Building a Penny Pusher Arcade Game

Gallery, Projects and General > Project Logs

<< < (2/23) > >>

Bluechip:
Hi Chris

Been there ... the motor armature is resonating at an audio frequency (ies).
Some motors just don't like PWM :bang:

Once had a Volvo truck radiator fan motor that gave no end of aggro.

Had to drop the PWM base freq. to 40-ish Hz, even then it was not good. Tried going up in freq. but all it did was squawk chromatic scales as you changed the revs. Amusing at first but then became irritating ...

Do you know the base freq. on the Maplin controller ?

Does it have a suppression diode on the O/P ?

You MIGHT try blunting the O/P by slapping a GOOD cap. across the motor, the resonating voltages can be very high, IIRC I once shut one up a bit with a 8uF motor start cap. Don't use an electrolytic, it may well have a brief and brutish life.

Bit of a bugger buying off-the-shelf stuff, you're more or less stuck with it.

Which is why I nail my own junk together.
At least if it works I can feel triumphant, or alternatively, smack it about until it does ... and if it never works, sling it under the bench and pretend it didn't happen ... :scratch:

Dave

raynerd:
Hey Bluechip

On the spec sheet for the regulator it says edge to edge frequency: minimum 1136hz, max 1250hz, average 1200hz - nothing about a base frequency!

Any more thoughts.... What do you mean by a "good" cap?

I'd prefer to use the Pwm controls if I can.

Chris

S. Heslop:
Hey this looks interesting. I've always secretly wanted to build an arcade machine similar to my hero's, Tim Hunkin, but the thing stopping me is visions of trying to goad reluctant family members into trying it out. The elaborate piggybank is a great way to give it a purpose beyond entertainment.

Bluechip:
Chris

Pic I got from here

http://abertoatedemadrugada.com/2013/04/o-que-e-o-pwm.html

Top row, if you imagine the pulses are right to left in time. the LH edges are all the same.
So, the frequency is F = 1/t. This is the base frequency, otherwise known as the PRF. ( Pulse Repetition Frequency ).

To take an easy eg. PRF is 1 kHz, if I have a pulse length of 1mS, the trailing edge of pulse 1 will be co-incident with the leading edge of pulse 2. ie. It's on all the time. 100% modulated.

If your doo-dah is 1.2kHz, then that is well within the audible range.

I do PWM with pic 16F886, and you can get a PRF of several kHz which takes ( hopefully ) most of the squawk well above audible.

BTW that motor is not on a desk top is it? Not got a sounding board effect have you?
I have some steppers that kick up the most hideous clatter until I pick them up ..

Good Cap ??

OK

If you shove a sharp edge pulse through an inductor ( your armature winding ) you will get a back e.m.f. the voltage dependent on the rate of change of current.

ie. -V = L x ( di/dt )

Numbers ??

Inductance = 10mH

Pulse rises from 0 to 5A in 100uS

-V = 0.01 x ( 5 / .0001 )

Some 500V - ish of nasty voltage spike ??

You need a cap. that is made to tolerate such nonsense ... not you average electrolytic eh ??

I have no idea what your motor inductance is or what the rate of change of current is either, but the effect IS there to some extent or other ...

The preceding stuff is from memory BTW, but I think it's right ... :scratch:

Poor 'owd codger ...

Dave BC

Bluechip:
Just in case you're not really interested, and seeing as it's too damned hot in the shop ... :thumbup:

Did a couple if piccys of the effect of driving a square wave current through a 22mH inductor.

Done on my venerable Hitachi V423 oscilloscope, vintage 1985 ( 1985 AD I might add. If it was much older it would be a fossilloscope, but I won't mention that. )

Using the 1kHz 0.5V pk calibration jack. which I doubt is capable of driving much in the way of amps ..

Shows the ringing which the choke causes, as you see they are several times the frequency of the driver pulse, so, if you make your PRF high enough, most of the junk is hopefully above audio ..

Dave BC

Navigation

 Message Index

Go to full version