I was hoping to pick your collective brains about an idea i had.
I am working with two friends on a shapeoko cnc kit (hopefully the very
start of a norwich hackspace) and we were chatting about adding
a position feedback mechanism. It obviously has to be accurate and
reliable, at least as much as the steppers. Cheap is also important.
We were talking about sticking a grid pattern along X & Y axis and using a
mouse to measure the progress of the gantry. After some googling it seems
this is good in theory but hard in practice.
So that lead us to a webcam + scale markings + opencv to measure where we
were and I thought seemed unreliable. I thought about sticking some
magnetically encoded tape (video or cassette tape ) and then attaching a
read head to the gantry. I could record a series of pulses on the tape and
count them back at the gantry moved.
Before i spend time testing this out does anyone have any opinion on the
practicality of this idea?
What are you trying to achieve? Zeroing at the limits and counting steps is
generally considered perfectly reliable, as long as you don't skip steps -
in which case you're in trouble anyway.
On Tue, Nov 6, 2012 at 1:05 PM, Toby Catlin <t...@korfball.com> wrote:
> Hello Everyone,
> I was hoping to pick your collective brains about an idea i had.
> I am working with two friends on a shapeoko cnc kit (hopefully the very
> start of a norwich hackspace) and we were chatting about adding
> a position feedback mechanism. It obviously has to be accurate and
> reliable, at least as much as the steppers. Cheap is also important.
> We were talking about sticking a grid pattern along X & Y axis and using a
> mouse to measure the progress of the gantry. After some googling it seems
> this is good in theory but hard in practice.
> So that lead us to a webcam + scale markings + opencv to measure where we
> were and I thought seemed unreliable. I thought about sticking some
> magnetically encoded tape (video or cassette tape ) and then attaching a
> read head to the gantry. I could record a series of pulses on the tape and
> count them back at the gantry moved.
> Before i spend time testing this out does anyone have any opinion on the
> practicality of this idea?
> What are you trying to achieve? Zeroing at the limits and counting steps
> is generally considered perfectly reliable, as long as you don't skip steps
> - in which case you're in trouble anyway.
> On Tue, Nov 6, 2012 at 1:05 PM, Toby Catlin <t...@korfball.com> wrote:
>> Hello Everyone,
>> I was hoping to pick your collective brains about an idea i had.
>> I am working with two friends on a shapeoko cnc kit (hopefully the very
>> start of a norwich hackspace) and we were chatting about adding
>> a position feedback mechanism. It obviously has to be accurate and
>> reliable, at least as much as the steppers. Cheap is also important.
>> We were talking about sticking a grid pattern along X & Y axis and using
>> a mouse to measure the progress of the gantry. After some googling it seems
>> this is good in theory but hard in practice.
>> So that lead us to a webcam + scale markings + opencv to measure where we
>> were and I thought seemed unreliable. I thought about sticking some
>> magnetically encoded tape (video or cassette tape ) and then attaching a
>> read head to the gantry. I could record a series of pulses on the tape and
>> count them back at the gantry moved.
>> Before i spend time testing this out does anyone have any opinion on the
>> practicality of this idea?
Although not mentioned in that listing, these have a data port where the
measurement can be extracted, they continuously spit out the data in
format very similar to SPI.
A bit expensive, especially as you probably won't need them.
That looks perfect and the price is not ridiculous at all.
I love having access to a hive mind. Not only do i have a better solution,
I also probably don't even need it which is the best solution of all.
thanks
toby
On 6 November 2012 13:37, Nigel Worsley <nig...@googlemail.com> wrote:
> Although not mentioned in that listing, these have a data port where the
> measurement can be extracted, they continuously spit out the data in
> format very similar to SPI.
> A bit expensive, especially as you probably won't need them.
They have quite a slow update rate, though. Some can be pushed into a
higher speed, but that's possibly only the report rate not the measurement
rate. It may be possible to move them fast enough that they lose position -
they're not an absolute measurement (even when called that) but I'm unsure
of the limits.
If they're fast enough, there are 150mm calipers often going cheap at Aldi
etc. These use the same position encoders as the machine-mounted scales and
may have the data port inside.
There's a company called US Digital which makes accurate magnetic scales as
you describe yet still at prices below the precision glass scales (also
available from the company Nigle linked). A Yahoo group called
cad_cam_edm_dro is a good place to get advice about them.
I've also wondered about using optical (pattern sensing) mice to measure
relative position. I'm not sure of linearity and it may be difficult to
keep them sufficiently clean on a CNC machine, but they're very cheap and
don't require a mechanical link to the surface.
> Although not mentioned in that listing, these have a data port where the
> measurement can be extracted, they continuously spit out the data in
> format very similar to SPI.
> A bit expensive, especially as you probably won't need them.
Just chuck an encoder on the other side of the stepper and be done with it.
Some cnc programs like mach3 actually support this and will restart/realign
the machine if your stepper stall.
On Nov 6, 2012 1:54 PM, "Adrian Godwin" <artgod...@gmail.com> wrote:
> They have quite a slow update rate, though. Some can be pushed into a
> higher speed, but that's possibly only the report rate not the measurement
> rate. It may be possible to move them fast enough that they lose position -
> they're not an absolute measurement (even when called that) but I'm unsure
> of the limits.
> If they're fast enough, there are 150mm calipers often going cheap at Aldi
> etc. These use the same position encoders as the machine-mounted scales and
> may have the data port inside.
> There's a company called US Digital which makes accurate magnetic scales
> as you describe yet still at prices below the precision glass scales (also
> available from the company Nigle linked). A Yahoo group called
> cad_cam_edm_dro is a good place to get advice about them.
> I've also wondered about using optical (pattern sensing) mice to measure
> relative position. I'm not sure of linearity and it may be difficult to
> keep them sufficiently clean on a CNC machine, but they're very cheap and
> don't require a mechanical link to the surface.
> On Tue, Nov 6, 2012 at 1:37 PM, Nigel Worsley <nig...@googlemail.com>wrote:
>> Although not mentioned in that listing, these have a data port where the
>> measurement can be extracted, they continuously spit out the data in
>> format very similar to SPI.
>> A bit expensive, especially as you probably won't need them.
