How would you measure rower / rowing machine sensor output?

Thread Starter

Crashcup

Joined Oct 11, 2017
17
I have a indoor rowing machine with a monitor that just doesn't work the way I want. I've used Concept 2 rowers at the gym before, and I like the way the data is presented on the performance monitor. But, $1000 for a Concept 2 isn't in the budget.

It would be a fun project to make a performance monitor for the rower I have. I'm not sure of all the details, but I think I could read the output from the two sensors with an Arduino and calculate various performance metrics, and drive a small LED or TFT display to show them. I'd have the flexibility to monitor the metrics that are important to me, and to design the display to show the most important ones more prominently.

Anyway, I think to start I'd need to know what kind of output the sensors give. There's one on the resistance wheel that has a magnet that passes by it once per revolution of the wheel. And a second sensor I haven't really gotten a look at, under the rail that the seat rolls on. I suspect that also has a magnet that passes over the sensor every time the seat rolls back or forward.

So (finally), my question is what do you think I'd expect the signal from each sensor to be? The standard monitor unit has two AA batteries in it, so I suppose the circuit has 1.5V or 3V on it. Would the wires from the sensors probably have a low, or no voltage on them normally, then a pulse of full voltage when the magnet passes by the sensor? Or the other way around, full voltage, and when the magnet passes over the sensor it drops to nothing?

Would I be able to tell just from a simple DMM and watching the voltage reading when I position the magnet next to the sensor? I'm not sure how small the pulse from the sensor would be. I do have an Arduino with a data logging shield.

How would you go about determining what that signal looks like?

Thank you for any help!
Keith
 

Sensacell

Joined Jun 19, 2012
2,518
To see what it "looks" like, connect an oscilloscope.

You might get some idea with a meter, but the results will be ambiguous.

Connect one lead to battery minus, probe the sensors with the other.
 

pmd34

Joined Feb 22, 2014
499
Hi Crashcup, the sensors are probably "Hall effect" magnetic sensors. As sensacell suggests an oscilloscope would give you the best idea of what the signal looks like. Or you might try seeing if you can identify a part number on the sensor itself. You might get away with lighting up an LED (through a resistor) from the output of the sensor if you dont have a scope.

For your own interface, you might consider a "nextion" screen interface, they are a bit more expensive than doing the LCD interface yourself, but i understand they are very fast and east to set up a nice interactive display.
 

AnalogKid

Joined Aug 1, 2013
8,226
The sensor on the seat rail counts reps, and the sensor on the wheel gives you 1/2 or the information you need to determine total effort (watts, kcal, whatever). To get that you need something that measures force. On elipticals, the big wheel drives a generator. Varying the resistor bank changes the force required to turn the wheel, and measuring the voltage across the resistors gives you (with some math) energy expended. Anything in your machine like that?

ak
 

Thread Starter

Crashcup

Joined Oct 11, 2017
17
....Anything in your machine like that?
ak
No, probably hall effect like PMD said. I don't see anything that could measure a force. The stock monitor does have calories displayed. I doubt it's very accurate, but I suppose they could roughly estimate effort based on the speed of the resistance wheel. Higher speed = higher effort, and they could have a lookup or an equation in the software to estimate force on the chain for any given wheel speed. Wouldn't take into account acceleration, though.

PMD, Sensa, I don't have a scope, and hadn't even entertained the thought of spending hundreds of $ for that piece of equipment.... Until... I just looked on Amazon and eBay and saw that there are cheap chinese made oscilloscopes for as little as $30-40. Not super precise, I'm sure, but for this kind of use they wouldn't need to be. I just might order one of those.
 

AnalogKid

Joined Aug 1, 2013
8,226
The stock monitor does have calories displayed. I doubt it's very accurate, but I suppose they could roughly estimate effort based on the speed of the resistance wheel. Higher speed = higher effort, and they could have a lookup or an equation in the software to estimate force on the chain for any given wheel speed.
My old Nordic Trak has the same thing. Pretty much useless.
 

pmd34

Joined Feb 22, 2014
499
Hi Keith, without a scope....
You could connect a multi-meter to the sensor and potentially you may see a voltage dependent on the speed at which the sensor gets repeatedly triggered, it sort of assumes that the existing electronics after the sensor is putting a load on it so its not a dead certainty. You can as I say also try using a 5mm LED with a series resistor of say 1k..this will limit the current the LED can take, so should be safe and not over power anything.
 
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