Simple servo ?

Hi, just after any comments/ ideas ...

On an old topic I mentioned I was thinking of making a simple chart recorder to go with my PicAxe weather station - just starting to think about it more now regarding the servo to drive the pen arm.

Been wondering about making a simple servo rather than just get a ready-built one. For the purpose of just moving a pen arm a ready-built servo seems a bit extreme and power hungry.

All (!) I think I need is:
- Cheap 'toy' motor
- Slotted disk with two (cheap) IR LED/Sensor pairs to detect rotation of the motor.
- L293D motor driver chip
- PicAxe 08M to control L293D chip plus read sensors. Would hold counter of motor revolutions and copare this to desired count and control motor accordingly.
- Worm gear on the motor shaft driving a bigger gear.

Although quite a list I think this could be a lot cheaper than ready-built servo/ or Stepper motor solution and I can design it for my purpose.

One issue I can see though is that the pen arm would need to be 'zeroed' on power-up unlike a ready-built servo that uses a Pot for feedback. Plus there might be issues with detecting revolutions at high speed - may need to gear down the slotted disk. Plus of course it's all complicated compared to a self-contained ready-built unit !

Still might be woth trying though .

<i>- Cheap 'toy' motor
- Slotted disk with two (cheap) IR LED/Sensor pairs to detect rotation of the motor. </i>

Having recently taken apart a PC's DVD Player, that's exactly what was used to move the head mechanism via a worm gear. A micro switch closure indicated the end stop.

Your idea sounds fine. In fact, I would NOT recommend a ready made 'hobby' servo because their accuracy is not too hot. Most only use gain in their loop so posistion is a function of force. Then their is gear backlass which can be quite high unless you buy a very expensive one. Even if you can get around all that, the usable range and hence resolution only equates to 7 bit. Don't get me wrong, hobby servos are great little devices, just not suited to your application.
You might like to consider analog feedback using resistance wire.

Thanks. Microswitch sounds good - could use another LED/Sensor pair though microswitch probably simpler.

Resistance wire is a possibility although I think I'm more attracted to the precision of a slotted disk.

I've got a bit to think about - thanks again

Also, thinking about the L293D chip to control the motor - I guess there's no reason why I can't pulse the input to not only control the direction but also the speed?

I'm thinking that there is likely to be overshoot and oscilaation of the motor unless I slow it down when it's approaching the right number of revolutions.

Would the mechanism which moves the head on a scanner work with a pen glued to it ?

That mechanism should come complete with a stepper motor and end stop detection built in.

Good idea Hippy, I expect it might work. Call me mad, but I think I want to try the difficult approach and build it from scratch - more sense of achievement that way ;-)

Cheap motors tend to cause problems mainly through noise that can be near impossible to filter out, A solar motor would be a better bet and give more grunt for less current.

The gear backlash can be reduced if a threaded rod is turned by the motor with a nut to carry the pen assy. If 2 nuts are fitted with a rubber washer or a spring between them you get rid of backlash.

You also get a bigger reduction ratio this way. The counter wheel can be mounted on the screw to give a lot of counts per mm moved.

To make the servo from scratch, first you get some copper ore and some iron ore...

Actually, we use Futaba S3004 servos with the 08M and find very little slop in the gears and accurate angular position for a given input. They might be considered a bit pricey at $10 US. But, of course, they are not homemade. I can understand wanting to make your own. Very cool.

Thanks guys. Well I didn't quite mean from the base metals ;-)

Yeah, I thought about a threaded rod and nut. There's really quite a few ways of approaching this and it's going to be a while before I get round to making it.

While it may eat up too much current given the drag of the things...what about getting one of those PC mount trimmer pots that are supposed to be turned with a fiddlestick, and put it over the motor (I would suggest stepper motor) shaft, and run that to the ADC iput on the picaxe. This will give you position feedback, and you can save codespace by running the step sequence &quot;loop until adc=blah&quot;. Not exactly a servo, but will give you absolute position ffeedback, all selfe contained in one unit, and only one extra wire into the picaxe.

Although an interesting concept for a Picaxe, at the end of the day you've still got the ancient chart recorder bugbear of paper reloading &amp; -argh- dried up pens. To make the point,feedback from a recent Hellscreiber article of mine lead to a &quot;wit&quot; emailing that perhaps the real reason the 1942/3 German army gave up in Stalingrad related to their Feld-Hell transceiver running out of paper tape...
( See =&gt; www.manuka.orcon.net.nz/hellrefs.htm )

If you really want a running display consider an LCD. Those home weather stations now abounding have great &quot;past day&quot; displays indicating how atmospheric pressure has varied. My own one runs for ~a year on a few AA cells. Stan

Rev-ed make a printer firmware chip that I have purchased but never played with (yet). It apparently prints out through a dot matrix printer or an older inkjet that has a parallel port. You can print out line by line so you could make it print a black box on each line which when looked at horizontally would show a graph over time.

But if you want to make a printer on your own, you may want to have a look at thermal paper going over say 10 separate selectable coils of fine wire. This would give you 10 discreet steps and have few moving parts. Could be used as a simple data logger of sorts.

Stan, I take your point, but I wouldn't have thought an LCD with high enough resolution to display a chart is that cheap ?

If you went to your local hobby shop j'm sure that they would be able to point you to an RC model flying enthusist in your area who may just have a few busted ones that they wouldn't mind giving away. I know I have a few busted ones but I'm a bit far away in Oz.

Another possibility is to use the carriage from an old inkjet printer. Attach a pen and use the picaxe to control the head and paper feed motors.

I am currently developing an LCD barograph display. I am using a Lascr SP5-GFX1 64 by 128 pixel display. It cost &#163;22 from Farnell, <A href='http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=4359549&amp;N=401' Target=_Blank>External Web Link</a>
It has been challenging to get the display working but I am near to completing the project. I plan to describe it here in detail when it's done.
Derek

That display looks very tempting. I'll be interested in your description.

In fact having now read up a little on the SP5-GFX1:

Are you also using an SPLC501c Display controller chip, as mentioned in the display datasheet? Or are you somehow controlling the display directly from a picaXe? Are you displaying fonts and if so is this via your own font data stored in memory?

Sounds very ambitious to me, but a nice little display - oh and no paper !

The thing about paper is that the record is still there after things have moved on. For example, when Michael Fish got the forecast wrong for Southern England.