A Basic Servo Tutorial

[westaust55]

I have recently done some searching in the internet for further information about Servo-motors in preparation for my own experimentation having already bought 2 from Jaycar, which did not have datasheets and the wire colours were not red, black and white as used by Futaba or referenced in the Rev Ed PICAXE manual 3.

I decided to compile the information I found into a single document for future reference.

As there has been a number of Servo-motor related queries on this forum in the recent past, I thought this might also be of interest to others.

 

[goom]

Very good and comprehensive document. Worthy of inclusion with the standard PICAXE documentation. Technical, how about considering that?

I have read numerous time of the advisability of having separate power supplies for the control electronics and the servo power. I have never found this necessary in practice. I do however always include decoupling capacitors close to the PICAXE power legs (usually 0.1uF and 10uF).

 

[westaust55]

Thanks goom.

I have spotted a small typo in section 7.1 being in
one program line which has a wrong comment

SERVOPOS 1, 200 ; move Servo toward the clockwise limit - can try higher values like 225

Can fix this later after see what other comments arise.

 

[ValueAdd]

A very useful introduction to Servos. Thank you WA55

 

[westaust55]

I have added a Part 2 under the first post to keep it all together . . .

The additional part covers more step by step with some photos how to endeavour to extend the rotation such as to achieve 180 degree rotation.

 

[lemaxnut]

Thank you very much for your time and effort in preparing such a clear and concise article - as a newbie to PICAXE (3 weeks) with experience with Electric RC, it is only natural to turn to the trusty servo as an actuator in my projects.
However, In the first reading, I did not notice any mention of the Throttle channel which has only positive values in the case of electronic speed controllers.
1) Is this something that is wired into the Electronic speed controller so that it's neutral or Off position starts at 75 ?
2) Also, some ESCs come with a Battery Elimination feature which feeds power back to the radio receiver - would this work with PICAXE ? ( ie could the BEC supply power back to PICAXE as it does with a receiver ?)
3) Am I being too simplistic in reading PICAXE, when it is being used as a servo driver, as being a direct replacement for "Transmitter" ?

 

[srnet]

On the point of powering 5V servos from a 7.2V supply being inefficient with a linear supply, this comes up often in the RC world as quite a few planes now are powered by a small 2S Lipo (7.4v nominal).

The actual differance in efficiency between a linear regulator and a switched for a 7.4v to 5V, is not actually that big, switched regulators are not that efficient when the voltage differance between input and output is small.

Given that linear regulators are electically quiet (compared to a switched regulator) the choice for small planes is often a linear regulator. This is not universal of course, a lot depends on exactly how much current is being drawn, but just be aware that switching regulators are not always the best choice.

 

[srnet]

1) Is this something that is wired into the Electronic speed controller so that it's neutral or Off position starts at 75 ?
2) Also, some ESCs come with a Battery Elimination feature which feeds power back to the radio receiver - would this work with PICAXE ? ( ie could the BEC supply power back to PICAXE as it does with a receiver ?)
3) Am I being too simplistic in reading PICAXE, when it is being used as a servo driver, as being a direct replacement for "Transmitter" ?

There is more or less a standard for the throttle, good job too, for obvious safety reasons.

A lot of ESCs wont arm (allow power to the motor) untill they have a pulse of close to 1ms for a particular period, an obvious safety feature. Many transmitters wont start transmitting until the throttle stick is 'off', again for safety reasons.

Then the motor may not turn until the servo signal is a bit more than maybe 1.2mS or greater.

Beyond that, and it gets more complex, as some ESCs have setup modes to allow you to set the max throttle position etc.

You can use a BEC to power a PICAXE circuit, but be aware that some BECs can be set to 6V, this is not good for a PICAXE. And dont expect the supply from a BEC in a RC setup to be electrically quiet, some BECs are switchers. And if you have a lot of current in use (20A to 50A or more is common) expect more electrical noise.

Although this topic in this very forum;

http://www.picaxeforum.co.uk/showthread.php?20513-Canon-Camera-Remote-Control-Interface-for-PICAXE-08M2

Is a device which plugs into an RC RX and hence gets power from the planes BEC. Never had a problem with this setup, my current aerial photogranphy plane runs at around 45A full throttle, total BEC current is around 1A (the plane has some big servos).