Common Voltage for PICAXE and Servo Question

[mackingu]

I posted a question here not too long ago about a project I have that rotates a servo back and forth (http://www.picaxeforum.co.uk/showthread.php?t=8397). I just recently bought a few more parts to expand on it, which has now prompted further inquiry. In Manual 3, page 6,the “Standard Circuit 2” shows a circuit that connects a PICAXE to a Darlington IC which is then used to drive a motor. On page 18 of the same manual, the example described to connect a servo to a PICAXE states that, because of the noise produced, the power source for the servo and the power source for the PICAXE should be separate. Does this mean that a servo creates more noise than a cheap DC motor? I’m asking because I was hoping to use a single 7805 to supply 5V to both the servo and PICAXE and cannibalize my other 7805 for a second PICAXE/servo combination (the current setup I’m using has a two 7805’s – one for the PICAXE, the other for a cheap HiTec servo)

Thanks

 

[goom]

For a 3-screw model boat (a Canadian icebreaker that was never constructed) I have built a Picaxe based rudder/throttle mixer. The output from the mixer feeds the rudder servo and 3 Picaxe based reversible speed controllers. A 7805 regulator (12V SLA battery source) powers all 4 electronics boards and the rudder servo. All Picaxe V+ pins are connected to ground via a .1mF capacitor as generally reccomended (and probably essential). The 7805 has 47mF capacitors to ground on both input and output, and an additional .1mF capacitor on the output.
No problems so far.
Simpler systems (just speed controller and rudder servo from a common 7805 power source) have also performed without problems.
Probably a good idea to keep the servo from close proximity to the microcontroller.
It would be quite easy to breadboard, so why not just try it.

Kevin

 

[pma32904]

Goom / Kevin,

Is there any additional information on your mixer and reversible speed controller? They might come in handy for a robot project I'm working on.

Thanks,
Paul

 

[goom]

Paul,
Private message sent to you. Email me for details.

Kevin

 

[mackingu]

Probably a good idea to keep the servo from close proximity to the microcontroller.
It would be quite easy to breadboard, so why not just try it.

Kevin

I'm going to try it out sometime on the weekend; however, my problem is space - the PICAXE/VR will be in very close proximity to the servo (around 2 inches) due to packaging.

Thanks

 

[profmason]

Most decent quality small servos will run fine off the same PS as the Picaxe. I assume you are setting up your voltage regulator with pair of filter capacitors on the input and output side (about 22uF and 0.1 uF) it also helps to put a 0.1 uF capacitor between power and ground to the servo (As physically close to the servo as possible) If you are trying to run big servos (>100 oz inch) you might run into problems with inductance and back emf causing your picaxe to crash.
Here is an old project using a cheap GWS 57 oz inch servo running off the picaxe power supply. Look at the picture with the can!
http://profmason.com/?p=97
The robot it ended up on still works
http://profmason.com/?p=188
and will probably be competing in the January RSSC robotics competition.
http://profmason.com/?p=408

have fun!

 

[mackingu]

Nifty 'bot, Profmason! I'm jealous.

I'm using two, previously loved, Futaba FP-S148 servos (42oz-in) and I setup a voltage regulator like the sample given in one of the PICAXE manuals, and the other like the sample provided in the 3rd edition of "Robot Builder's Bonanza" - both seem to work fine. I'll probably stick with the 'AXE version as it is the one supplied by RevEd.

As I'm rather new to the electonics/robotics/uController scene, I was wondering if a diode, like the 1N4001 that's always mentioned in the PICAXE manuals, would curb the back emf problem.

 

[slurp]

I guess the suggestion to split the supply is based upon you running servo's on 6v.

From experiance, it's not so much the noise that causes problems when running from the same battery supply - it's the brown-out under load as the batteries become "worn".

You may find the battery can happily drive the picaxe circuit or the motor/servo but over time torques may be just enough to dip your voltage and reset your controller. Bulk capacitance can only go so far but if you've room for a 9v battery and regulator for the control side it'll make all the difference in reliability.

I have found that even with a fresh Li-poly battery, capable of 10A delivery, there has been cause for picaxe on a 1A motor - the cause not discrimitaing the regulator and motor supply effectively such that voltage drops still occured.

regards,
colin

 

[ljg]

YMMV, but I have been running 2 Futaba S3003 servos with an 08M without splitting the supplies. Power is a cheap wall wart putting out around 8V.

the board uses a 7805 regulator that is filtered with a 330µF electrolitic cap on the supply side (biggest I could fit) and a 10µF tantalum on the output side and a .1µF ceramic right between the power and ground of the 08M socket. Servos , chip, and an IR remote decoder all run run off 5V with no problems.

I've never stalled the servos, but in normal operation there is no glitching or resetting. Two other fellows are using the same circuit without problems under testing.

(The application is for model railroad scene automation )

 

[slurp]

YMMV, but I have been running 2 Futaba S3003 servos with an 08M without splitting the supplies. Power is a cheap wall wart putting out around 8V.

My problem was needing to break free of the wall... a transformer will have a much lower internal resistance and will give a much better supply than battery.

In the first instance I had AA type batteries, before taking up the 9v separation.

To make the li-poly and regulator work well I needed to address the size of conductors to avoid resistance effecting the voltage drop and ensure that I didn't have a -ve loop bypassing the the low side of the regulator. The low drop-out regulator I used could not operate effectively of their was another current path. (Doh!)

regards,
colin