Hall effect sensor

nem',
Thanks for all the very relevant info! I did get in a sample from Allegro
and with a 8mm neo magnet am getting 15mm range. I'll have to play around
with magnet size and sensitivities but I think it will work.
t


"Nemo" <Paul@nospam.nospam.nospam.nospam.co.uk> wrote in message
news:3eoCd$c+lWhHFwoh@furfur.demon.co.uk...
Check out www.allegro.com
where you will find many unipolar Hall Effect sensors.
A word or two of advice from my own experience. When I used them I found
they worked OK, but the range was poor, even when using an expensive
neodymium magnet [See note 1]. Say 3mm. We got another mm or so by putting
a ferrite bead behind the magnet. Mind you these were small magnets -
about 3mm diameter, 3mm long.
You gotta worry about the magnetic field strength dropping with time
(years) and temperature (I forget if the magnets get weaker as temperature
goes up or down, the manufacturer will tell you).
So, just be careful about separation between sensor and magnet. If you
have a rotating thingy, and the environment is rugged, you may need to
mount the sensor & magnet rather far apart to allow for vibration, knocks,
misalignment etc (the same would be true of an opto sensor).

If cost is no object I know of another technology being developed at a
start up company near me, they're looking for apps. This one sounds
suitable but would need to be a mass production or high value one to
justify them developing a custom solution for you.

I wouldn't go for a reed switch as they're electromechanical and I just
don't trust such things' failure rates when cycled a zillion times. Also I
have seen some which were orientation sensitive. They do have the
advantage of not caring what polarity they're facing, though.

Something to consider: why does the rpm sensor need to be mounted near the
gear teeth?

Note 1 - neodymium magnets
=====================
Neodymium alloys make the best magnets because (a) they're strong (b) they
retain their strength even without a keeper forever. Other magnet types'
strength decays significantly within a year or two.

Marvellous! So why are these magnets so expensive? Because it's a rare
element. There used to only be 2 mines in the world, but the US one closed
down for environmental reasons. The remaining one is in China (actually
they get the neodymium by processing the waste tailings from a mine which
is mainly extracting some other mineral). However a year or two ago that
mine had a fire and temporarily closed down, so the price of neodymium
magnets shot up. I imagine it's back down again by now.

Just thought that was interesting...
--
Nemo
Click to expand...
 
Check out www.allegro.com
where you will find many unipolar Hall Effect sensors.
A word or two of advice from my own experience. When I used them I found
they worked OK, but the range was poor, even when using an expensive
neodymium magnet [See note 1]. Say 3mm. We got another mm or so by
putting a ferrite bead behind the magnet. Mind you these were small
magnets - about 3mm diameter, 3mm long.
You gotta worry about the magnetic field strength dropping with time
(years) and temperature (I forget if the magnets get weaker as
temperature goes up or down, the manufacturer will tell you).
So, just be careful about separation between sensor and magnet. If you
have a rotating thingy, and the environment is rugged, you may need to
mount the sensor & magnet rather far apart to allow for vibration,
knocks, misalignment etc (the same would be true of an opto sensor).

If cost is no object I know of another technology being developed at a
start up company near me, they're looking for apps. This one sounds
suitable but would need to be a mass production or high value one to
justify them developing a custom solution for you.

I wouldn't go for a reed switch as they're electromechanical and I just
don't trust such things' failure rates when cycled a zillion times. Also
I have seen some which were orientation sensitive. They do have the
advantage of not caring what polarity they're facing, though.

Something to consider: why does the rpm sensor need to be mounted near
the gear teeth?

Note 1 - neodymium magnets
=====================
Neodymium alloys make the best magnets because (a) they're strong (b)
they retain their strength even without a keeper forever. Other magnet
types' strength decays significantly within a year or two.

Marvellous! So why are these magnets so expensive? Because it's a rare
element. There used to only be 2 mines in the world, but the US one
closed down for environmental reasons. The remaining one is in China
(actually they get the neodymium by processing the waste tailings from a
mine which is mainly extracting some other mineral). However a year or
two ago that mine had a fire and temporarily closed down, so the price
of neodymium magnets shot up. I imagine it's back down again by now.

Just thought that was interesting...
--
Nemo
 
T

thomas

Guest
I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of a
2.5" rotating cylinder. It rotates at 20 RPM resulting in 120 pulses per
minute (I'll calculate speed from this). This will be a damp environment so
a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder. I have NOT found small inexpensive
ring magnets that are magnetized diametrically (N and S are on the same
face).

There are many many types of Hall Effect sensors. Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.
Is there a type that can detect the coming and going of just a S field (the
field will never be totally absent)?

I found a Gear Tooth Sensor type that senses a ferrous target. Can I just
use the S facing ring magnet instead of the ferrous target?


TIA
 
"thomas" <thomas@yahoo.com> wrote in message
news:26f28$4774190a$d8447b12$25112@FUSE.NET...
I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of
a 2.5" rotating cylinder. It rotates at 20 RPM resulting in 120 pulses
per minute (I'll calculate speed from this). This will be a damp
environment so a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder. I have NOT found small
inexpensive ring magnets that are magnetized diametrically (N and S are on
the same face).

There are many many types of Hall Effect sensors. Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.
Is there a type that can detect the coming and going of just a S field
(the field will never be totally absent)?

I found a Gear Tooth Sensor type that senses a ferrous target. Can I just
use the S facing ring magnet instead of the ferrous target?


TIA
Click to expand...
Why worry about a ferrous target in the damp - it doesn't worry many car &
motorcycle makers, you could also apply protective lacquer. In any case
ferrous alloys used for magnetic applications tend to form a thin film of
rust and then stop corroding unless fully exposed to weather, note also that
hall sensors are favoured for their immunity to environments that would foul
up opto sensors.
 
thomas wrote:
I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of a
2.5" rotating cylinder. It rotates at 20 RPM resulting in 120 pulses per
minute (I'll calculate speed from this). This will be a damp environment so
a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder. I have NOT found small inexpensive
ring magnets that are magnetized diametrically (N and S are on the same
face).

There are many many types of Hall Effect sensors. Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.
Is there a type that can detect the coming and going of just a S field (the
field will never be totally absent)?

I found a Gear Tooth Sensor type that senses a ferrous target. Can I just
use the S facing ring magnet instead of the ferrous target?


TIA


I think the gear tooth sensor will do just fine.
Click to expand...
You can also use any small tranformer,from which you can
remove the iron on one side of the coil.
Unless of course your speed can go to zero, in which case
you need the hall type tooth sensor.
You could also try a reed relay, they have a long lifetime
as long as they only have to switch a small current.
 
On Dec 27, 1:26 pm, "thomas" <tho...@yahoo.com> wrote:
I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of a
2.5" rotating cylinder.  It rotates at 20 RPM resulting in 120 pulses per
minute (I'll calculate speed from this).  This will be a damp environment so
a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder.  I have NOT found small inexpensive
ring magnets that are magnetized diametrically (N and S are on the same
face).

There are many many types of Hall Effect sensors.  Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.
Click to expand...
Here's a link that helps explain the types of hall sensors:
http://www.allegromicro.com/en/Products/Design/an/an27705.pdf

Is there a type that can detect the coming and going of just a S field (the
field will never be totally absent)?
Click to expand...
That's called a unipolar sensor; sensitive only to one magnetic
polarity, typically south.
Example: Honeywell's SR13C-A1. I used that sensor to convert a
motorcycle distributor from points to solid state. I installed
quarter inch diameter neo magnets in a fixed aluminum disc above the
base plate, one hall sensor on the base plate directly below each
magnet, and had a steel shutter disc rotating between the magnets and
the sensors, to break the magnetic field. Some ancillary circuitry to
drive the ignition coil. It worked the first time I turned over the
engine.
I found a Gear Tooth Sensor type that senses a ferrous target.  Can I just
use the S facing ring magnet instead of the ferrous target?

Well, I've never used that kind of sensor, so I can't say it would
Click to expand...
work. But I would encourage you not to give up on the hall sensors.
I had an easy time with them. If you use a unipolar sensor and get it
installed the right distance from the magnets you shouldn't have any
problem. This is as much a mechanical job as an electronic one.
 
On Thu, 27 Dec 2007 16:26:46 -0500, "thomas" <thomas@yahoo.com> wrote:

I'd like to use a Hall Effect sensor to detect 6 magnets on the outside of a
2.5" rotating cylinder. It rotates at 20 RPM resulting in 120 pulses per
minute (I'll calculate speed from this). This will be a damp environment so
a ferrous 'target' is not an option.

I have found small inexpensive ring magnets (with the S pole facing out)
that I can easily mount on the cylinder. I have NOT found small inexpensive
ring magnets that are magnetized diametrically (N and S are on the same
face).

There are many many types of Hall Effect sensors. Most need alternating N
and S fields: either with 2 opposite poled magnets, or with a single N/S
ring magnet.
Is there a type that can detect the coming and going of just a S field (the
field will never be totally absent)?

I found a Gear Tooth Sensor type that senses a ferrous target. Can I just
use the S facing ring magnet instead of the ferrous target?
Click to expand...
As others have mentioned, a gear tooth sensor should be just fine.
The standard "reluctance" sensor is just a magnet plus a coil of
wire around a ferrous core. The coil senses changes in the
magnetic field caused by the gear teeth concentrating the flux
momentarily as they pass by.

But if you really want to mount rotating magnets, then all you
need for a sensor is a coil of wire around a core. The whole
thing can be potted in plastic if you are worried about moisture.

If you have a junkyard nearby, you might cannibalize an old
automotive distributor from the days before electronic timing.
(But which still used electronic ignition.)

The GM design used an 8-tooth "gear" that rotated inside a
ring with 8 teeth pointing inward so that they came very close to
the "gear" teeth. That gave much higher output than the
conventional single geartooth sensor (used by Ford and others).

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
 
nem',
Thanks for all the very relevant info!
Click to expand...
Glad it was of help! I'm a bit of a newbie on this newsgroup and wanted
to contribute something. Good luck.
--
Nemo
 
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