Tachometers with hall proximity sensor question

The manual has one notation about the 24VDC power supply being 200mA but no mention or description anywhere else, or the schematic connections. It could also be a misprint in the manual. Connections would be CM and 24 but may also be effected if it is setup for sink or source. The other supplies like the speed pot is 20 mA. Try a small 9VDC battery to eliminate a power supply issue.
 
A couple of comments.

1) The Hall Effect produces a voltage that is proportional to the magnetic field. If the field direction is changed by the magnet orientation then the voltage sign changes. The sensors you are talking about have a electronics in them (this could be why they are so physically long) and most of them use uni-polar electronics so they do not amplify one the Hall voltage in one direction. Hence, you are forced to get the magnet orientation correct. The effect is not speed sensitive at all, but some sensors electronics have drift in their DC levels or the out puts might be AC coupled. This is not commonly specified, so I assume that most sensor do not use AC coupled outputs. The outputs usually just swing from near 0 to a few volts so as to be compatible with other digital electronics. So when a magnet goes by, the Hall voltage rises as the field intensity increases and then goes back down as the field intensity goes back down. In side the electronics amplifies the signal, then thresholds it to yield a digital output which looks like a rectangular wave. Most likely, these are made from silicon directly on a silicon integrated circuit. While Silicon does not have the largest Hall coefficient, being integrated into a chip has real advantages. (One of the most sensitive hall materials is indium antimonide (InSb) but there are others and good Hall Effect meters use these.) Most of the displays are designed to trigger on one edge of a rectangular wave, rising or falling, but not both so that they do not double count.

2) Anything that a magnet sticks to decreases the field from the magnet. Hence, sticking it to steel actually lowers the field available to be sensed. Burying it inside steel really reduces the external field as the steel shorts out the field. So when we stick the magnet to a spindle etc we have to compensate for this by getting closer to the magnet or using a bigger magnet. The actual Hall effect device is actually quite small and can be sub-millimeter in size or smaller. The packaging, electronics and mounting makes it large, but you can buy quite small sensors in a plastic case that looks like a small transistor. Likewise, putting the sensor inside or on the other side of soft magnetic materials, like a cover etc or even the Hall Effect package will lower the field reaching the actual Hall Effect device. So the sensors we usually use have a metallic mount but the sensor is right out at the end.

3) If you can get to the packaging around the sensor correct, in concept you could put the magnet on the back side. Then the field going through the sensor would create Hall voltage all of the time. Then when some soft magnetic material like a gear tooth comes close to the device it would modulate the field to possibly yield a signal. However, I have not seen any cheap commercially available units like this.

4) Some non-Hall Effect sensor work on the principle of picking up a inductively produced signal in a coil. These would be speed sensitive since they work on the principle of a voltage being produced which is proportional to the time rate of change of the field. I am not for sure, but I think some anti-lock brake sensor work this way. They probably put a magnet inside the coil and then bring in up close to a gear so that it can detect the moving gear teeth passing by. In concept you could also do this with the Hall Effect devices if make like topic 3 above. In concept you have access to the spline teeth you could consider these the gear teeth and could sense them directly, without a moving magnet. By the way, so called inductive position sensors are different than what I just mentioned. They use an oscillating current in a coil to generate an oscillating field at the magnetic material surface and then measure the change in the inductance of this coil. They might be too slow for your higher speeds.

5) I put 10 magnets in a non-magnetic holder and fastened this on my lathe spindle. So I get RPM*10 out of it. I wanted to know the value when it was turning very slowly. I also used a uncommon small package 6 digit RPM meter/display, not that my lathe would ever go that fast. The small package would fit in a smaller panel, but is still quite readable. If you want to check it out you can find it by, search Aliexpress with "6 Digits Digital Tachometer" . The seller is "DIGITEN Official Store" The price is $13.49.
 
B2 said:
A couple of comments.

1) The Hall Effect produces a voltage that is proportional to the magnetic field. If the field direction is changed by the magnet orientation then the voltage sign changes. The sensors you are talking about have a electronics in them (this could be why they are so physically long) and most of them use uni-polar electronics so they do not amplify one the Hall voltage in one direction. Hence, you are forced to get the magnet orientation correct. The effect is not speed sensitive at all, but some sensors electronics have drift in their DC levels or the out puts might be AC coupled. This is not commonly specified, so I assume that most sensor do not use AC coupled outputs. The outputs usually just swing from near 0 to a few volts so as to be compatible with other digital electronics. So when a magnet goes by, the Hall voltage rises as the field intensity increases and then goes back down as the field intensity goes back down. In side the electronics amplifies the signal, then thresholds it to yield a digital output which looks like a rectangular wave. Most likely, these are made from silicon directly on a silicon integrated circuit. While Silicon does not have the largest Hall coefficient, being integrated into a chip has real advantages. (One of the most sensitive hall materials is indium antimonide (InSb) but there are others and good Hall Effect meters use these.) Most of the displays are designed to trigger on one edge of a rectangular wave, rising or falling, but not both so that they do not double count.

2) Anything that a magnet sticks to decreases the field from the magnet. Hence, sticking it to steel actually lowers the field available to be sensed. Burying it inside steel really reduces the external field as the steel shorts out the field. So when we stick the magnet to a spindle etc we have to compensate for this by getting closer to the magnet or using a bigger magnet. The actual Hall effect device is actually quite small and can be sub-millimeter in size or smaller. The packaging, electronics and mounting makes it large, but you can buy quite small sensors in a plastic case that looks like a small transistor. Likewise, putting the sensor inside or on the other side of soft magnetic materials, like a cover etc or even the Hall Effect package will lower the field reaching the actual Hall Effect device. So the sensors we usually use have a metallic mount but the sensor is right out at the end.

3) If you can get to the packaging around the sensor correct, in concept you could put the magnet on the back side. Then the field going through the sensor would create Hall voltage all of the time. Then when some soft magnetic material like a gear tooth comes close to the device it would modulate the field to possibly yield a signal. However, I have not seen any cheap commercially available units like this.

4) Some non-Hall Effect sensor work on the principle of picking up a inductively produced signal in a coil. These would be speed sensitive since they work on the principle of a voltage being produced which is proportional to the time rate of change of the field. I am not for sure, but I think some anti-lock brake sensor work this way. They probably put a magnet inside the coil and then bring in up close to a gear so that it can detect the moving gear teeth passing by. In concept you could also do this with the Hall Effect devices if make like topic 3 above. In concept you have access to the spline teeth you could consider these the gear teeth and could sense them directly, without a moving magnet. By the way, so called inductive position sensors are different than what I just mentioned. They use an oscillating current in a coil to generate an oscillating field at the magnetic material surface and then measure the change in the inductance of this coil. They might be too slow for your higher speeds.

5) I put 10 magnets in a non-magnetic holder and fastened this on my lathe spindle. So I get RPM*10 out of it. I wanted to know the value when it was turning very slowly. I also used a uncommon small package 6 digit RPM meter/display, not that my lathe would ever go that fast. The small package would fit in a smaller panel, but is still quite readable. If you want to check it out you can find it by, search Aliexpress with "6 Digits Digital Tachometer" . The seller is "DIGITEN Official Store" The price is $13.49.
Click to expand...
WOW WOW WOW. You have said a lot, and I thank you for explaining in very much detail,but like I previously mentioned, and didn't understand a lot of it, I am a complete novice at this. Can I make an assumption that if I changed the steel ring to say some kind of polymer, I would get a better result? I am sorry if you might feel that you waisted your well explained reply on me and that is on me. What else can I do in your opinion in conjunction with lowering the voltage to get an reading on my display?
 
I do not fully understand why you are not getting a reading. It could be a number of things. I do not know what model sensor or display you have, but if you got any specs with it check see what power supply voltage each wants and how much current do they need. It probably wants 12 or larger volts and >50mA or so, but there are a lot of different models on the market. I think you said you could get it to work by hand, but did not say if all the other VFD electronics were running orhow much power they might be taking. They might need the current and so there was not enough left for the Hall sensor electronics and the display. I have a little digital 5 digit panel volt meter and the display itself needs about 15mA and it varies a bit depending upon what number is being displayed. It wants to see 12 Vdc, not 24. My Hall meter/display will accept higher voltages but I just give it some of the same 12Vdc. You can purchase a small 12 Volt by 1 Amp supply for about $20 on Amazon. search Amazon with "MEAN-WELL-MDR-20-12-DIN-Rail-Supply". Anyway, test it again by hand and make sure it is not burned out.

There is a second approach you can use for the supply if the VFD has plenty of power, but I still prefer the external supply. They are cheap and easy to replace if something goes wrong and you burn it up. The VFDs are a lot more money and if the supply dies perhaps it is then useless. So the other approach is to use the VFDs 24 volt and drop it down to 12 volt with an integrated circuit three termail voltage regulator. google "LM312 voltage regulator" and you can see them and specs. Butthis will require a little circuit board and wiring on your part. The DIN supply is much quicker. In my Lathe VFD conversion I used a 24V DIN supply to power a the control circuits and took it down to 12 with an LM312 for the displays and sensor.

With respect to the magnet, mine are Neo magnets (neodymium, iron and boron to form the Nd2Fe14B, at ~> 30 mega-gauss-oersted energy product) at about 12mm dia and about 2-3mm thickness. I built a ring out of 1 inch thick Nylon that I had laying around. I milled a do-nut shape and then milled a slot in the flat face as near to the outer edge as possible and slipped the magnets into the slots. No glue required but I did put a small plastic screw in so that the head just lies on the milled hole. The magnet is trapped. You could cover it and then no magnetic particles would ever reach the magnet and wiping any off the nylon would be easy. Position the Hall sensor end as close to the magnet as possible. These magnets are usually oriented North and south across the thickness of the coin shape. So the sensor end faces the flat surface of the magnet. Yes, get it off the steel will increase the signal as the field will be stronger farther out from the magnet. I figure the field is maybe 50% stronger with the gap from the steel. (my holder slides onto my lathe spindle.) There Neo magnets which are oriented across the width, usually rectangular. I have some that are about 1"x1.5"x .25. These are very strong and really much bigger than needed for Hall sensing. Aliexpress has lots of Neo magnets for sale for little money. Any orientation will work. It all about getting a field at the sensor going axially down the sensor. Anyway, spacing of the sensor from the magnet is the key. Kept it small or get a bigger field. It is also good to keep the field edge sharp so that the hall signal to the hall sensor electronics is sharp (on and off the field). I will stick a drawing and a picture of my magnets, holder, and sensor mount up. I had lots of room. I made the holder extra large in OD so that I could come back and cut the inner diameter out so that I could reattach it later to a spider which is yet to be made. The do-nut /hockey puck design is thicker than need be, but I used a nylon bold in the back half to squeeze down on the spindle. I turned the ID to fit the spindle and after drilling an taping for the squeeze bolt I cut the through one side of the puck so that the bolt could squeeze the diameter.

Oh I almost forgot. I put a little wire with insulation around the magnets to fill the edges of the slots so that the magnets remained centered in their holes.

You can make a the holder out of almost anything non-magnetic (Nylon, Teflon, etc, Al, brass, etc.) , but to avoid Eddy currents, it is better if it is not metallic. You could even 3D print one. I have a friend who 3d printed a spline wrench!

Glad to help if I can. Just ask.

Dave L.
 

Attachments

  • Fig PM1440GT Hall Magn Mount L823_1913.pdf
    61.7 KB · Views: 8
  • IMAG3146 Hall Spindle IMAG3146.jpg
    IMAG3146 Hall Spindle IMAG3146.jpg
    2.6 MB · Views: 13
Last edited:
B2 said:
I do not fully understand why you are not getting a reading. It could be a number of things. I do not know what model sensor or display you have, but if you got any specs with it check see what power supply voltage each wants and how much current do they need. It probably wants 12 or larger volts and >50mA or so, but there are a lot of different models on the market. I think you said you could get it to work by hand, but did not say if all the other VFD electronics were running orhow much power they might be taking. They might need the current and so there was not enough left for the Hall sensor electronics and the display. I have a little digital 5 digit panel volt meter and the display itself needs about 15mA and it varies a bit depending upon what number is being displayed. It wants to see 12 Vdc, not 24. My Hall meter/display will accept higher voltages but I just give it some of the same 12Vdc. You can purchase a small 12 Volt by 1 Amp supply for about $20 on Amazon. search Amazon with "MEAN-WELL-MDR-20-12-DIN-Rail-Supply". Anyway, test it again by hand and make sure it is not burned out.

There is a second approach you can use for the supply if the VFD has plenty of power, but I still prefer the external supply. They are cheap and easy to replace if something goes wrong and you burn it up. The VFDs are a lot more money and if the supply dies perhaps it is then useless. So the other approach is to use the VFDs 24 volt and drop it down to 12 volt with an integrated circuit three termail voltage regulator. google "LM312 voltage regulator" and you can see them and specs. Butthis will require a little circuit board and wiring on your part. The DIN supply is much quicker. In my Lathe VFD conversion I used a 24V DIN supply to power a the control circuits and took it down to 12 with an LM312 for the displays and sensor.

With respect to the magnet, mine are Neo magnets (neodymium, iron and boron to form the Nd2Fe14B, at ~> 30 mega-gauss-oersted energy product) at about 12mm dia and about 2-3mm thickness. I built a ring out of 1 inch thick Nylon that I had laying around. I milled a do-nut shape and then milled a slot in the flat face as near to the outer edge as possible and slipped the magnets into the slots. No glue required but I did put a small plastic screw in so that the head just lies on the milled hole. The magnet is trapped. You could cover it and then no magnetic particles would ever reach the magnet and wiping any off the nylon would be easy. Position the Hall sensor end as close to the magnet as possible. These magnets are usually oriented North and south across the thickness of the coin shape. So the sensor end faces the flat surface of the magnet. Yes, get it off the steel will increase the signal as the field will be stronger farther out from the magnet. I figure the field is maybe 50% stronger with the gap from the steel. (my holder slides onto my lathe spindle.) There Neo magnets which are oriented across the width, usually rectangular. I have some that are about 1"x1.5"x .25. These are very strong and really much bigger than needed for Hall sensing. Aliexpress has lots of Neo magnets for sale for little money. Any orientation will work. It all about getting a field at the sensor going axially down the sensor. Anyway, spacing of the sensor from the magnet is the key. Kept it small or get a bigger field. It is also good to keep the field edge sharp so that the hall signal to the hall sensor electronics is sharp (on and off the field). I will stick a drawing and a picture of my magnets, holder, and sensor mount up. I had lots of room. I made the holder extra large in OD so that I could come back and cut the inner diameter out so that I could reattach it later to a spider which is yet to be made. The do-nut /hockey puck design is thicker than need be, but I used a nylon bold in the back half to squeeze down on the spindle. I turned the ID to fit the spindle and after drilling an taping for the squeeze bolt I cut the through one side of the puck so that the bolt could squeeze the diameter.

Oh I almost forgot. I put a little wire with insulation around the magnets to fill the edges of the slots so that the magnets remained centered in their holes.

You can make a the holder out of almost anything non-magnetic (Nylon, Teflon, etc, Al, brass, etc.) , but to avoid Eddy currents, it is better if it is not metallic. You could even 3D print one. I have a friend who 3d printed a spline wrench!

Glad to help if I can. Just ask.

Dave L.
Click to expand...
Thanks again for all this information, I will make some changes next week and see what happens. I just want to know one thing. I was provided with one 10mm×3mm neodymium magnet with the sensor,my question is,do yourealy need 10 or is one sufficient?
 
@Suzuki4evr you only need 10 if you want to measure very low RPM, like below 60. For most normal RPMs one magnet is usually sufficient.
 
WobblyHand said:
@Suzuki4evr you only need 10 if you want to measure very low RPM, like below 60. For most normal RPMs one magnet is usually sufficient.
Click to expand...
Thank you Wobblyhand. I see you mentioned my name Suzuki4evr now. How dis you do that again to mention a members name so it shows in blue? I forgot :D
 
Just use an @ immediately before the member's name. "@"Suzuki4evr with no quotes. So if there was a member called noname, you would do @noname.
 
You must log in or register to reply here.
Back
Top