Anybody used a hall-effect sensor to accurately measure small distances? - Page 1
Forum section: electronics / projects

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: March 16, 2016, 12:02:59 am
content: I'm a newbie--did a search and did not see anything, so forgive me if I missed something.As the title says I have concocted a scheme whereby an Allegro A1302 gives me a voltage when placed between two magnets (which are cleverly at 45 degrees to the sensor and 90 degrees to one another. The sensor voltage varies evenly as I vary from one extreme (at south pole to the other (north pole). The voltage is 2.5 volts (Vdd is 5volts) when the sensor is between the said magnets. This is for a feedback scheme whereby said feedback is going to be used to make sure that the hall effect sensor stays positioned right in the middle of the magnetic region. Region is roughly 20mm across. Hopefully, physical setup is clear.I'm hoping for something like 0.1mm sensitivity.Does this sound like it will work? Pitfalls?Allegro has better sensors with lower noise, like the A1366, would that work? Or should I ditch the magnets and use optics of some sort?Thanks,Randall

author: nctnico
author_rank: Super Contributor
timestamp: March 16, 2016, 12:31:08 am
content: AFAIK ultrasonic is the way to go for distance measurement but 0.1mm resolution may be on the edge. Otherwise you can look at optical time-of-flight devices.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 16, 2016, 12:34:27 am
content: Can you touch things?LVDT

author: exmadscientist
author_rank: Frequent Contributor
timestamp: March 16, 2016, 02:23:45 am
content: Your plan sounds good enough to try prototyping.I stronglydo not recommendultrasonic sensors. They never quite work right. 80% of the time you get a reasonable reading, 20% of the time random crap happens and completely throws things off. That can be tolerated in a lot of applications, but using an ultrasonic in a feedback loop is just asking for trouble.TI also has some inductive sensors (LDC1000, LDC161x, and friends) that may be suitable, but I'd try the magnetic sensor first. Magnetic sensing is quite robust if you can get the magnets arranged properly, which it sounds like you've already done.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: March 16, 2016, 03:34:53 am
content: linear Hall sensors and magnets were used in a variety of displacement and angle sensors at the company I last worked atin fact we even made load cells with Hall sensorsmagnetic "circuit" design isn't obvious to most, the field doesn't like to curve sharply and often "reaches out" into space a good distance making for possible errors introduced by any ferromagnetic material nearbyand the permanent magnets have tempcos which can give environmental sensitivitybut they can work quite well in some applications and are one of the more "hackable" technologies

author: jh15
author_rank: Frequent Contributor
timestamp: March 16, 2016, 05:34:23 am
content: Also used in clamp on dc ammeter probes I've come across, would make a great video to test this question.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: March 16, 2016, 04:38:44 pm
content: Thanks for the pointer to LVDT. Did not realize that technology even existed. Unfortunately, I cannot "touch things". Also, i did not mention this but it is important in my application to measure along the X axis and be *insensitive* to Y axis movement.I came across this app note from Allegro AN27702:http://www.allegromicro.com/~/media/Files/Technical-Documents/an27702-Linear-Hall-Effect-Sensor-ICs.ashx?la=enWhich encourages me and shows a few specific (probably better) magnetic field designs to try out. So I will do some experimenting.Thanks,Randall

author: miguelvp
author_rank: Super Contributor
timestamp: March 16, 2016, 05:48:01 pm
content: AFAIK Xbox One controllers use hall effect sensors for their analog triggers. Not sure if accurate to 0.1mm but I doubt it travels too much within that controller.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: March 16, 2016, 06:11:19 pm
content: Found this articlePosition Measurement with Hall Effect Sensors

author: Fungus
author_rank: Super Contributor
timestamp: March 16, 2016, 07:41:30 pm
content: Quote from: RandallMcRee on March 16, 2016, 12:02:59 amIt won't be very accurate. The fields coming out of magnets are far from perfect.(ie. The reading will be a bit wobbly, not nice and linear).Youcouldcalibrate it and use a lookup table for sensor->position but it'll be messy to do without a way to accurately position the sensor while you make the table.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: March 16, 2016, 08:29:40 pm
content: yes 3-D mag fields are going to be complicated, nonlinearclever choice of magnet size, shape and orientation can give regions of usable linearity but cross axis motion makes it harder - you could add a cross axis sensing hall sensor to have additional input for any nonlinear correction formula or tablefor calibration, or even as an alternative computer vision based on today's highly precise camera chips can be very good - up to the precision of the optics, mounting - and some simple geometric/optics errors can be calibrated out

author: Spike
author_rank: Regular Contributor
timestamp: March 16, 2016, 08:33:40 pm
content: Shahriar uses a hall effect sensor in a control loop in his magnetic levitation experiments video. You may want to check that out.https://youtu.be/LaGv2FHS5zg

author: AlfBaz
author_rank: Super Contributor
timestamp: March 16, 2016, 11:46:15 pm
content: Perhaps TI'sLDC1000may be feasible for your application.Check to see if they still have the evaluation board for it. It has a coil etched on to the PCB, the chip measures it's inductance and provides digital data. This data is sent to their demo software on the PC via USB.If I recall correctly the eval board I have was able to detect minute changes in distance

author: Johnb5012
author_rank: Newbie
timestamp: March 17, 2016, 01:00:29 am
content: What about non-contact capacitive sensor? Link Engineering uses them in a rotor mapping machine capable of making very small displacement measurements between the probe and a conductive surface (brake rotor).http://www.linkeng.com/docs/default-source/featured-items/Rotor_Mapping_Station_spec.pdf?sfvrsn=0

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 17, 2016, 01:15:39 am
content: How about OpenCV?

author: AndrewDojo
author_rank: Contributor
timestamp: March 17, 2016, 01:47:59 am
content: Quote from: RandallMcRee on March 16, 2016, 12:02:59 amIf I am getting this right, the objective is to maintain a precise position within the field rather than to measure the deviation as such. If so, then this is a much simpler problem as linearity is not so much of a concern.