Motor Noise, Decoupling techniques

"Peter Q." wrote:
> 
> I'm building a small vehicle that uses a couple of optical sensor
> as navigation. There are two MCUs, both Freescale HC08 family. The
> motors are just small 5V DC brush ones that's been driven at 7.0V,
> PWMed to 70% (PWM Frequency 800Hz). There are two motors for a
> differential drive setup.
>
... snip ...
> 
> Therefore I'm wondering what are some of the techniques to
> decouple the motors from the electrical system. I'm about to
> separate the circuit VCC out from the 5 AA batteries and use a
> single 9V batt to supply the electrical power. I'm wondering if
> this is a standard technique?

Definitely consider a totally isolated power supply for the motor. 
You should also carefully isolate all the logic from ground return
drops and noise.  Brush systems are inherently noisy, and will also
radiate.  The brushs are interrupting inductive circuits.

-- 
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: <http://cbfalconer.home.att.net>
            Try the download section.



-- 
Posted via a free Usenet account from http://www.teranews.com

On Sun, 24 Feb 2008 15:49:49 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Sun, 24 Feb 2008 11:09:22 -0800, Peter Q. wrote:
>
>> Hello All,
>>     I'm building a small vehicle that uses a couple of optical sensor
>> as navigation. There are two MCUs, both Freescale HC08 family. The
>> motors are just small 5V DC brush ones that's been driven at 7.0V, PWMed
>> to 70% (PWM Frequency 800Hz). There are two motors for a differential
>> drive setup.
>>     I'm measuring a total idle current of 800mA on the motors, the
>> load current is about 1.2A, and the stall current at 3.5A.
>>     I'm supplying this system with 5x1.5V AA alkline batteries, the
>> electrical system is regulated using a 5V regulator. The motors are
>> driven via a H-Bridge.

...

>For a prototype system it's not a bad idea to power the signal processing 
>and power electronics from different batteries.  This is probably the 
>least-engineering way to get the job done.  One battery pack is certainly 
>cleaner from a product design point of view, but I see a lot of RC toys 
>that separate the two, so I can't say it's not an accepted method.

As others have pointed out, those small alkaline batteries might not
be able to deliver such currents. NiCd accumulators (if you can still
get hold of them) have a low internal resistance, reducing the voltage
drop.

With such loads, the PCB layout is also important. Make sure that
there are separate wide tracks from the motor battery to the H-bridge.
Use a separate ground system for the control system and run it from a
separate 9 V battery. Add a single  connection from the control system
ground to the ground side of the H-bridge for the return path of the
gate drive signals only. This should keep the ground noise out of the
control system.

If you later want to get rid of the 9 V battery, install a small
inverter, driving a transformer followed by a floating rectifier and
regulator to run the control logic. With the isolated control system
power supply, the only galvanic connections between the control system
and the high power system is the gate drive signals and the single
return path between the control system ground and the high power
system, in which only the gate signal flows. 

This should be sufficient to keep any resistive coupled interference
from the control system, but of course, there could still be
capacitively or inductively coupled interference running through the
single ground coupling between the systems (and in the logic system
ground). If this a problem, use optoisolators to drive the H-bridge
and remove the connections between the grounds, so that the control
system can float freely and no interference currents will flow in the
control system ground.

Paul

In comp.arch.embedded,
Peter Q. <pmouseca@gmail.com> wrote:

In addition to what others have written.

Motor brushes generate a lot of noise, standard practice is to add a small
capacitor directly between the motor terminals on the back of the motor.
Do you motors have this? Other confgurations are capacitors from each
motor terminal to the motor housing or a combination of these two (so you
have 3 caps in total). Value can be something like 100pF-10nF or even
larger, but be carefull with higher values if your PWM frequency goes up.


-- 
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

"Computers in the future may weigh no more than 1.5 tons."
(Popular Mechanics, 1949)

Stef wrote:
> 
> In addition to what others have written.
> 
> Motor brushes generate a lot of noise, standard practice is to
> add a small capacitor directly between the motor terminals on the
> back of the motor. Do you motors have this? Other confgurations
> are capacitors from each motor terminal to the motor housing or a
> combination of these two (so you have 3 caps in total). Value can
> be something like 100pF-10nF or even larger, but be carefull with
> higher values if your PWM frequency goes up.

Such noise suppression capacitors should be in series with a
resistor.  The resistor size will depend on the actual motor, but
generally small values (such as 47 ohms) will be suitable.  Not
critical, but this provides a place to dissipate the noise energy.

-- 
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: <http://cbfalconer.home.att.net>
            Try the download section.



-- 
Posted via a free Usenet account from http://www.teranews.com

In comp.arch.embedded,
CBFalconer <cbfalconer@yahoo.com> wrote:
> Stef wrote:
>> 
>> In addition to what others have written.
>> 
>> Motor brushes generate a lot of noise, standard practice is to
>> add a small capacitor directly between the motor terminals on the
>> back of the motor. Do you motors have this? Other confgurations
>> are capacitors from each motor terminal to the motor housing or a
>> combination of these two (so you have 3 caps in total). Value can
>> be something like 100pF-10nF or even larger, but be carefull with
>> higher values if your PWM frequency goes up.
>
> Such noise suppression capacitors should be in series with a
> resistor.  The resistor size will depend on the actual motor, but
> generally small values (such as 47 ohms) will be suitable.  Not
> critical, but this provides a place to dissipate the noise energy.

They never are, atleast I have never seen it. I know of one motor that
can be supplied with a rather large capacitor (220nF on a 5W motor)
across the brushes then 2 inductors and 2 smaller capacitors to the
body after those. 

All other motors I have seen use the 1, 2 or 3 cap method without
resistors. Open up any toy/appliance with a DC motor and I expect you
to find caps only, 1 cap being the most common.


-- 
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

"Computers in the future may weigh no more than 1.5 tons."
(Popular Mechanics, 1949)