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Recently I have been trying to do a project for my end of year project, it involved making a humanoid arm.

I am still a newbie at these things, I usually have my friend do the wiring, but since he is away for the moment can anyone help me with it?

I am currently using 6X MG996R Servos for each finger and 1 for the wrist. I am also using a Arduino Mega with a Adafruit 16-Channel 12-bit PWM/Servo Shield - I2C interface.

I have already searched through the web for the best way to power it but I only found people trying to power it with a 5v adaptor, my problem is I'm trying to make the hand mobile so I need to use a battery. I have tried 2x UltraFire 18650 3000mAh 3.7V rechargeable battery in series (still doesn't work only 1 servo moved), also using a 9v battery (Only moved 2 servos).

So i have been wondering if I could utilise the Adafruit 16-Channel 12-bit PWM/Servo Shield - I2C interface..

  • Take a multimeter and test the voltages and amps while the servos are being used. If there is voltage drop you will need more batteries in parallel (equalize the voltages before connecting them). – ratchet freak Jan 16 at 13:38
  • Ok ratchet ill try that soon, oh and BTW do you think the Adafruit 16-Channel 12-bit PWM/Servo Shield - I2C interface will make a difference since as of now im suing a breadboard to control all the servos.. – Purnawan Brillian Jan 16 at 13:49
  • You do have the servo power lines connected in parallel, yes? All Power lines together at the battery, same with all Gnd lines, and the Gnd from the Mega as well. Some folks have mistakenly connected the power lines in series, that will not work. – CrossRoads Jan 16 at 17:39
  • OK then i guess i will try to do both in series and parallel – Purnawan Brillian Jan 17 at 0:12
  • read what @crossroads wrote .... do not do a series connection – jsotola Jan 17 at 3:31
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Here is a spec-sheet for the MG996R servos. Notice at the top of the 2d page, they require a supply of 4.8 - 7.2 volts, and can draw .5 - .9 Amps running and 2.5 Amps when stalled.

You'll need a supply of 6 volts (+/-) that can handle that current, times however many servos you need to operate simultaneously. The 3.7 volt batteries don't even meet the minimum voltage for the servos (unless you have two of them in series, but then they're slightly over the maximum).

You need to figure out your power budget first, then how your are going to meet it. You might be able to use a lighter supply than the worst case (6 stalled motors) would require or even than 6 motors at full operating torque (0.9 Amp each), if you monitor the current on each motor and shut down the or sequence the one(s) with the highest draw. You might even try monitoring just the battery-pack voltage, which won't tell you which motor(s) are stressing, but will let you know that you're unable to supply the total requirement for that present move.

If it were me, I'd use battery voltage sensing first (easier, simpler) to see if I have a problem that I can't solve by sequencing the motors. I might find I don't need to measure each motor's current.

Update:

So its either i use 3.7 bat or 9v. Or.can you suggest any other power source?

A single 3.7v battery won't work well) for a motor that needs 4.8v minimum. Two of them in series will be slightly over the max voltage spec (7.4v vs. 7.2v). Will that slight overage damage or overheat the motor? Will be OK for intermittent use? Maybe, if you're careful to shut down a stalled motor quickly. The alternative is to build a voltage regulator that will drop the 7.4 to around 6v, but the regulator must also be able to meet your power budget, too.

A 9v supply is definitely too high and will need to regulated to within the motors' spec. (I assume you're not considering the four or five cm high ones with snap-on connectors, usually used in smoke-detectors? They have very little capability to deliver current and will be totally unsuitable for your application.)

My first try would be to experiment with 1 motor at 7.4v with a typical (for your project) mechanical forces on it, and see if it overheats and how soon, with how much load. You'd be taking a bit of a risk with that motor but the potential advantage is you might save yourself having to build a high-current voltage regulator.

Update2:

NiMH AA cells are 1.2V

Good call. Just know that Nickle-based batteries have the twin disadvantages of long charge cycles (or degradation of capacity with faster charging) and higher self-discharge rate of about 1%/day. Their advantage to you is that it's easier to make a battery pack whose voltage falls within the motor spec. If this is a one-time project, this may well be more important to you than their disadvantages.

  • Ooo ill keep that in mind Jrobert, but my problem is in my country i cant seem to find any 5v NiMH battery which was my 2nd option if the 3.7 ultrafire bt wont work. So its either i use 3.7 bat or 9v. Or.can you suggest any other power source? And keep.i mind im trying to make the hand mobile – Purnawan Brillian Jan 16 at 14:23
  • NiMH AA cells are 1.2V, use 4 of them in series to make 4.8V, 5 for 6V. – CrossRoads Jan 16 at 17:41
  • @CrossRoads ok i will try to find those in my country – Purnawan Brillian Jan 16 at 23:23

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