Run Amok Combat Robotics
Example Drivetrain Analysis: The Tentacle Torque Calculator
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Example robot drivetrain analysis using the Team Tentacle Torque Calculator."

This example shows a drivetrain analysis for a builder examining the potential performance of a 1-pound (antweight) robot powered by four Pololu Micro HP 50:1 gearmotors running at 7.4V with 1.25" diameter wheels. The robot will compete in an 8-foot square arena. The analysis will use Team Run Amok's version of the Team Tentacle Torque / Amp-Hour Calculator.
Tentacle calculator example screen The entry fields highlighted red in the example screen are the drivetrain entries:
  • 'Motor' (pull-down list): Select the Pololu Micro HP 50:1 motor from the pull-down list. The motor performance figures in the top section of the calculator will auto-fill.
  • 'Operating Voltage': Replace the default motor votage (6.0) with the desired voltage (7.4). The 'Stall Amperage' will recalculate automatically.
  • 'Motors per Side': Enter '2', representing 4 total motors.
  • 'Wheel Diameter (inches)': Enter '1.25'.
All other entry fields are left at default values in this case. The 'Help' button at the right of the entry fields offers explantions of the data inputs and outputs.

Looking down to the 'output' fields at the bottom of the page:

  • The 'Amps (per motor) to spin wheels' should generally be less than half the 'Stall Amperage' of the motor. The design being evaluated has the wheelspin amperage (0.23 amps) less than 1/8th the motor stall amperage (1.97 amps).
That current draw is indicates that the motors are very lightly loaded and may be a sign that the wheels are too small or the gear reduction is too great. We'll investigate that possibility next using the 'Acceleration Calculator'.

Tentacle acceleration calculator screen Clicking on the 'Acceleration Calculator' button at the right of the input section on the main screen brings up a new window showing the time and distance the robot will take to achielve top speed, as well as the speeds expected within an arena of any given size. The default arena size is 8 feet square, so no change is requires for this analysis. The 'Help' button in this window provides explanations of all the output fields.

The 'Distance to Top Speed' output is of interest. A robot is typically designed to reach top speed in roughly half the width of the arena, but the proposed drivetrain leaps to its top speed of 2.8 MPH in less than one foot! Combined with the low motor current draw noted above this indicates that the 50:1 gear reduction loads the motor too lightly to achieve best performance from the robot.

Returning to the main calculator screen, clicking on the 'Gear Ratio Tips' button brings up the Optimum Gearing for Combat Robots page that explains the steps used by the Torque Calculator to select the proper gear reduction for a robot. Scroll down to the 'Adjusting gearing for special conditions' tab and follow the instructions to try the different gear ratios available for the Polulo motors and different wheel sizes to optimize the robot's performance for an arena of any given size.

In our example, replacing the motor selection with the Pololu Micro HP 30:1 gearmotors improves the performance of the robot:

  • The the maximum amp draw per motor rises to an acceptable 0.45 amps.
  • Pressing the 'Recalc' button on the Acceleration Calculator updates the window and now shows the robot now accelerating to 4.4 MPH in a comfortable 3.5 feet. That's a considerable gain in performance over the 50:1 motors.

Now that you know what to look for you can evaluate other drivetrain options for your robot.

Q: I'd like to see if a pair of 36 Volt 1000 Watt 3000 RPM Electric Scooter Motors will work for my 'bot, but the motors have no specs. Is there any way I can use the Tentacle Torque Calculator to see what the performance of my 'bot would be with these motors?

A: You have more motor information than you think. Take a look at Calculating Motor Specifications.

Copyright 2016 by Mark Joerger -- all rights reserved.