Pololu G2 current sense output for Spindle load meter?

More
15 Dec 2018 19:06 #122452 by 3D-Master
Hello,

i am currently using a Pololu G2 PWM driver to control my Servos and it has a current sense output (approx. 20 millivolts per amp) could i use my 7i73 analog input for this? what resolution do the 7i73 analog inputs have? If not what would be suitable/easy to implement and fast enough? I already use 2 of the 4 analog inputs for spindle and feed override using Potentiometers but the voltage coming out of the Pololu driver will never be 3,3v (that would be 165 amps, maximum is 21 amps continuous/current chopping @50amps). I want to implement a Spindle Load meter and maybe also axis load meters as well. (if there is a board out there compatible with Mesa hardware which has a minimum of 7 analog inputs please let me know)

thanks

Please Log in or Create an account to join the conversation.

More
15 Dec 2018 22:02 #122456 by PCW
It sounds like a direct signal from the current sense resistor
(too small signal), plus theres probably quite a bit of common mode
noise (since the PWM power ground = current sense common)
and enough ground bumping that you might damage the 7I73
analog inputs if directly connected.

One solution is to add a differential amplifier (OpAmp+ 4 resistors) with a gain of 10 or so you can reject the common mode noise, and amplify the full scale signal, and protect the 7I73 analog inputs from voltages < -.6 and > +5V

Please Log in or Create an account to join the conversation.

More
17 Dec 2018 00:51 #122498 by andypugh
I am currently playing about with one of these, though I am having some trouble with the SPI comms.
www.ebay.co.uk/itm/ADS1256-8-Channel-24B...ion-ATF/192428635947
I suspect that it might not be very easy to interface with a Mesa BSPI, due to the setup required so probably an Arduino and a UART would be required.
(I keep meaning to look in to the possibility of an Arduino acting as a Mesa Smart Serial slave)

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 16:47 #129265 by 3D-Master
i have a Arduino Mega 2560 laying around, would this work? It has 16 analog in pins. I once had a program on the Arduino which sent signals to linuxcnc via USB so i guess sending float values to Linuxcnc should be doable as well?

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 16:57 #129267 by andypugh
Yes, that should work.
I guess you used: emergent.unpythonic.net/01198594294 ?

But, amplifying and using the 7i73 seems like an easier answer given that the analogue input ranges of the 7i73 and the Arduino are about the same.

You can buy off-the-shelf op-amp amplifier boards if you don't want to build your own (but it is very do-able with dead-bug construction) www.sparkfun.com/products/9816

That might have more gain than you want, you would have 2V / A into a 3.3V ADC

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 18:52 #129274 by 3D-Master
so basically you connect the current sensing output from the Pololu driver into the input from the opamp and the output of the opamp into the analogin from the Mesa card?

thanks

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 19:18 #129276 by andypugh

3D-Master wrote: so basically you connect the current sensing output from the Pololu driver into the input from the opamp and the output of the opamp into the analogin from the Mesa card?


Yes.

If you use a bare op-amp ship with a pair of resistors it can fit in a cable under heat-shrink even.

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 19:54 #129280 by Mike_Eitel
Quite sure it has to be a rail to rail opamp.

Please Log in or Create an account to join the conversation.

More
22 Mar 2019 21:41 #129298 by andypugh
Yes, 5V and rail-to-rail.

Please Log in or Create an account to join the conversation.

More
23 Mar 2019 14:08 #129362 by 3D-Master

Please Log in or Create an account to join the conversation.

Time to create page: 0.091 seconds
Powered by Kunena Forum