Servo basics, position hold etc

Projects General Guidance
1

I have an ambitious plan to change my car's heater control from a muscle driven version to a servo&arduino driven version. First idea was to use stepper motors, but since very low accuracy is needed, I changed the plan to servos. And, I guess, the stepper motors do not have any mechanical "home" position, which I think I'll need in this case. Needed torque is also quite low, just to move the heater flaps.

Since the servos are new to me (not counting one RC monster that I had years ago) there are couple of questions.

  • If I move a servo to some position, will it hold it's position? Will the Arduino still keep sending the pulse or how?
  • Are there any current limitations etc. in how many servos can operate simultaneously? I think four servos are needed in this plan.
  • Is there any sense to use 12V servos (are there any?) and can they be controlled with Arduino PWM?
2

  • You can easily find 5-8v servos, also 12v versions are available.

  • The Arduino servo library can handle 12 motors. Your servo power supply must be able to supply the maximum currents these servos require.

  • The Arduino tells the servo to go to a position then keeps sending that PWM value for the servo to maintain a position.

3

Ok, I see. What happens when power is switched off? Will they hold their position? And what happens when power is switched on? Do they initialize themseves into some kind of "home" or what?

4

When the servo is detached and servo power is turned OFF, the servo position stays as is.

When servo code is detached, then the servo power turns ON (when the servo PWM is maintained then attached) the servo goes to the PWM position. If the servo was at that PWM position then there is no movement.

5

How about those movement physical limits? How to determine them? I mean when power is off and servo position may be whatever it is. When power will be on and I want to drive it, let's say, into middle position. Does the servo know what was its starting position? Or how...

Or do I need to first drive it the max movement amount into one direction to be sure that limit has been reached to get the accurate position?

Or do they have a "home" position?

6

The servo compares where it is to the PWM position it is told to go to.

Movement results from differences in these two.

Read this:

Use myservo.writeMicroseconds(testPosition); to calibrate to the hard stop limits.

BTW, lots of videos on YouTube on servos.

7

Thanks!

8

the trick is to save the last servo position command in EEPROM or flash and resend that position during setup()

9

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