, analysis was done using a method of current measurements and data sheet torque characteristics to linearize the torque parameters to determine the actual torque produced by the Head and Tail Servo.

Methodology

The most difficult part of the analysis is creating a best fit line between the specification torque for a certain voltage and the torque for the voltage that we are operating at, which is 3.7V. To accomplish this, I analyzed a linear fit between the two voltages listed on the data sheet to see if the fit between the torque and the voltage was linear. After that, I performed an exponential fit of the data sheet values to see if an exponential fit would be a better approximation.

 

4.8 V Torque	6.6 V Torque	6.6 V Linear	6.6 V Exponential
1.8 kg-cm	2.2 kg-cm	2.48 kg-cm	2.37 kg-cm

 

Because the exponential fit was the more accurate fit, the fit between the spec sheet voltage level and the actual voltage level will be used.

Results

For the analysis, stall current was measured and recorded as well as current at 50% margin and 50% margin with a 6.5 Degree incline.

Mass	With Margin
72 g	108 g

 

INoLoad	IFullLoad	INorm	IMargin	IMargin,Inline
3.2 mA	365.6 mA	43.5 mA	113 mA	185 mA

 

τNoLoad	τFullLoad	τNorm	τMargin	τMargin,Inline
1.14 mN-m	130.57 mN-m	15.54 mN-m	40.36 mN-m	66.07 mN-m

 

Conclusion

The torque generated with 50% margin is less than that of the limits of the servo and very close to the predicted torque values listed in an earlier blog post. This successfully fulfills the Head and Tail servo requirements.

References