# RC Servo, Measuring feedback and current

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#### lompa

##### New Member
i have made a servo controller but i need to obtain some results.

I need to measure that the feedback is working, i have the following ideas

plot weight vrs current draw - were would be the best place to measure the current drawn by the servo?

also

also i may plot different weights being lifted to a specific angle, there for this will prove that the feedback is compensating for the added weights

Can you think of any other methods of obtaining results for feedback or any other kind of results for that matter? (i have scope print outs of the various pulse widths)

#### Russlk

##### New Member
Measuring the current will only tell you the power used. What are you trying to accomplish with the servo? If it is a constant speed, then measure the time to lift a given distance with different weights. If the time is constant, the feedback is working. Depending on the type of feedback, the speed may vary some with weight. You need to explain how that happens.

#### lompa

##### New Member
i am moving 2 servos left to right and vice versa via push switches. i need to obtail some results that prove the feedback is working. i was going to measure the current drawn lifting various weights, there for more current will be drawn when lifting a bigger weight but it should still end up in the same position as a smaller weight

any ideas on any better experiments?

#### Russlk

##### New Member
OK, so it is a position servo. What are you using for feedback? How do you get it to stop at the right position?

#### lompa

##### New Member
its a RC servo so the feedback is built into the unit

#### Nigel Goodwin

##### Super Moderator
lompa said:
its a RC servo so the feedback is built into the unit

I don't really see what you are trying to achieve?, an RC servo is a very mature device, and they work extremely well.

The servo simply positions itself at the point determined by the pulse supplied to it - as long as it's not too large a load for the servo to move (but they are incredibly powerful for their small size).

#### bmcculla

##### New Member
It sounds like you want to make sure the servo actually reaches the setpoint when you load it with various weights. In one of my college classes someone opened up the servo and soldered a wire to the feedback pot - he needed constant information about the current position of the servo. You could do the same thing to measure the actual servo position for given weights.

#### lompa

##### New Member
bmcculla said:
It sounds like you want to make sure the servo actually reaches the setpoint when you load it with various weights. In one of my college classes someone opened up the servo and soldered a wire to the feedback pot - he needed constant information about the current position of the servo. You could do the same thing to measure the actual servo position for given weights.

yes thats what i may have to do. Do you know where abouts i need to add a wire?

I theory the voltage increase from the pot would increase as i add more weight to the servo, am i right?

I also was thinking that to lift a bigger weight the servo would draw more current. Could i not measure the current drawn for different lift weights to prove that the motor in the servo is trying to compensate for the weight increase?

I know the servos work well i just need some proof that they are doing there job

any other ideas?

#### Nigel Goodwin

##### Super Moderator
lompa said:
bmcculla said:
It sounds like you want to make sure the servo actually reaches the setpoint when you load it with various weights. In one of my college classes someone opened up the servo and soldered a wire to the feedback pot - he needed constant information about the current position of the servo. You could do the same thing to measure the actual servo position for given weights.

yes thats what i may have to do. Do you know where abouts i need to add a wire?

I theory the voltage increase from the pot would increase as i add more weight to the servo, am i right?

NO! - the voltage from the pot just indicates the position of the servo, it's unrelated to the load.

I also was thinking that to lift a bigger weight the servo would draw more current. Could i not measure the current drawn for different lift weights to prove that the motor in the servo is trying to compensate for the weight increase?

You could try?.

As far as I understand what you are wanting to do, you just need to attach a mechanical pointer to the output shaft (to indicate how far it travels), and a paper scale under the pointer.

Set a number of repeatable points for the servo, and mark them on the paper scale - then repeat using different weights loading the servo.

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