Concept 2 rowing machine sensor puzzle

[jpb]

Here is a puzzle.
I have an old (2003) Concept II rowing machine (or ergo) and a week ago the Performance Monitor failed to  register any output (stayed on zero).
The sensor system (as I've found out in the process of fixing it) consists of three small magnets in the fly wheel and a sensor which  is just a coil of wire around a metal pin (see attached pictures).
My sensor produced no output but noise when I attached a scope. The magnets on the fly wheel were ok (not rusted and could hold a paper-clip).
I've replaced the sensor today with a new sensor which now works nicely and produces output as shown in the pictures.

My puzzle though is that the impedance of the broken sensor is nearly the same as the new sensor (110 ohms instead of 109 ohms) - that is the wire hasn't gone open circuit which is what commonly happens (according to forums I've looked on).

So how can the coil have broken without changing impedance?

(Apologies for the rather out-of-focus scope shots - I had to shut the garage door to get rid of reflections and then it was a little dark.)

[maginnovision]

The resistance you're reading is probably a resistor, not a coil. Just look up variable reluctance sensors. My model E uses a different type of sensor so I couldn't tell you exactly how your is made but you could open it up yourself.

[mikerj]

A shorted turn or two could have negligible impact on the measured resistance, but would significantly reduce the output of the sensor.

[ace1903]

Maybe there is a bad solder connection or intermittent contact. I had similar issues with other sensors.
When you disassemble the sensor cable has bent in different directions and connection is reestablished.
Try to take measurements while bending the cable and tapping on the coil with some plastic hammer.

[mjkuwp]

Maybe there is a bad solder connection or intermittent contact. I had similar issues with other sensors.
When you disassemble the sensor cable has bent in different directions and connection is reestablished.
Try to take measurements while bending the cable and tapping on the coil with some plastic hammer.

I think it is this ^^

[jpb]

I don't think it is a bad solder joint because the measured resistance of 109/110 ohms was consistent yet when a scope was attached only noise came out.
I wondered if it was a bad connection to the monitor so I took the back off and re-plugged in the old sensor and I could measure the 110 ohms on the circuit board (the same as measuring it on the plug).
I did try a number of times to get it to come back to life before ordering the new part and there was never a sign of life.
Also, it had been a good connection for 17 years - so if it was a bad joint I'd expect it to turn up before then.
It wasn't intermittent - one day it was working and then when I had my next session it came up as all zeros and stayed that way.

I guess it is probably movement through thermal cycling (it now lives in the garage - I used to have room for it in the house in my old house) has either exacerbated a weak joint or even worn through some of the enamel insulation on the coil wire.

Someone on the concept 2 forums who repairs these things in New Zealand uses a multi-meter to check them (he stated that the resistance should be 105 ohms plus or minus 3 ohms so my meter is reading a bit higher), clearly this test does not work in cases like mine.

I've just had my first session with the new sensor and it all went fine - I just hope this sensor lasts another 17 years (I suspect that I won't!).

I'm very impressed that Concept 2 is still able and willing to supply spare parts for the model C which was superseded shortly after I bought it in 2003 - they may be expensive but they are solid and well supported, as an engineer I really like that.

[jpb]

I suddenly thought to measure the two sensors with my LCR meter (DE-5000).

I hoped to see a difference in inductance but, again the readings are very similar e.g.

At 1kHz measuring as series RL (I tried various combinations):
OLD L= 44.57 mH ESR = 147.0 ohms
NEW L=43.15 mH ESR = 150.9 ohms

So I'm as puzzled as ever.

[ace1903]

Can happen iron core to became magnetic and to annulate the effect of moving magnet.
That is why the screwdriver demagnetizer tool exists.
From picture when coil is out of the holder inductance and resistance seems OK.
But fot that many turns I expect larger induced EMS. Either core is magnetizied or magnets on moving part is weak.

[jpb]

Can happen iron core to became magnetic and to annulate the effect of moving magnet.
That is why the screwdriver demagnetizer tool exists.
From picture when coil is out of the holder inductance and resistance seems OK.
But fot that many turns I expect larger induced EMS. Either core is magnetizied or magnets on moving part is weak.

The moving magnets are the same for both the old and new sensors so don't explain the difference.

The core becoming magnetized is an interesting theory - I'm not sure how I can test it though as I don't want to remove the new sensor.

I guess if this is the problem then it could be demagnetized with an AC signal ?

If I get the chance at the weekend, I might experiment with the old sensor, my scope and a magnet held in my hand to act as the signal and try different cores.

Note, the moving magnets are not that close to the core when they pass - it is several mm (it is a bit hard to measure as it is behind the flywheel) so the magnetic field is not that strong.