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hitec servos true power ?

hi,

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

http://www.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? or the controller (ssc32- from lynxmotion) ?

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

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One quick google and a quick read of the product description...

 ...HS-7980TH is designed to operate on a two cell LiPo Pack. 

Other features in the HS-7980TH include a 7.4V optimized coreless motor...

That would be 7.4v boys and girls. Even with your 6v supply putting out 6.8v (which I am sure was measured with no load and thus, will drop when a load is applied) you could be as much as 2+ volts below what those servos need.

Add all this to what OddBot had to say about battery chemistry and internal resistance and well, there you go...

Your battery is only 6V and it's a lead acid so it has a relatively high internal resistance.

Use a 7.4V 5000mAH battery with power cables at least 1.5mm². Use screw connectors or gold plated connectors.

Your starving the motor of the current it needs.

one more question:

why a (Li-Ion) battery would be better than a lead-acid ?

only because it has a lower internal resistance ?

 

Batteries, connectors and wires all have a small amount of resistance. For low power applications this resistance is so small it is ignored. When you start working with high current devices like big motors, switchmode power supplies etc then these small resistances can severely limit current drain and cripple the device.

I have replaced the cables ... and now I can lift over 1 kg ...

here is a video ...

http://www.youtube.com/watch?v=T50Fuh-vxyU

Looking at your video, I see (although it's hard because you move the camera too fast over the wires) that the wires that are connected to the battery use crocodile clamps and thin wires. Especially the positive lead. At the SSC I see thick wires going in, so my question is, why did you not use some thick wires like the ones found in automobile stores with quick disconnect plugs on them to connect to the battery instead of the crappy crocs wires? I bet your current is limited by the thin wires.

I have built a 6DOF arm some time ago, you can see a picture of it here: http://letsmakerobots.com/node/31698 At the time I was using a similar SLA battery and the arm was able to lift an empty can of Pepsi. To make it work as intended, I need to shorten the forearm (the first segment from shoulder to the elbow) by 3 cm. Right now, the length of the segments are (from center of rotation to center of rotation and to middle of gripper) 15cm, 11cm, 10cm. The servos have 42kg/cm at shoulder (plus 11kg/cm for side movement), 11kg/cm at elbow, 7kg/cm at wrist (and another 7kg/cm for rotate) and 3kg/cm at gripper, so in theory I should be able to lift a 330ml can of Pepsi with it. However, the shoulder servo is not able to lift that weight. 

Take a look at this Robot Arm Calculator http://www.societyofrobots.com/robot_arm_calculator.shtml 

Good luck with your project!

wooow !!!

cables seems to be a problem!

I have replaced them with bigger cables ... and now I can lift a grapefruit of 400 grams ...

more later ...

thanks,

mihai

cables replaced ... now I have lifted over 1 kg:

 

http://www.youtube.com/watch?v=T50Fuh-vxyU

 

thanks a lot for help,

mihai

I am curious, did you take into account the mass of the parts that are part of the arm?

the mass is very small ... all parts there are printed with a 3d printer.

the gripper (including the motor) has about 100 grams.

 

so, what I lift there is 100 grams (the gripper) + 300 grams the orange.

at that distance I expect to lift over 1 kg ... but when I tried an grapefruit of 400 grams ... the motor is not moving anymore.