Powerup state of cam sensor?

[Healey3000]

Hi,

The datasheets of typical hall effect cam sensors I've seen don't specify what the state of the output is at power on. Some Hall devices (the small IC package types) do state this. For example, many will start in a high or low state regardless of whether they are facing a tooth and will toggle to the correct state after seeing the first edge transition. Others are True Position types (TPOS, like Allegro's ATS627) that will power up to match the presence or absence of a tooth.

Which leads to a question - how does MS3 deal with this? Does it ignore the cam sensor state until it has toggled at least once?

Alternatively, are there certain gear tooth sensors that folks have used that do power up correctly?

Thanks!

[jsmcortina]

The Megasquirt (2,3) cam input is normally edge triggered. i.e. it responds to a change from high to low or vice-versa.

James

[racingmini_mtl]

James,

Couldn't the half-moon on cam be problematic if the state is wrong for the first crank rotation since it's not edge triggered?

Jean

[jsmcortina]

James,

Couldn't the half-moon on cam be problematic if the state is wrong for the first crank rotation since it's not edge triggered?

Jean

It certainly would have been, but the new cam sync method in the current code should be checking for a low-high-low-high (or vice versa) of the cam sensor with phase before declaring full-sync.
However, I would recommend using a sensor that doesn't require a change of state before giving the true value.

James

[walterclark1]

The common industrial/automotive (Cherry and Honeywell) gear tooth hall sensors and OEM (VW) hall sensors I have dealt with for my MS project contain no latching circuits of any sort, only a differential amplifier and Schmitt trigger, so they can only power up reflecting the presence or absence of ferrous metal (tooth/window). The type sensor you describe is called "latching". You can avoid any indeterminate power on state issues by not using a latching sensor.

[kaeman]

good to know I was looking at getting a sensor and changing to ignition control using wasted spark on a 36-1 trigger wheel. I didn't even think about latching sensors... thanks for the heads up.

[Healey3000]

The common industrial/automotive (Cherry and Honeywell) gear tooth hall sensors and OEM (VW) hall sensors I have dealt with for my MS project contain no latching circuits of any sort, only a differential amplifier and Schmitt trigger, so they can only power up reflecting the presence or absence of ferrous metal (tooth/window). The type sensor you describe is called "latching". You can avoid any indeterminate power on state issues by not using a latching sensor.

Hi,

My understanding of "latching" sensors appears to be different from yours. From what I can tell, latching characteristics apply to Hall devices that require a magnet to actuate them, that is, they don't have an integrated back-magnet. In this context, a latching sensor will toggle in the presence of a North pole and stay in that state, i.e., latched, until a South pole comes into proximity.

Latching behavior does not define power-up behavior, per the data sheets I've perused. It may be that the Cherry and Honeywell gear sensors use TPOS Hall sensors but I find it curious that the datasheets make no mention of power-up behavior. Perhaps, in their typical use as speed sensors, this is irrelevant.

If anyone has actually tested the GT type sensors and determined whether they are TPOS or not, that would be very useful information. Alternatively, if these have worked well with MS3, that would be indirect confirmation as well. I'm going to try and contact both Cherry and Honeywell to ask. May also ask Hamlin (Mouser has the Hamlin version of the GT101 sensor for around $12 in unit quantities, quite a steal).

Thanks.

[walterclark1]

I have a couple of the Hamlin that are a look-a-like to the Honeywell GT and they work well in the crank positions senor application for the MS. I have never seen one power on in a state different than what the gear teeth (crank tooth wheel) suggested they should. In my somewhat limited experience they seem better suited to the wheels like that sold thru DIYAutoTune than the Honeywell GT101, which appears to need a tooth wheel made from metal 1/4" or greater thick to perform reliably.

[billr]

Some of you folks weren't involved in my thread about this two years ago, you may want to take a peek:

http://www.msextra.com/forums/viewtopic ... ro#p350231

I did a lot of bench-testing, using a good scope, before I came to those conclusions. As already stated in this thread, all sensors based on the Hall-effect are not "created the same", testing of your particular one is always a good idea if there are any doubts.

[Healey3000]

Some of you folks weren't involved in my thread about this two years ago, you may want to take a peek:

http://www.msextra.com/forums/viewtopic ... ro#p350231

I did a lot of bench-testing, using a good scope, before I came to those conclusions. As already stated in this thread, all sensors based on the Hall-effect are not "created the same", testing of your particular one is always a good idea if there are any doubts.

Thanks Bill,

Looks like the ATS627 is a popular device. I just wish it was easier to mount - perhaps epoxying it in place is adequate...

[Healey3000]

Hi,

I checked with Hamlin regarding their 55505 Hall Effect Gear Tooth sensor (the mechanical twin of the Honeywell GT101) and it will NOT be a good choice for a polling style cam sensor.

The device powers up in the OFF state, regardless of whether a tooth is present or not. With a pullup resistor, that means the output will always be high at power-up. Might be problematic for MS3, based on James' assessment. Makes you wonder if the Honeywell and Cherry ones behave the same way. I have not been able to contact them yet.

I'm liking the ATS627 for this application. In my design, I will end up having it "look" through an aluminum wall of around .05" thickness. I hope the presence of the aluminum does not degrade performance too much, since I'll have a total gap of 2 mm.

[walterclark1]

This has me curious now. I am currently using a 55505 at the crank and the original VW distributor sensor (magnet opposite the hall with a vane moving thru the slot between). I wont be able to get to it or the spares with a scope for a couple weeks.

[elaw]

If I can toss my $0.02 in here... it looks like among the Allegro sensors, the ATS627 would not be the best choice for this application.

If you look at http://www.allegromicro.com/en/Products ... r-ICs.aspx, the 627 is listed as a "crank sensor", and while they say it has zero-speed capability, they also say it has a fixed power-on state. So it sounds like the output state is not accurate until it's seen at least one edge.

Allegro has a page that lists cam sensors at http://www.allegromicro.com/en/Products ... r-ICs.aspx. On that page I see the ATS675 which looks easy to use (no weird features like programmable switch points) but has what they call "TPOS" which is "true power-on state". I think that means the output reflects the presence of a target at power-on, without waiting for an edge.

[Healey3000]

If I can toss my $0.02 in here... it looks like among the Allegro sensors, the ATS627 would not be the best choice for this application.

If you look at http://www.allegromicro.com/en/Products ... r-ICs.aspx, the 627 is listed as a "crank sensor", and while they say it has zero-speed capability, they also say it has a fixed power-on state. So it sounds like the output state is not accurate until it's seen at least one edge.

Allegro has a page that lists cam sensors at http://www.allegromicro.com/en/Products ... r-ICs.aspx. On that page I see the ATS675 which looks easy to use (no weird features like programmable switch points) but has what they call "TPOS" which is "true power-on state". I think that means the output reflects the presence of a target at power-on, without waiting for an edge.

Oops! My mistake.

I meant to say the ATS675, not the 627. I had both datasheets downloaded and mixed them up. Thanks very much for catching the error.

TPOS is key, unless MS3 does indeed accommodate non-TPOS.

[elaw]

Well keep in mind that MS3 *can* work with either type of sensor. "Polled" mode (for the cam sensor) will get sync most quickly if you have a sensor that outputs the correct state beginning at power-on. But the MS can also be set to edge-triggered mode, where it considers the cam phase to be unknown until it sees a hi-lo or lo-hi transition on the cam signal.

[dontz125]

I have a couple of the Hamlin that are a look-a-like to the Honeywell GT and they work well in the crank positions senor application for the MS. I have never seen one power on in a state different than what the gear teeth (crank tooth wheel) suggested they should. In my somewhat limited experience they seem better suited to the wheels like that sold thru DIYAutoTune than the Honeywell GT101, which appears to need a tooth wheel made from metal 1/4" or greater thick to perform reliably.

Bear in mind that the issue would only be there for the first tooth transition. In no more than the very first 10* of crank rotation (assuming a 36-1) and likely less, the sensor would reset itself and send the proper signal.

[billr]

I don't know if I didn't make it clear in my old thread, or if folks just don't believe me. I found (then) that the ATS667 could be "out-of-phase" as long as it was powered up, that condition didn't correct itself after the trigger wheel started moving. If that is true, it is a very ugly subtlety of those "gear-tooth" sensors that we all should be aware of. If it is not true, then I need to figure out what I was doing wrong. It was two years ago that I did that testing, but I am going to set one up and see if I can repeat my previous findings. If I do get the same results, I would sure like somebody else to test a 667 and see if they can duplicate.

It wasn't clear to me that the 627 would behave any different, so I ended up using an Allegro A1120 with a magnet, a much simpler Hall switch. I'll have to take a look at the specs on the other device (675?) mentioned.

[Healey3000]

Well keep in mind that MS3 *can* work with either type of sensor. "Polled" mode (for the cam sensor) will get sync most quickly if you have a sensor that outputs the correct state beginning at power-on. But the MS can also be set to edge-triggered mode, where it considers the cam phase to be unknown until it sees a hi-lo or lo-hi transition on the cam signal.

Hi Eric,

That would certainly be an option. Please tell me if I have understood this correctly:

Use a cam sensor target with a single tooth as opposed to a half-moon.
Set MS3 to edge trigger mode
Set mode for falling edge - assuming that it powers up incorrectly, the leading tooth edge may or may not produce a signal edge. The next tooth edge (falling), would always be in sync. and correct at all times from that point on.

Sound right?

Thanks!

[Healey3000]

I don't know if I didn't make it clear in my old thread, or if folks just don't believe me. I found (then) that the ATS667 could be "out-of-phase" as long as it was powered up, that condition didn't correct itself after the trigger wheel started moving. If that is true, it is a very ugly subtlety of those "gear-tooth" sensors that we all should be aware of. If it is not true, then I need to figure out what I was doing wrong. It was two years ago that I did that testing, but I am going to set one up and see if I can repeat my previous findings. If I do get the same results, I would sure like somebody else to test a 667 and see if they can duplicate.

It wasn't clear to me that the 627 would behave any different, so I ended up using an Allegro A1120 with a magnet, a much simpler Hall switch. I'll have to take a look at the specs on the other device (675?) mentioned.

Hi Bill,

I have not used the 667 so I can't say for sure but the datasheet suggests that it always powers up in the high state and switches from there. What you may have encountered is the directionality behavior of this device, where the polarity can invert if running backwards. If using such a device, a backfire would really mess up the MS3!

You'll like the ATS675. I'm curious how you mounted the A1120. I have designed a dizzy replacement where the device would end up having to be potted and exposed to possible oil spray. Not sure if that's a good idea.

[billr]

No, it wasn't the polarity reversal due to direction of tooth travel, as described in the datasheet. I'll set up a test again and see if I get the same results. The easy explanation is that I was wrong before! For mounting those things I use JB Weld to set them into a pocket in aluminum. Although the JBW is viscous "as mixed", it flows very well into small clearances when it sits for a few minutes; heat rating on it is 450F, as I recall. In my case, I machined a small holding bracket for both the sensor and magnet with a ferrous vane passing between. This sensor assembly fits in where the ignition points used to be.