Large Brushed servo setup on the cheap (WIP)

[samco]

Slooowly but surely..

http://www.electronicsam.com/images/...t/mounting.JPG

http://www.electronicsam.com/images/...rt/ampmess.JPG

Finally closed the loop with the pluto last night. (small servo to play with that I mounted a 2540 line encoder on the back). Did the Ziegler-Nichols method here http://en.wikipedia.org/wiki/PID_con...Nichols_method
That seemed to work pretty good (good start anyways). I got a sustained oscolation period of .036ms at a P of 460. So
P= .6*460 = 276
I= 2*276/.036ms = 15333
D= 276*.036ms/8 = 1.242

sam

[unterhaus]

Did you make that h-bridge board on a T-Tech Quickcircuit?

Really looks good so far.

[samco]

Heh - no. I made it on our limp-noodle gantry.. I ended up having to slow it down to 7ipm so that it wouldn't oscillate too much. With emc2 and a microswitch to set tool lengths - that was really slick. Program pauses - swap out to the next mill/drill - machine runs over and touches the microswitch and away it goes.

http://www.electronicsam.com/images/...ostart/top.JPG

Originally Posted by unterhaus Did you make that h-bridge board on a T-Tech Quickcircuit? Really looks good so far.

[acondit]

Sam,

I am really interested in the HBridge board you are building. I downloaded the Eagle files, but the component values for most of the resistors and capacitors are missing. Can you either provide the values or give some guidance as to how you decided what values to use? I am thinking of using much smaller servos so the values may need to change?

Thanks,
Alan

[samco]

Sorry - I am not good at actually dressing up circuits. The gate resisters are 10ohm the resisters feeding the optos are aprox 270ohm. The pull up resisters on the back side of the optos are 10Kohm. The mosfet looking thing on the lower left is a 3 pin 15volt regulator. All the caps are .1uf except the one in parrellel with the boot strap cap which is 10uf.

FDH44N50 N - MOSFET transistors 500V / 44A for the power mosfets.
RURG5060 Ultra Fast Diodes 600V 50A for the free wheeling diodes
IR2111 for the half bridge driver.
MUR120RLGOSCT-ND DIODE ULTRA FAST 1A 200V AXIAL for the boot strap.
LM340T-15-ND 15volt regulator
TLP2530-ND PHOTOCOUPLER DUAL 6N135 8-DIP
.1uf caps and
iirc 10uf electrolytics.
The big cap is 1900uf I think at 350volts (I think)

Couple things
- when there isnt any pwm signal applied to the bridge - both lower mosfets are on. (braking)
- I had a lot of help from a emc developer that does high power circuits for his real job.
- there is no current limiting yet.
- you should find lower Rds mosfets.
- if your going to run higher than 200v you need higer voltage boot strap diodes.
- this cannot have 100% duty cycle pwm signal other wise the boots strap doesn't get charged.
- use at your own risk (of cource)
- great care was made to have a short power circuit loop - to help get rid of stray inductance.

[samco]

This is the little servo I have been playing with to teach myself a bit more tuning.
http://www.electronicsam.com/images/...rt/ampmess.JPG

Here is a picture of the following error just goofing around.
http://www.electronicsam.com/images/...art/ferror.png
This would work great for a mini/micro mill - Tempted to buy one for home.

Now I finally got a decent read on the large servos votage/rpm=.125
So at 150v = 1200rpm <- which is its maximum rating. That would give us a max speed of 200ipm with 2:1 and 3tpi.. (we where going to direct drive it but for ease of retro we are going to belt drive it.) Plus the 150v falls right into the operating range of the diy h-bridge.

sam

[acondit]

Sam,

I am thinking about trying out some servos. I have some 63v 15amp peak servos. Do you think that your boards would be a reasonable match?
I also have some smaller 100 watt servos 30 to 36v (I think). Would these work with your board design?

Are you working on the design more (ie, current limiting)?

Alan

[samco]

The drive 'should' be able to handle 20a continuous at around 150v and close to 40a peak.. But I have only tested it so far at 10a at around 60v.

Remember - no current limiting yet. I was planning on using some fast blow fuses to try to save the mosfets in case of an oops.. I don't know how well in practice this will be.

sam

[JBV]

You could perhaps add something like this + a comparator and a little logic to shut down the output when it draws to much current!?

[samco]

I am a bit leary about adding current limit yet as the guy who helped be design the low inductance current loop of the h-bridge said there are right ways and wrong ways to add current limit. He suggested a aprox $30 magic chip that you would hook in series with the motor that would be used as the current sense circuit. Not messing up the h-bridge by adding a series resister high wattage resister.

But I have not had much time to look into that.

sam

[epineh]

Hey Sam, have you looked into/heard of high side current sensing as opposed to using sense resistors in series with the motor (Low side)? I cannot say I really know what is involved, but I wonder if it involves measuring the Vce Sat voltage of the output FET's.

EDIT :

I did a quick Google and found this :

http://www.national.com/nationaledge/

Don't know if something like this would work, not enough hours in the day to find out right now .

Russell.

[bigeye3107]

hello,
I am a new user of emc.
I bought a pluto-p card.
Do you have a schematic of the mosfet board ?
I learn electronics for this project !

PS : Excuse my poor english, but i'm french guy...