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'Motor Control help...'
1998\11\29@023850 by marc

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Hi there all.
I'm going to control an electric kart using PIC16F84 in my school
project.
What I'm going to do is to control the speed. Do anyone here can tell me
how to increase the speed of the motor proportionally with time? I want
the motor to reach it's maxmium speed in 10 sec.

Here comes another problem that when a driver decelarated but the kart
is still moving, what will be the speed of the motor if he step on the
foot switch again??

Thanks in advance.
marc.

1998\11\29@034442 by marc

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Hi there all.
I'm going to control an electric kart using PIC16F84 in my school
project.
What I'm going to do is to control the speed. Do anyone here can tell me

how to increase the speed of the motor proportionally with time? I want
the motor to reach it's maxmium speed in 10 sec.

Here comes another problem that when a driver decelarated but the kart
is still moving, what will be the speed of the motor if he step on the
foot switch again??

Thanks in advance.
marc.

1998\11\29@095950 by Brian Duran

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Hello

On Sun, 29 Nov 1998, marc wrote:

> Hi there all.
> I'm going to control an electric kart using PIC16F84 in my school
> project.

> What I'm going to do is to control the speed. Do anyone here can tell me
> how to increase the speed of the motor proportionally with time?

Just how proportionally do you want it?  Usally motors are relatively
unproportional in and of them selves.  So first you have to figure out
what the curve is for the motors speed.  This will also depend on what is
driving the motors, H-bridge (Mosfets-TIP).

But is this really important to you, or would an estimate be ok.  Maybe
you can assume that voltage is proportional to speed, which would make
life easy.

If you motors are controlled by an H-bridge, mostlikely you can hook your
Pic directly up to it.  Otherwise they sell motor control chips, or some
buffer will do.
If you have a Pic with a D/A converter you would be able to output a 0-5
volt swing, or you can hook up a converter to the 84.

I think your first step need to be how are you going to interface you Pic
with the motor, and just how important in proportionality to you.

>
> Here comes another problem that when a driver decelarated but the kart
> is still moving, what will be the speed of the motor if he step on the
> foot switch again??

When the driver is decelarating are the motors on or costing.  Is his form
of decelaration breaking of slowing down the motors.  I would guess that
the break discontects the power to the motor(thus stopping quicker).  Thus
you want some information to be sent back tot he pic telling how fast the
motor is turning and were to start on the ramp.  Since when the motor is
turning wit will generate EMF, you might be able to measure this and use
it as you reference.

I would bet that there is a better way then this, but I don't know it.
Maybe someone else will inform both of us.

Good luck and if you have more questions....

Brian

1998\11\29@101022 by Ing.Andreas Pockberger

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Hi !

The easiest way to determine the motorspeed is some indexer on the motorshaft
or on some other parts which are turning.
With this information you are always in the situation that you can control
the actual speed of the motor, that means if the torque is to high your normal

speed settings will not produce the speed you need so with an indexer you
can measure the speed and accordingly set the output voltage to achieve the
desired speed. Also with this approach you can set a high current at the
start and then decreasing the current as the speed goes up.
To make an indexer its very simple, there are different approaches.
I found out that the easiest way is to put a small magnet on a turning part
and position an hall effect sensor, this sensors give you clear 5V output when

the magnet is passing.

I hope this helps you
with best regards
Andreas pockberger


Brian Duran wrote:

{Quote hidden}

1998\11\29@153630 by Michael J. Ghormley

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Ing.Andreas Pockberger wrote:

> I found out that the easiest way is to put a small magnet on a turning part
> and position an hall effect sensor, this sensors give you clear 5V output when
> the magnet is passing.

This is the smallest of points and maybe not germane, but all of the HED's I hav
e worked
with give you a 0V output when the magnet is passing and +V (usually +Vdd via a
pull-up
resistor) the rest of the time.  I just didn't want the newbie to repeat my firs
t
experience, when I got a whole tube of "bad" HED's that never had any output.  T
hat is
until I RTFM and put a pull-up resistor on the output!

Of course, there may be other types that I have not run across.

Michael

*************************************************************************
When the way of the Tao is forgotten, kindness and ethics must be taught.
Men must learn to pretend to be wise and good.  --  Lao Tzu
*************************************************************************

1998\11\30@003448 by Eric Borcherding

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These are open collector HEDs?

Eric Borcherding


'Motor Control help...'
1998\12\17@143331 by John Payson
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> What I'm going to do is to control the speed. Do anyone here can tell me
> how to increase the speed of the motor proportionally with time?

|Just how proportionally do you want it?  Usally motors are relatively
|unproportional in and of them selves.  So first you have to figure out
|what the curve is for the motors speed.  This will also depend on what is
|driving the motors, H-bridge (Mosfets-TIP).

I'll assume for the moment that you're dealing with a motor that
will be run in one direction (or else are using a relay for reversal)
so that you need only focus on driving voltage in one polarity; I'll
further assume that one leg of the motor is tied to ground (to simpl-
ify descriptions, I'll refer to the other leg as the "motor voltage"
or "motor output").

When PWM'ing a motor, there are two methods which can be used, which
I'll call voltage-based PWM and current-based PWM.

For voltage-based PWM, the motor output is switched between "active
high" [a positive voltage source] and "active low" [ground].  Ground-
ing the motor output will actively brake the motor, so the motor will
be switched between full-on and full-stop.  This method will cause
the motor to move somewhat jerkily (if the PWM is at an audible freq-
uency, the motor will "sing") but has the advantages of decent torque
output and (for the slower part of the range) a speed output which is
reasonably proportional to the duty cycle.  The big disadvantage of
this PWM method is that it is a *MAJOR* power hog (running a motor at
1/2 speed with no load will use about 1/4 as much power as a stalled
motor receiving 100% power) but if that's acceptable the performance
can be quite good.

For current-based PWM, the motor output is switched between "active
high" and "floating" [but diode-clamped to absorb flyback pulses].
This method has the advantage that it's much more efficient than the
voltage-based PWM, especially at high PWM rates.  Unfortunately, the
motor speed is very sensitive to motor loading; a motor with excellent
bearings and no load may run almost as fast at 10% duty cycle as at
100%, but adding any mechanical load will slow it down considerably.
The advantage of this PWM method is its design simplicity (no device
is needed to switch the output to ground) and its efficiency (if the
motor's load causes it to run at half-speed when the duty cycle is at
10%, then it will use 1/20 as much power at that speed as a stalled
motor receiving 100% power, a savings of 5:1 vs the voltage-based PWM).
Unfortunately, it requires some sort of feedback from the motor to
obtain any sort of predictable speed response.

When used in feedback control systems, current-based PWM's can work
very well.  Nonetheless, it may sometimes be useful to add a sinking
transistor; this is to allow for rapid changes in motor speed.  If
the motor has little mechanical loading, there will be no way for it
to slow down quickly absent a sinking transistor.  If the "active
high" output is equivalent to a car's gas pedal, the "active low"
output is the brake.  Although braking should be minimized when it's
not needed, it's hard to slow down or stop without it.

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