Servos optocouplers and milliseconds

[D n T]

Playing with RC servos again/still, did a search of the forum, all 12 pages that came up( "servo"), time to ask.

I am running 5 servos off a 40X1 PICAXE and they don't play well together.
Basically when I turn a 10K pot the servo will turn, all the maths is good and it works perfectly when I use a single servo
I know servos make lots of noise (signal), i want to seperate the servos from the rest of the circuit.

First question : Do I have to assign ADC 0 on a 40X1 (what is ULPWU?).

Second question: Has anyone used an optocoupler to to reduce the noise?
Ive go a handful of 4N28 optocouplers to try.
Any pearls of wisdom from those who have been before?
Thank you

 

[MPep]

The only way I can see to reduce the noise is to run off completely seperate power supplies. Then the 4N25 is a great way to go. Maybe use a seperate DC-DC converter (5V-5V)? Something like this http://www.meanwell.com/search/spr01/default.htm.

My 2c worth.

MPep.

 

[BeanieBots]

The 'noise' associated with servo is power line dips.
Running from seperate supplies will help a lot but only if the grounding is correct. The servo power must not share any 0v line that is common to the PICAXE or another servo.
Opto coupling is one way around the problem but well thought out power line routing is a cheaper option. I've got many servo projects running up to eight servos from a single PICAXE and a single common power supply with no ill effects so it's certainly possible with a little care. (remember to fit those decoupling caps)

 

[D n T]

Might I ask BB?

"The servo power must not share any 0v line that is common to the PICAXE or another servo."
Do you mean that there should be a capacitor to stop or reduce noise before anything that makes noise joins the 0v rail?

 

[BeanieBots]

Both.

Proper decoupling will help a lot, but not allowing high current fluctuations to pass down any connection that is between PICAXE and source will help more.

To get your head around the idea, draw every single connection between every single device as a resistor and then look/think about what might happen to the voltage at the end of that connection when a high current passes. The same applies when you come to fit your decoupling caps.

Electrical noise can be thought of in the same way as audio noise.
You can block audio quite easily and effectively with the double glazed windows can't you? Putting the double glazed window on the lawn outside your house actually has little effect on the noise. Although that sounds a little silly, placement is just as critical as it is with caps.

 

[D n T]

Thanks BB for you help, agian

I went to my local bits supplier and had a little chat about my problem and I had a look into values for decoupling.
It was suggested that not only should I follow your advice without fail, but that I should also place several extras around the board to moderate fluctuations throughout the circuit, especially with the amount of noise the servos make.

Now my breadboard has a 1uF tantalum where ever current enters the board, a component or even just along the power rails for good measure, and a couple just sitting there looking menacing.
My breadboard looks "tantalumising",or is that tantalumount to a pun?

No optocouplers yet.
The BL##*Y servo don't use 75 - 225 pulses either, these ones prefer to run from 66 - 200, I'm sure I saw a thread dealing with extending the range, but was it worth it? Not yet.

Thanks again

 

[BeanieBots]

A couple of low ESR electrolytics wouldn't be a bad idea but don't just throw caps at it thinking they will solve all your problems. Don't forget that a cap is just as capable of "coupling" as it is of "decoupling". Put in the wrong place, they can make things worse.

Above all, make sure you have a good low impedance power supply. Don't go thinking you can run half a dozen servos from a 7805 regulator!

 

[MPep]

Having too much capacitance is also bad.
Will 'look' like a short-circuit with the initial charging current. Something to be aware of.

 

[D n T]

7805 for running the PICAXE but I have a 3.5A wall plug for the servos at the moment and they don't all run at the same time.
Caps across the supply, the PICAXE power pins, the pot rails and each servo has its own across the power.

 

[Dippy]

Do the power lines +V and Gnd run separately back to PSU?

Have you tried the setup on another PSU?
What quality is the one you are using? Some can get a bit scrappy when loaded up.
Have you tried powering just the PICAXE section from a battery pack or completely independent psu? (Commoned grounds oc)

I'm scared to say it, but a precise schematic posted may help...

 

[D n T]

Hold on to you, whatever, a diagram

I decided to use ceramics instead of tantalums and I am using a fully charged 4.5 volt batteery pack for the PICAXE.
I changed the operating range from 75- 225 pulses to 60 - 200, the servos didn't like anything above 205 but do respond to 58.
It seems to be working better
Did a caveman test on the power supply.
I connected a motor to it as a load and applied a load to the motor, 6 volts, 2.3 amps for 30 seconds (my hand was getting hot), so I think the supply is ok
Precision, measured with a vernier, marked with a chalk and cut with a chainsaw, cave man. Perhaps not, but a diagram for your viewing.
Thank you for taking the time