[Help] Issues with servo reliability

Other Hardware Motors, Mechanics, Power and CNC
1

I recently bought some new servos, but I am having some issues getting them to run.

I bought some "Tower Pro MG958 Digital Servo's" with the following specs:

  • Weight: 55g
  • Actuator Type: digital servos
  • Servo size: 40.7 × 19.7 × 42.9mm
  • Rotational speed: 0.18sec / 60degree (4.8v); 0.15sec / 60degree (6.0v)
  • Operating voltage: 4.8 ~ 6.0v
  • Stall Torque (4.8v): 18.0kg.cm
  • Stall Torque (6.6v): 20.0kg.cm
  • Stall Current 2.5 A (6V)
  • Running current 500mA - 900 mA (6V)

More information can be found if helpful at the following URLs:

While I am relatively new to Arduino, I have used servos before and have not had this issue.

I have set up a simple circuit as shown in this fritzing diagram:

1626×1389 268 KB

As you can see there are 4AA batterys which (to my understanding) should supply around 6volts of power for the servo.
While I am attempting to control the servo from pin 9.

I am using the following simple source code to try and get the servo to rotate:
http://pastebin.com/raw.php?i=gBqZj88k

When I use other, less powerful servos the source code works as expected and the servo will rotate back and forth.

However, when I connect the Tower Pro MG958 the following things will/can happen at random (note I have 4 and have seen this behaviour for all):

  • The servo will rotate as expected
  • The servo will rotate sometimes
  • The servo will rotate not at all

I have also noticed that when the battery source power is turned on the servos will make a buzzing / vibrating / clicking sound.

Does anyone have any idea what is going on here?

Any suggestions of things I can try to fix this?

Or have I simply been ripped off?

2

For the future please post code in this Forum so we don't need to go to another website.

It may be worth trying to slow the process by changing delay(10) to (say) delay(50) or delay(100) to see if that has any effect.

Are you using brand new batteries ?
While AA NiMh cells produce a lower voltage than alkaline cells they can produce a lot more current.

If neither of these suggestions helps I would suspect poor quality servos - have you any other way to prove that they are OK ?

...R

3

Yes the batteries are new.

I tried playing with the delay but it has no effect on the outcome.

I dont have anyway to prove they are OK, other than attempting to use them.

4

eutasfa:
I dont have anyway to prove they are OK, other than attempting to use them.

I was wondering if you have a non-Arduino way to test them - perhaps with a regular Radio Control system?

Have you tried fully charged AA NiMh cells ?

Are all the connections in your battery holder in good condition ?

...R

5

Normal battery holders won't handle 2.5A reliably in my experience, you have to use
quality battery holders with welded metal connectors, not riveted.

That or the batteries cannot handle 2.5A. Have you measured the voltage
under load?

6

It is probably a power issue or poor wiring connections as the servo behaves properly at times. If you are using a breadboard in your circuit, than its connections could be the issue.