Servos true power?

Help & Support Robot Parts
1

hi,

I’ve built a simple arm like the one that you see in this movie:

youtube.com/watch?v=XtF54QEjZsE

the arm has 2 HS-7980TH motors and one 7985MG for gripper.

all parts are printed from plastic … so it is very light. The gripper with the motor has less than 100 grams.

I have tried lifting different objects, but it seems that I cannot reach the limits from the specifications of the motors.

7980TH is supposed to lift at least 38kg/cm … so at a distance of 10 cm it means 3.8kg/cm and at 20 cm means 1.9 kg/cm

but I can lift only about 330 grams with it (at a distance of 20 cm from the motor [the closest to the gripper]).

if I increase the weight, the motors are not moving any more.

why is that ? I was expecting to be able to lift 1.9 kg at 20 cm.
why I can lift only 330 grams?
from where is this limitation ? motors? ssc32 ?

I use a 6V acid lead battery (12Amps) from Panasonic.
I’ve measure the voltage of the battery and it is 6.8Volts

thanks,
mihai

2

Make sure your servo power wires are of sufficient gage to the SSC-32. Measure voltage at the servo connections, with the arm moving. But I doubt if this is your problem.

Alan KM6VV

3

You may want to make a seperate heavy duty power bus for the servos instead of using the one on the ssc-32.

4

I’ve found the problem.

cables were too thin … now I have replaced them and I can lift much more …

here is a video when lifting over 1 kg:

youtube.com/watch?v=T50Fuh-vxyU

thanks for help,
mihai

5

Good to hear! Did you actually get around to measuring the available voltage before and after?

Alan KM6VV

6

no, I just replaced the cables with thicker ones … and everything seems to be ok … as you have seen in my movie I can lift over 1 kg at 20 cm distance.

regards,
mihai

7

also, it seems that acid-lead batteries are not perfect for this due to some internal resistance which makes the voltage to drop under load.
I’ll try with a 7.4V LiPo battery as recommended by Hitec.

mihai

8

Sure! If Hitec spec’d a 7.4 V LiPo battery, you were already at a disadvantage.

I hadn’t heard of the Panasonic lead-acid failing to supply good 6 V, they should be OK for 6 V loads.

Alan KM6VV