go_away

Author Topic: Opto Isolating Motors  (Read 2242 times)

0 Members and 1 Guest are viewing this topic.

Offline dualTopic starter

  • Jr. Member
  • **
  • Posts: 43
  • Helpful? 0
Opto Isolating Motors
« on: November 03, 2009, 01:04:24 PM »
I have questions regarding optoisolating my motors.

I am using the TB6612FNG Mosfet Driver from Sparkfun to drive my two 6V DC Motors. Here is a link to the product description of the Motor Driver from SparkFun.
http://www.sparkfun.com/commerce/product_info.php?products_id=8905
I will be using a 6 pin Opto Isolator called H11L2 to isolate my motors.
Here is the datasheet that explains more:
http://www.datasheetcatalog.org/datasheet/motorola/H11L1.pdf


The problem is the motors reset the board when I try to run the motors at some pretty high speed. So I will be implementing some opto isolation circuitry using what I have from above. However, the opto isolators that I have inverts the input. I’m thinking of maybe using a NOT gate (74HC04) in either the input or the output of the opto isolator or maybe an inverting op amp in the output of the opto isolator or maybe just another opto isolator to invert the inverted output from the first opto isolator in the sequence.

I have attached a schematic that includes how I was thinking of inverting the PWM input before the opto isolator inverts it.
Please give me some comments on this.

Thanks,
Yan.

Offline chelmi

  • Supreme Robot
  • *****
  • Posts: 496
  • Helpful? 15
    • Current projects
Re: Opto Isolating Motors
« Reply #1 on: November 03, 2009, 01:55:01 PM »
Well I think you don't need to invert anything, you can manage it in software: a 100% pwm will become a 0% speed and 0% will be full speed.

Offline waltr

  • Supreme Robot
  • *****
  • Posts: 1,944
  • Helpful? 99
Re: Opto Isolating Motors
« Reply #2 on: November 03, 2009, 01:59:57 PM »
1- You will need a current limiting resistor in series with the Opto's LED to set IF (see the Opto's data sheet).
2- You can have the LED/resistor connected to Vcc and the uC pin pull low to turn on the Opto's LED. This will eliminate the inverter (NOT gate).
3- If you need to isolate the PWM lines then you also need to isolate the direction control lines to fully isolate the power and ground.
4- The Opto isolation does require have two power supplies or batteries. Not always a bad thing.

5- You may be able to keep the processor from resetting by keeping the battery connection to the motors heavy and short. A large value Cap at the motor control board should also help as well as at the processor.
6- A small value cap (1000pF) directly across the motor terminals could also help suppress high frequency spikes.

Offline dualTopic starter

  • Jr. Member
  • **
  • Posts: 43
  • Helpful? 0
Re: Opto Isolating Motors
« Reply #3 on: November 03, 2009, 02:47:04 PM »
Thanks waltr, that really helps. One thing though, if you would notice the 270Ohm resistor in the opto isolator that ties Motor VCC to Vo which is an open collector output. I believe the pulls the Vo high. Should I tie that to Motor GND instead when I use number 2 in your response?

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Opto Isolating Motors
« Reply #4 on: November 03, 2009, 03:42:17 PM »
Hi,

No need for opto-isolators (which are cumbersome and might not even help).
Try this instead: http://That.Homepage.dk/PDF/Motor Noise Suppression.pdf
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline waltr

  • Supreme Robot
  • *****
  • Posts: 1,944
  • Helpful? 99
Re: Opto Isolating Motors
« Reply #5 on: November 03, 2009, 04:12:01 PM »
Thanks waltr, that really helps. One thing though, if you would notice the 270Ohm resistor in the opto isolator that ties Motor VCC to Vo which is an open collector output. I believe the pulls the Vo high. Should I tie that to Motor GND instead when I use number 2 in your response?
No, the Opto output side of the schematic is correct.
The 270 Ohm resistor does pull the output high, an open collector can only pull low so the resistor is needed.

My #1 & 2 responses are for in Opto input side only.

My #5 & 6 implied (sorry I didn't state it clearer) what Soeren suggests. Don't use the Optos and suppress the motor generated noise.

Offline Webbot

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 2,165
  • Helpful? 111
    • Webbot stuff
Re: Opto Isolating Motors
« Reply #6 on: November 03, 2009, 09:46:47 PM »
I can't see any reason why you would need to isolate the PWM line or any other input line
The chip has its own Vcc and the motors have their own supply.

Presumably you are powering the motor driver Vcc with the same regulated power supply as the microcontroller. Are you using a separate battery pack for the motor supply?

So always attach the 'capacitors' as Soeren said (should increase motor life anyway) to restrict noise.

If you are only using one battery pack then it may be that the motors are taking all the current and so the regulated power to your mcu and motor controller dips and maybe even resets the microcontroller.

« Last Edit: November 03, 2009, 09:48:52 PM by Webbot »
Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

Offline dualTopic starter

  • Jr. Member
  • **
  • Posts: 43
  • Helpful? 0
Re: Opto Isolating Motors
« Reply #7 on: November 04, 2009, 07:51:14 AM »
Soeren, question about the Noise Suppression diagram that you have attached: The L1. I don't quite get the Powdered Iron Ring description on top of it. How or where do i get my hands on one? Why do you say the Opto Isolators might not even work?

Offline Webbot

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 2,165
  • Helpful? 111
    • Webbot stuff
Re: Opto Isolating Motors
« Reply #8 on: November 04, 2009, 12:56:04 PM »
The PWM and other input pins all work at TTL levels - low current. Your circuit may work - but its not required.

You can think of the chip as just turning a relay on and off. The motor supply goes via the relay to the motor. So the amount of current used by the motor is going through the relay but makes no difference at all to the PWM logic that is turning the relay on and off. So effectively the relay is already 'isolating' the input pins anyway.

The main thing on the diagram is the 3 capacitors. One between the motor terminals, and one from each motor terminal to the motor housing.




Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

Offline dualTopic starter

  • Jr. Member
  • **
  • Posts: 43
  • Helpful? 0
Re: Opto Isolating Motors
« Reply #9 on: November 04, 2009, 01:48:03 PM »
Okay, by the way, on the Diagram, to what does the B+ correspond to? is that the Motor Battery + ?

And i think i know how my opto isolators work anyway, I'm still concerned about that inductor in case I actually implement that circuit. What inductor is that?

Offline chelmi

  • Supreme Robot
  • *****
  • Posts: 496
  • Helpful? 15
    • Current projects
Re: Opto Isolating Motors
« Reply #10 on: November 04, 2009, 02:09:33 PM »
And i think i know how my opto isolators work anyway

If you're referring to the last post from Webbot, he is not talking about the optoisolator but about he motor driver.
And it is showing why the optoisolator is useless in this case. Read his post again and it should be clearer :)

Chelmi.

Offline Webbot

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 2,165
  • Helpful? 111
    • Webbot stuff
Re: Opto Isolating Motors
« Reply #11 on: November 04, 2009, 02:12:47 PM »
Okay, by the way, on the Diagram, to what does the B+ correspond to? is that the Motor Battery + ?

I've never had to use that inductor technique - so not sure.
The only other comment I'd make is that the diagram says the capacitors should be at least 16v - it should really be 'at least 2 x the motor voltage'
Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Opto Isolating Motors
« Reply #12 on: November 05, 2009, 04:03:31 PM »
Hi,

The only other comment I'd make is that the diagram says the capacitors should be at least 16v - it should really be 'at least 2 x the motor voltage'
Motors are 6V, so I think you'll agree to what I wrote, considering that caps usually come in voltages of 6V, 10V, 16V, 25V etc. (approx. 1.6 x the previous one in the row).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Opto Isolating Motors
« Reply #13 on: November 05, 2009, 04:16:11 PM »
Hi,

The L1. I don't quite get the Powdered Iron Ring description on top of it. How or where do i get my hands on one?
If you had only read the parenthesis ;)
Go rip one from an old PC power supply unit - the ring cores used in  a PC-PSU are powdered iron, recognized by the yellow coating usually visible through the windings on the outside, if they're not wound in several layers.
It just need as many windings that you can comfortably fit and you may be lucky to find one allready wound with a suitable gauge copper wire, so that all you have to do is desolder it and use it as is.


Why do you say the Opto Isolators might not even work?
Because I'm fairly sure that your noise problem is transferred on the power line, not the signal lines.


B+ is short form of: Battery, Positive Terminal.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Webbot

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 2,165
  • Helpful? 111
    • Webbot stuff
Re: Opto Isolating Motors
« Reply #14 on: November 05, 2009, 04:26:10 PM »
Hi,

The only other comment I'd make is that the diagram says the capacitors should be at least 16v - it should really be 'at least 2 x the motor voltage'
Motors are 6V, so I think you'll agree to what I wrote, considering that caps usually come in voltages of 6V, 10V, 16V, 25V etc. (approx. 1.6 x the previous one in the row).


I'm not disagreeing - just trying to state the 'generic' case in case someone finds this thread but has different motor voltages.
Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

Offline Webbot

  • Expert Roboticist
  • Supreme Robot
  • *****
  • Posts: 2,165
  • Helpful? 111
    • Webbot stuff
Re: Opto Isolating Motors
« Reply #15 on: November 05, 2009, 04:28:17 PM »
Because I'm fairly sure that your noise problem is transferred on the power line, not the signal lines.

Dual - can you confirm whether you are using the same battery to power the motors and the motor controller - and what about your microcontroller board?
Webbot Home: http://webbot.org.uk/
WebbotLib online docs: http://webbot.org.uk/WebbotLibDocs
If your in the neighbourhood: http://www.hovinghamspa.co.uk

 


Get Your Ad Here