From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Jerry B. Hillman" 
>
>Hi all,
>Since the military has seen fit to keep my son overseas, it is up to me to
>figure out how to motorize my scope.  Not wanting to be encumbered by a
>computer and all that entails, I decided to build a simple motor drive
>system.
>Months of researching the electronics involved in controlling a stepper
>motor system, versus a dc motor system led me to decide to use the dc motor.
>I finally found a simple circuit to control the motor. Pulse Width
>Modulation.  Now, I have very little electronics experience, and zero
>experience in putting a circuit board together.  However, after about two
>hours of soldering, preceeded by numerous hours on the internet learning how
>to read the circuit diagram, and all those little symbols, I was ready to
>test it.  I hooked up the motor, then the battery, turned it on and
>proceeded to test the system.  It worked first time.
>Now, two questions.  I first noticed that the motor produces a tone that
>increases in pitch as the motor speeds up.  It disappears completely at
>about 3/4 speed(except perhaps in the range dogs can hear. Any answers as to
>why this happens?

Hi
 This is a resonance and filtering issue. If it is really
loud, you need to change some parameter of the signal you are using or the
resonance could cause motor failure due to things rattling and fatiguing.
Using pulse modulated can cause some additional sound.

>Secondly, how much torque should the motor produce.  Even at full speed, I
>can stop the motor shaft with my fingertip, although it is a little
>difficult.  At it slowest speed, it is quite easy to stop. My understanding
>was that PWM control didn't lose torque at the slower speeds.  That was one
>of the reasons I chose this control system. This is a 6volt motor running
>off a 5 amp battery.
>Any of you electronic experts out there that can answer this would be much
>appreciated.

 Pulse modulating improves low speed action but doesn't effect
the torque all that much. It overcomes some of the issues with stiction at low speed.
 For DC motors, you need to include a feedback of the back EMF
of the motor to the controller in order to maintain speed. In doing this,
you need to subtract out the voltage drop caused by the IR drop of the
motor. You can put a small resistor in series with the motor and measure
the current to the motor. For small motors you can just measure the
resistance of the motor with an Ohm meter and use that to determine the
voltage that would have been added because of the IR drop of the motor. You
sum this feedback to the control voltage as a positive feedback for the IR
and a negative feedback for the entire voltage across the motor. The trick
is that the back EMF is proportional to the motor speed. The total voltage
across the motor leads is the back EMF plus the IR loss. If you make the
controller so that the control strength is controlling only the back EMF,
you have a nice constant speed. The pulse width is then only related to
keeping the control voltage equal to the back EMF.
 I hope this makes some sense.
 This method is used in many high quality cassettes drives and
some of the direct drive turn tables. These usually use pure analog drive
signals but one can use PWM drive with the right amount of filtering.
 This is also used on Orion's small battery Accu-Track drives.
I use these drives on my smaller scopes ( some of the early units were not
correctly made and had an incorrect resistor in them ).
Dwight

>Thanks, Clear skies,
>Jerry
>
>
>

--- BBBS/NT v4.00 MP