SpiderBot - Test Leg

Summary of problem

SpiderBot burns out servos.

Thoughts

• I need to build a test rig to determine how much weight a motor can take safely.

• The servos are supposedly rated to a stall current of 2.4-3A.

• Therefore, I should in theory be able to put up to 2A safely into the whole leg (comprising 3 servos).

• If I hook up an ammeter and measure weight applied to the test rig, then i should be able to prove 2A is a good safe limit for each SpiderBot leg.

Rig

• 3x 18650 lithium cells

• 5A fuse

• XL4015 5A 75W DC-DC step down module @ 7.4v

• USB ammeter / voltmeter

• Servo tester

• Luggage scales

• Carrier bag for adding weight

Results

Weight	Amps (min)	Amps (max)
0	0.03
scales	0.1	0.6
Scales + 1kg	0.5	0.7
Scales + 2kg	1.0	1.0
Scales + 5kg	1.5	1.8

Problems

1. One servo became very heated (thankfully didn't burnout).

2. Bracket bent.

Conclusion(s)

1. The servos in SpiderBot configuration cannot handle 5kg (evident from heat).

2. The brackets in same configuration cannot handle 5kg (evident from distortion).

3. At 1.8A or less, it is possible that a single motor on the leg is drawing too much power and burning out.

Possible next steps

• Limit to 1A? At 7.4v: V = IR 7.4 = 1 * R R = 7.4 Ω

Gallery