Help to tune this variable reluctance VR amplifier - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: October 14, 2019, 07:10:01 pm
content: Hello Mates,got this circuit reversed engineered from the Siemens VDO/Continental SIM32 fuel ECU.This VR amplifier is amplifying signal from a ABS sensor (not shared with the ABS). The goal is to read both wheel speeds, the trigger whell has 100 gapsThe circuit works very very well but, in very low speeds a lot of fast switch is generated at the output. IMHO because the very low voltage from the sensor. I do not need to read the whell speed in low frequency thus.Where to modify to get better performance?Greetings

author: bob91343
author_rank: Super Contributor
timestamp: October 14, 2019, 07:23:56 pm
content: The signal will be proportional to speed, so at slow speeds you get very little signal. If the noise is too great, you will get erratic operation.I would think the best solution would be to tear it apart and redo with more noise safe methods. Perhaps some filtering and more gain and even a different method of sensing. Or just don't read anything slower than some level.

author: rvalente
author_rank: Frequent Contributor
timestamp: October 14, 2019, 07:26:04 pm
content: I've done a software filtering, worked great but, it is not the right thing do to.I do not understand the circuit I'm using, them, adjusting it makes it very complicated. For a low peak-to-peak I would like to not read anything.Where to start?

author: max_torque
author_rank: Super Contributor
timestamp: October 14, 2019, 08:45:21 pm
content: Modern ABS sensors are not VR sensors, they are smart 7-14mA hall effect ones! They let 7mA pass when there is no target present gap) and 14mA when there is (tooth) and the length of the pulse they send depends on the passing direction, so the system can work down to zero wheel speed and tell which way the wheel is turning.if you are stuck with an old type VR sensor, then you can use off the shelf VR amplifier ICs to do the hard bit for you ie MAX9924 or LM1815 etc!

author: rvalente
author_rank: Frequent Contributor
timestamp: October 14, 2019, 09:22:36 pm
content: Tks by the information, but I'm stuck with the old VR sensors, they are reliable and cheap.The MAX is a good option but, is costly and this simple dual comparator is working fine, just need some adjustments.

author: duak
author_rank: Super Contributor
timestamp: October 15, 2019, 09:46:50 pm
content: R3 & R4 (22K1) in conjunction with R11 & R12 (10K) attenuate the input signal by ~70% at all frequencies. I would consider reducing the value of R3 & R4 or eliminate them entirely to increase the signal. If filtering is still required then I would consider reducing the loading caused by R11 & R12 either by using higher value resistors and a better op-amp, buffers or a single chip instrumentation amplifier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2019, 03:33:10 am
content: Perhaps R16 + R19 could be reduced, to increase the hysteresis band, or dynamically anyway (whereas R15 + R17 control static hysteresis). But that reduces sensitivity, too.How much does it really matter? Is this a problem at idle? During cranking? Surely not while cruising.Cranking and idle are typically different operating modes, as I understand it. Cranking might be crudely timed, good enough just to get things going (and shooting enough fuel to ensure starting, even if the resulting mixture is technically awful). Idle might be extra rich to maintain temperature. Timing probably doesn't matter as much, just as long as it's not so far out it's trying to backfire or detonate. How much does precision timing actually matter? Can a different, more lenient assumption be made? Can it be timed, or another (more crude) sensor used? (Or something like that. I know little about ECU operation.)Tim

author: langwadt
author_rank: Super Contributor
timestamp: October 16, 2019, 03:42:46 am
content: Quote from: T3sl4co1l on October 16, 2019, 03:33:10 amsurely the ABS sensor doesn't have anything to do with cranking and running the engine

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2019, 03:46:26 am
content: Dunno why I read "crank sensor" when it says "ABS".Seems to me, ABS doesn't work at low speeds anyway, and this would seem to be a good explanation as to why. Also, the stick-slip motion should ensure some signal during operation.Tim

author: moffy
author_rank: Super Contributor
timestamp: October 16, 2019, 08:18:34 am
content: Quote from: rvalente on October 14, 2019, 07:10:01 pmSo the circuit is oscillating at low speeds? The LM393 can get very noisy when switching if you don't have a solid ground. Ran into the same sort of problem where the output switching was being fed back to the input. Make sure your ground is solid. How is it constructed, I hope not veroboard. Also make sure you decouple the LM393 supply pins with caps.

author: rvalente
author_rank: Frequent Contributor
timestamp: October 16, 2019, 11:30:36 am
content: Quote from: duak on October 15, 2019, 09:46:50 pmIll will explored these changes and check if the circuit behavior improves and feedback.

author: rvalente
author_rank: Frequent Contributor
timestamp: October 16, 2019, 11:36:08 am
content: Quote from: T3sl4co1l on October 16, 2019, 03:33:10 amI do not need to have readings at low speed, the trigger wheel has 100 gaps. The wheel at 4Km/h (about one 1mile/h) will pulse at 26Hz.I just want to eliminate the fast switching noise that occurs under low frequency thus low voltage. Lets say under 500mV peak-to-peak the comparator do not need to switch. How should I implement that?

author: rvalente
author_rank: Frequent Contributor
timestamp: October 16, 2019, 11:39:47 am
content: Quote from: moffy on October 16, 2019, 08:18:34 amI do not have a solid ground and it is yet a perfboard, is version pre alpha prototypes hehehe...I've checked that under very low vpps (direct reading at the sensor) the circuit gets to oscillate around 1.36MHz and this fast switching makes my capture peripheral crazy.As I said before, how to add a hysteresis input?

author: rvalente
author_rank: Frequent Contributor
timestamp: October 16, 2019, 11:47:42 am
content: Guys,setting the middle voltage for the comp around .7V would help?

author: moffy
author_rank: Super Contributor
timestamp: October 16, 2019, 12:09:55 pm
content: Quote from: rvalente on October 16, 2019, 11:39:47 amTry using a ground plane. Perhaps if you tried the classic Bob Pease dead bug construction, you could get around any grounding or parasitic capacitive feedback issues. If you have CAD software do a layout for a PCB with a ground plane (fill in the bottom layer with a ground pour, you only need 2 layers.). I've used PCBWay and including DHL shipping costs between $30-$50 US, 5-7days. Well worth it. Also make sure your input wires are tightly twisted and maybe shielded also.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2019, 02:09:08 pm
content: Like I said, decrease the feedback resistors.Tim

author: rvalente
author_rank: Frequent Contributor
timestamp: October 16, 2019, 02:35:29 pm
content: Will totally do that, 100K give or take? What do u think?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2019, 02:48:25 pm
content: Sure. And if that's not enough, try increasing R9/10?Tim

author: rvalente
author_rank: Frequent Contributor
timestamp: October 31, 2019, 11:41:56 pm
content: Hello mates,Reduced R17 and R15 to 10K and R19 and R16 to 47K, is working better know.I'll do some tests in the field and report.Thanks a lot for all suggestions

author: max_torque
author_rank: Super Contributor
timestamp: November 01, 2019, 11:59:32 am
content: Quote from: rvalente on October 14, 2019, 09:22:36 pmif you are stuck with the passive VR sensors then you cannot sense either direction or down to low speed, which is why OEMs moved to active 7-14mA sensors about 15 years ago.If your "cheap" circuit is indeed working fine, why is this thread here??IME, the "cost" of an IC is never the true cost of the task, and for anything prototype, it's always better to just buy the IC designed to do the job! How many hours have you spent trying to get your solution to work? The only exception to this "use the right part" rule is when you aren't trying to deliver anything else, and just want to learn about how things are done. Otherwise your project becomes a project about making a VR sensor interface, rather than a project about doing what ever was actually the original goal of the project...........

author: SilverSolder
author_rank: Super Contributor
timestamp: November 01, 2019, 12:19:23 pm
content: Quote from: max_torque on November 01, 2019, 11:59:32 amBroadly agreed, but - in fairness - you sometimes have to spend many hours figuring out how a particular IC likes to be fed and cared for - and many ICs have "gotchas" that ends up costing even more time after you thought you had figured it out.I like the idea of the hall effect sensors, though. Do they actually cost more than the VR ones?

author: rvalente
author_rank: Frequent Contributor
timestamp: November 01, 2019, 10:49:12 pm
content: Quote from: max_torque on November 01, 2019, 11:59:32 amI do not need to sense direction or low speed, but customer requested to use this ABS sensor because he already uses it in many products.Its the reuse of knorr bremse and Wabco sensors so, gotta stick with that for now.I'm not sure if the max is expensive for the project, possibly no, it is not, there is a dual version. The VR amplifier is working fine now, after some good tips from you guys, which I appreciate a lot.

author: contatto_decollo
author_rank: Newbie
timestamp: February 09, 2022, 04:31:13 pm
content: Quote from: rvalente on November 01, 2019, 10:49:12 pmHello everybody,I am in a similar situation like rvalente : specialized IC like 1815 or 9925 to amplify VR sensor signals are already out of BOM budget.Our scope is instead little bit different: very low speed is the main feature to achieve.First attempt was to start testing a common solution with two op. amp stages: an integrator and a Smith Trigger.Now I came accross this circuit and I would like to test it and make a comparison.Do you think that this circuit from rvalente would be suitable for the low speed purpose?Btw, I am not getting the VR signal from an automotive toothed wheel but from a paddle flowmeter...Thanks in advance