Hall-effect sensor at a distance? - Page 1
Forum section: electronics / beginners

author: Dave Wave
author_rank: Contributor
timestamp: June 18, 2012, 10:37:40 pm
content: I want to use a Hall-effect sensor to count the RPMs on a shaft. The catch is the shaft is about 200 feet from where the microcontroller would be.What is the best way to send a signal from the sensor to the ? I need to use a hard-wired solution.Any ideas appreciated.Thanks!-Dave

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2012, 10:49:04 pm
content: Make a little circuit that is located with the sensor that amplifies the signal to a more usable level and provides a differential output for the signal.Cable the two wire differential signal 200feet to your microcontroller and then convert it back to ground reference.You could also do it digitally but analogue is probably easier.If you need to power the sensor over the cable as well then use a higher voltage to what you need, say 12v or 24v. Put a 7805 regulator at the sensor to provide VCC for the amplifier/line driver board. Doing that you'd need a 4 wire cable, GND +12v D- D+

author: codeboy2k
author_rank: Super Contributor
timestamp: June 19, 2012, 01:50:33 am
content: You can still power your circuit with 2 wires DCandsend the RPM signal back. Here's how:nothing digital here. just op amps. send 12V or so down the wire to the sensor what ever your sensor needs.At the sensor end, convert the RPM to a current range and sink a current representing the RPM, for example 20-40ma = 0 - 2000 RPM. The current range you choose depends on the accuracy of your current sink and the other end current sense. You can sink more current if you need to, so 20-120mA = 0 - 2000 RPM.At the host side 200ft away, sense the current with a standard current sense op-amp configuration, turn that into a voltage, and get your RPM through an ADC on a micro. Your current will be offset by the operating current of your sensor, so you need to take that into consideration.200 feet is a long way, it will be noisy. The voltage drops will be significant. Differential signalling with a wide enough voltage margin or current sense is the only way to go. Even if you go digital, you will still need differential signalling.

author: Rufus
author_rank: Super Contributor
timestamp: June 19, 2012, 03:15:59 am
content: Quote from: Dave Wave on June 18, 2012, 10:37:40 pmDo you want to use a hall sensor or do you want to count turns of the shaft or do you want to measure the rotational speed of the shaft?A hall sensor is probably not a good choice. An inductive pick up could drive 200' of cable with no electronics or power required.

author: Dave Wave
author_rank: Contributor
timestamp: June 19, 2012, 01:21:11 pm
content: Quote from: Rufus on June 19, 2012, 03:15:59 amI want to determine the RPM's of the shaft...Could you explain a little more about the inductive type sensor? Any examples links would be great.Thanks,-Dave

author: Psi
author_rank: Super Contributor
timestamp: June 19, 2012, 01:55:16 pm
content: I think he means a magnet on the shaft and a fixed coil to pickup the magnetic field as it rotates past.Same kind of thing you see on a lawnmower to generate the sparkplug voltage from the engine rotation.Though obviously you don't need quite that much voltage for thisNoise on the cable isn't such an issue then, since the pulses generated are relativity high voltage and your only interested in "was there a pulse" rather than trying to measure the pulse.

author: Dave Wave
author_rank: Contributor
timestamp: June 19, 2012, 02:25:11 pm
content: I did a bit more research and came up with this hall-effect sensor:Honeywell 103sr13a-1http://sensing.honeywell.com/product%20page?pr_id=36086If I am reading this right...This unit outputs a sink everytime the magnet goes past. So would it be work if I ran 5 volts through one line of my 200 foot cable to the sink of the sensor. At the other end (base station), I had an inverter on that 5 volt line. When the sensor starts sinking, the 5v line goes close to zero and the inverter goes high, then I can read that with a microcontroller? Is noise still an issue with a setup like this?Thanks,Dave

author: Psi
author_rank: Super Contributor
timestamp: June 19, 2012, 02:52:42 pm
content: That has to be one of the worse websites for finding a datasheet on one of their products.

author: Rufus
author_rank: Super Contributor
timestamp: June 19, 2012, 04:47:37 pm
content: Quote from: Dave Wave on June 19, 2012, 01:21:11 pmThe VRS series herehttp://sensing.honeywell.com/speed_range_guide_005911-5-en_lowres.pdfare examples.Probably rip off prices from honeywell.The are basically a nail with a coil wound around it and a small magnet on the back - so nothing to be expensive, you could probably make your own. The presence of ferrous material in front of the 'nail' changes the reluctance of the magnetic circuit. Rapid changes of the amount of ferrous material produce quite large voltages on the coil.They are also called geartooth sensors because they are often used to detect the teeth of ferrous gears. One of my motorbikes uses one for the speedometer looking at the heads of the bolts which fasten the rear brake disk.They produce a large balanced differential signal (one of the honeywell ones up to 190V pk-pk - lol) which can easily give you a usable signal at the end of a long twisted pair cable.

author: Dave Wave
author_rank: Contributor
timestamp: June 19, 2012, 06:40:59 pm
content: Here is the data sheet.http://sensing.honeywell.com/index.php?ci_id=50343I never did manage to get one from Honeywell, that search engine is worthless.Upon further thought, why not just have a +5v sources with a current limiting resistor attached to an input pin of the microcontroller. This line would run to the 103SR through the 200 foot cable. When the 103SR goes low, it would pull down the +5v, which I could read with the microcontroller.Would there still be a noise/ringing issue with this. The max freq is only 10hz or so (600 RPM)?Thanks for all the help...-Dave

author: codeboy2k
author_rank: Super Contributor
timestamp: June 20, 2012, 04:45:27 pm
content: Yes, it could work, you could pull the micro input pin to ground via the sensor, at the far end of the wire. It might not be reliable all the time, and it might be susceptible to noise, but you can dampen any ringing with enough series resistance to make it critically damped. That's hard to calculate without knowing the exact inductance and capacitance of your long wire, but a few hundred ohms is more than enough. You will need three wires. PWR, GND and the SENSOR.If you want a two wire approach, using current signalling as I described, I included a spice model for you to play with. In my spice simulation I chose a 12V source down the wire and a 5V sensor as you wanted a 5V sensor, but it might make sense to use a 12V sensor instead. Sending 5V down the line to a 5V sensor, and then using current signalling doesn't work because the voltage will drop with the current draw and the sensor might fail to operate since the voltage drops down to about 4.1V. It also doesn't see a nice clean operating voltage but a pulsating voltage. So my simulation sends 12V to a 5V device and uses a 5.1V zener diode to keep the voltage clean. The device doesn't see any fluctuation because of the current signalling.In my simulation, 200 feet of 22 AWG zip cord or line cord has about 600-800pF of capacitance, and for inductance, I don't know, I guessed about a few nH.The sensor is the 5V model, with a 5.1V zener diode and capacitor for a small filter. The sensor sinks 15mA of current through the 680 ohm resistor. This is sensed at the Microcontroller end with Q1 and Q2, and outputs a 5V pulse at RPMOUT. There is a 4.7V zener at the output limit the pulse voltage to the micro. You can use a 3.3V zener if your micro is 3.3V.Whether you use 3 wires or 2 wires, you should protect all 3 inputs at both ends from static and voltage spikes. The long wire will act as an antenna and pick up static, so you probably want 1M bleed resistors to ground on power and signal lines. You want zeners on all inputs to limit input spikes the wire might pick up. Without input protection you will likely kill the sensor and/or the micro in a year or so.I've attached the LTSpice file.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 20, 2012, 08:12:12 pm
content: It's also interesting to note that a simulation of a 3 wire system with PWR, GND and the SENSOR pulldown as you described generates higher negative voltages in the cable due to the distributed capacitance and inductance.The simulation I did is just a switch simulation, but if you add any kind of current draw, then the pulsing swing in voltage charges and discharges the mutual capacitances of the long wire, and together with the inductance can generate hundreds of volts that your micro input pin will see. These need to be clamped at the micro with diodes.The current sense solution I gave earlier doesn't have these voltage spikes because the voltage is relatively constant on the wire. Only the current changes by a few mA to signal the micro.LTspice simulation is in the zip file attached.edit: I just did the calculations, it turns out the inductance of 200 feet of 22AWG wire is about 158uH at 10hz , so in the spice simulation you should make each coil about 75uH .. when you do that the voltage spikes are a little higher.