Mg996 Power supply issue

Other Hardware Motors, Mechanics, Power and CNC
1

Hi there,

I have been working on a walker concept using 3d printed parts and a selection of servos.

It has six legs. with two servos to operate each leg, a total of 12 Servos.

the shoulder section of the bot is controlled using MG996 Servos.

The Elbow sections of the bot is controlled using standard 9g Servos.

Power is distributed via a distribution board (PDU) made of vera board.

i have created a debug sketch to allow me to test the individual servos via serial commands.

i will send a command that involves a letter and a number. The mega will peek at the serial bus and use parsing to extract the necessary angle it needs to move the servo.

The 9g servos worked perfectly without fault however the MG996 Servos have not moved at all.
i have tried a number of power supply set ups.

First was a 12V fed 7805 with decoupling capacitors fed straight to the PDU in this test the 9g servos worked fine even when sending a string with data to move all 6 9g servos.
But no response from the MG996's.

I assumed a current supply issue as 7805's only supply 1-1.5 Amp constant and also there was a heating issue as well.

Next was a small Adjustable SMPS that i adjusted to 5V which could supply 5 Amp max. no response from anything. I assumed this was faulty.

Finally i moved onto a salvaged Computer Power supply that says it can supply 5V at 14 Amps max. This would cut out straight away as the turned the 5v standby on the Computer Power supply High.

I then started to remove Servos from the my vero board PDU and it turns out that as soon as i connected any of the MG996 Servos to the PDU the Computer Power supply would cut out. all the 9g Servos Still worked perfectly fine.

(have used MG996 Servos before but only two on a robotic arm and a 5V power supply and they worked fine(i have tested this set up with one of those two servos that i know worked with no success)

Has anyone had any issue with supplying MG996 Servos like this before?
Or anyone got any Techniques to supplying power to this things that work well.

Any Information would be fantastic.

Thanks,

Joe.

2

It does sound like a current supply issue rather than a voltage supply.

I proper higher current supply rather than a recycled computer PSU may be in order.

Also a single 7805 would certainly not suffice for all those servos so something capable of handling the total current at maximum draw would be in order

Add the max draw for each item together to get the number you need and then add a safety factor of around 10% (minimum) to that.

Many people have reported similar types of issues with multiple servos which in most cases is resolved with better power supplies.

Oh and dont forget your common ground back to the Arduino as that too can cause random issues.

3

Are you attaching all 6 of the Mg996s within a few milliseconds? If so, that will cause a humongous current drain, do a 200 millisecond delay between them.

for(int i = 0;i < 6;i++)
{
   servo[i].attach();
   delay(200);
}
4

Thank you for your quick responses.

I found i was only getting about 5.363 Amps out of the Computer Power supply before it would cut out.

Im gonna try get dedicated 10 Amp and 20 amp SMPS and see if that resolves the issue permanently.

And that's an interesting idea, it hadn't occurred to me to eek out the initial current draw so i will give it a go and update my result.

I have also just found in another post that some mg996 servos may be packaged in their cases too tightly enacting extra load on the motor so i will open one up and investigate that as well.

Thanks again,

Joe.

5

Your power supply must be able to provide at the very least the servo start/stall current value times the number of servos that are moving at once.

For some of the MG996R servos, the start/stall current is as high as 2.5 Amperes, so a 10 Amp power supply can handle only four at once. With the smallest servos, you still need to budget 1 Amp per servo.

6

HAZOPjoe332:
I found i was only getting about 5.363 Amps out of the Computer Power supply before it would cut out.

Which 5volt output.
The 5volt standby pin can't provide significant current.
The main 5volt output, providing you have set the power-enable pin, can usually provide dozens of amps.
Ratings are on the sticker of the supply.
Leo..

7

So I received my 20amp and 10amp power supplies.

Th 20 amp power supply had no issue with initiating all servos at once.

The 10 amp struggled but with JCA34F's staggered power up addition it worked no problem. I have a feeling however that when I have them all triggering at once to walk their will be issues but I have to redo the positions as the shoulder servos are too tightly packed together.

Thank you everyone for your help.

Joe.