Motor precision stopping- encoder
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Posts: 46,084
--- In basicstamps@yahoogroups.com, "ch2surf" <sachacrompton@v...>
wrote:
> I will be getting a motor with a built in tacometer and gear box to
> slow it to 25RPM, but I don't know the gear ratio or motor RPM
> before the gearbox. I was wondering if it was possible to make the
> motor stop at a precision of 1 degrees or less usign a basic stamp?
>
> The motor would have a preset number of revolutions to make (17 624
> for example) and it would have to stop exactly at 17 624, I was
> thinking of making it slow down as slow as possible when it's about
> 100 revolutions from the destination to allow it to stop at exactly
> the preset number.
>
> Would this be possible?
>
> Thanks
The only way to get precision is to measure the process. that would
mean an encoder on the shaft or drum or wheel, or some other feedback
from the process (wheel).
for instance, you might be winding wire and if it snags, it could
miss some revolutions until it freed itself or created a jam.
my rule of thumb is to be 4 times more accurate than your desired
measuring unit. for 1 degree, that is 360 tick per revolution, and
that times 4 is an encoder of about 1,500 ticks per revolution.
with 360 ticks, you could be off 1 degree plus 1 count so 2 degrees.
with 1,440 ticks, you could be off 1/4 of a degree plus one tick or
1/2 degree.
you didn't mention the speed of the motor, so using this method, you
may be able to use just the stamp, or if it is really too fast, you
might need some external high speed counter support chips.
Dave
wrote:
> I will be getting a motor with a built in tacometer and gear box to
> slow it to 25RPM, but I don't know the gear ratio or motor RPM
> before the gearbox. I was wondering if it was possible to make the
> motor stop at a precision of 1 degrees or less usign a basic stamp?
>
> The motor would have a preset number of revolutions to make (17 624
> for example) and it would have to stop exactly at 17 624, I was
> thinking of making it slow down as slow as possible when it's about
> 100 revolutions from the destination to allow it to stop at exactly
> the preset number.
>
> Would this be possible?
>
> Thanks
The only way to get precision is to measure the process. that would
mean an encoder on the shaft or drum or wheel, or some other feedback
from the process (wheel).
for instance, you might be winding wire and if it snags, it could
miss some revolutions until it freed itself or created a jam.
my rule of thumb is to be 4 times more accurate than your desired
measuring unit. for 1 degree, that is 360 tick per revolution, and
that times 4 is an encoder of about 1,500 ticks per revolution.
with 360 ticks, you could be off 1 degree plus 1 count so 2 degrees.
with 1,440 ticks, you could be off 1/4 of a degree plus one tick or
1/2 degree.
you didn't mention the speed of the motor, so using this method, you
may be able to use just the stamp, or if it is really too fast, you
might need some external high speed counter support chips.
Dave
Comments
control the h-bridge to run the motor and when you see a count you like want
the motor feeback then turn off the h-bridge and turn on the lower two
transistors in the bridge for a short period of time. This will short the
leads of the motor and the motor will brake itself and stop immediately.
Adjust the position by changing the count required to stop at a certain
point. In other words, if you want to stop at a position represented by a
count of 1000, begin the process at 999 or 998 or whatever to allow time to
turn off the bridge and short the leads as described above. Once set up, it
is very consistant. I've used this arrangement to move metal tags into
position for laser marking and was able to place them consistantly with
.003" of the intended point. If you are moving to a fixed point, use
instead a sensor to detect the point and then stop the motor as above and
you don't need any direct motor feedback.
jim
http://www.geocities.com/jimforkin2003/
Original Message
From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=CQ_wSsMOTGzurplNvdC_gkjD9mdPOPnepBQ1uMc8zj2QinMQ19D0c-sf101dH6j5Oh7kOF43X9M]davemucha@j...[/url
Sent: Monday, August 11, 2003 11:27 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Re: Motor precision stopping- encoder
--- In basicstamps@yahoogroups.com, "ch2surf" <sachacrompton@v...>
wrote:
> I will be getting a motor with a built in tacometer and gear box to
> slow it to 25RPM, but I don't know the gear ratio or motor RPM
> before the gearbox. I was wondering if it was possible to make the
> motor stop at a precision of 1 degrees or less usign a basic stamp?
>
> The motor would have a preset number of revolutions to make (17 624
> for example) and it would have to stop exactly at 17 624, I was
> thinking of making it slow down as slow as possible when it's about
> 100 revolutions from the destination to allow it to stop at exactly
> the preset number.
>
> Would this be possible?
>
> Thanks
The only way to get precision is to measure the process. that would
mean an encoder on the shaft or drum or wheel, or some other feedback
from the process (wheel).
for instance, you might be winding wire and if it snags, it could
miss some revolutions until it freed itself or created a jam.
my rule of thumb is to be 4 times more accurate than your desired
measuring unit. for 1 degree, that is 360 tick per revolution, and
that times 4 is an encoder of about 1,500 ticks per revolution.
with 360 ticks, you could be off 1 degree plus 1 count so 2 degrees.
with 1,440 ticks, you could be off 1/4 of a degree plus one tick or
1/2 degree.
you didn't mention the speed of the motor, so using this method, you
may be able to use just the stamp, or if it is really too fast, you
might need some external high speed counter support chips.
Dave
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