AT-ST/Fall/2018- Back of the Envelope Calculations and Motor Specs
Back of the Envelope-Motor Specs
Author/s: Wendy Guerra (Project Manager)
Table of Contents
Introduction
In order to determine what motors to buy for the AT-ST, there are certain tests that can be done. Considering the AT-ST model from Spring 2017 could not give many accurate readings, these tests were mostly done on the Theo Jansen-style Biped (TJB) Robot. Things that had to be calculated and measured were the gear ratios and the distance traveled for walking (stride). The other things that were considered to determine what motor to purchase were weight, size, speed, current, voltage, torque, and budget.
Stride
When measuring the stride, we first placed a ruler along a table and lined the back of the foot of the Theo Jansen-style Biped Robot (figure 1) to the edge of the ruler at the zero mark.
Figure 1- Theo Jansen-style Biped Robot (ruler placement missing to demo how to measure stride)
Gear 4 (figure 2) was turned just enough to move leg one step forward, which was about half a rotation, resulting in 2 cm as the stride (step length from heel to heel for 1 step).
Figure 2- Gears of TJB
Considering we were limited on time to order the motors, we glided the TJB across a table slowly while timing one minute to see what distance it would take. We came to the conclusion to a speed of 105cm/min.
Measurements
There are 2 ways that we calculated the gear ratios. The first way to count the number of teeth of each gear and compare gears as ratios which is the number of rotations it takes for one gear to turn the other gear. The second way was to measure the radius (or diameter in this case) and compare those values as gear ratios as well. The two methods should give the same ratio, as shown in Table 2. An example would be the interlocked gears 3 and 4, refer to figure 2, and divide the number of teeth of gear 4 with gear 3. It takes gear 3 about 2.74 turns in order to rotate gear 4 one whole rotation (2.74:1).
Table 1- Gear Teeth & Diameter
Table 2- Gear Ratios
Conclusion
Considering these calculations and tests had to be done in a short amount of time, and we did not want to spend too much on motors, we ended up purchasing 2 micro metal geared motors with built-in encoders from DFROBOT for $11.50 each, plus fast shipping ($12). The total amount spent was $35.
Metal Geared Motor specs:
- Rated Voltage: 6.0 V
- Motor Speed: 15000 RPM
- Gear Reduction Ratio: 150:1
- No-Load Speed: 105 rpm@6v
- No-Load Current: 60 mA
- Rated Torque: 1.0 kg.cm
- Rated Speed: 60 rpm@6V
- Current Rating: 170 mA
- Instant Torque: 2.2 kg.cm
- Weight: 18 g