Help! Exercise Bike & Elliptical Magnetic Pedal Resistance Controller Circuit Explained

[vestaviascott]

On older exercise machines, a "H Bridge" circuit is used to control the motor/magnet position for the resistance setting. See this excellent video tutorial on a teardown and fix of a failed transistor that caused the resistance circuit to fail:

However, on more modern exercisers, it does not appear that the "H bridge" configuration found above is present. Perhaps over time this "H bridge" circuit has been consolidated into a single IC? For example, this instructable illustrates the same issue as above, failed resistance circuit at the console board:

https://www.instructables.com/Fix-your-NordicTrack-Exercise-Bike/

As you can see looking at the board, there does not appear to be an "H bridge" on the board (that I can see). Does it appear that a single IC chip has replaced this circuit? If so, I'm seeking to ID that chip in order to fix several consoles I have on hand that appear to have this same issue.

Here is another good video that illustrates how the circuit works along with expected voltages back and forth from the console circuit to the tension/servo motor and potentiometer:

 

[kellys_eye]

Perhaps over time this "H bridge" circuit has been consolidated into a single IC?

Most certainly they do make integrated h-bridge controllers - sometimes with more than one bridge in a package too.

Trace the motor wires back to the board and from there to the IC that 'may' be controlling it then look up the datasheet for that chip - it will tell you what it is.

 

[vestaviascott]

Here are a few photos to help. There are not many ICs on the board actually:

Photo 1: Resistance up and down are the two buttons bottom right:

Overall shot of board:

Close Up:

 

[vestaviascott]

I will make an inventory of the few ICs on this board and do a lookup to determine if any are possible candidates for the resistance controller bridge circuit.

 

[vestaviascott]

I think I may have a candidate IC. The IC marked "U5" just above and the the right side of the 12 pin connector is listed as a "Weltrend WT7901". The product detail sheet is here: http://www.weltrend.com/en-global/product/detail/66/86/273

It reads:
The WT7901 is a monolithic bridge driver power MOSFET for its driver stage. It is provided with forward, reverse and brake functions and is ideal as a driver circuit for a motor that winds or rewinds the film in a camera.

 

[vestaviascott]

I suppose if I can get a replacement chip for this, I can just replace it and test that it fixes the resistance circuit! Otherwise, I'm open to suggestions on how I may test this IC to determine if it's faulty.

 

[vestaviascott]

After some research, it appears that this specific chip, WP7901, is in very short supply unless I order min 50 qty. I suppose other "monolithic bridge driver power MOSFET" chips potentially work providing that the specs are the same? That's assuming that the IC is to blame for the issue with the circuit.

An "E-3" error showing on the console after the resistance is fiddled with, but the behavior of the resistance system is that it does not appear to be able to properly read the position of the resistance motor position/potentiometer.

When incrementing the resistance via the control panel, the resistance servo motor, rather than simply rotating a small amount and moving the magnet to the correct relative position reading on the console, "level 2 for example", it rotates a near full revolution and moves the magnet all the way closest to the flywheel (as if "level 12", the highest resistance level where asked for).

Eventually an E3 error is flashed on the console and some times the resistance motor keeps moving back and forth in tiny increments as if the console is sending it in a +/- voltage loop.

 

[vestaviascott]

Here's a closer view of the circuit area where I suspect the resistance logic is. The 12 pin connector is along the left side. Starting from the bottom of the connector, there are Red, Black, Yellow, Green and Blue wires. These wires are discussed in the 2nd video I posted in the opening thread and are directly connected to the resistance motor.

It's not exactly clear to me how to fully follow the traces from those pins. But there do appear to be dark trace lines coming from that area of the connector and going across the U5 ic as well as the electrolytic capacitors and surface mounted resistors & capacitors in this area.

From top to bottom, the wires are:

PURPLE
PINK
BROWN
WHITE
GRAY
YELLOW
GREEN
BLUE
YELLOW
BLACK
RED

The last 5 wires are directly referenced in the 2nd video I posted in the opening thread. Red, Black, Yellow, Blue and Green.

 

[kellys_eye]

The H-bridge controller chip can only move the motor. It does nothing to determine the actual position of the motor (as reported back to the controller) so there has to be some form of feedback.

When the motor moves, does it move anything else i.e. a potentiometer? A faulty feedback potentiometer will result in the symptoms you are seeing, rather than a faulty motor controller as the motor IS actually moving.

 

[vestaviascott]

The H-bridge controller chip can only move the motor. It does nothing to determine the actual position of the motor (as reported back to the controller) so there has to be some form of feedback.

When the motor moves, does it move anything else i.e. a potentiometer? A faulty feedback potentiometer will result in the symptoms you are seeing, rather than a faulty motor controller as the motor IS actually moving.

Thanks, thats good to know!
Yes, the electric motor is geared to a potentiometer that is part of the servo motor assembly. When the motor moves, it turns the wiper/shaft of the potentiometer. On the end of the potentiometer wiper shaft is a black round plastic wheel that is part of the linkage that moves the magnet relative to the flywheel. You can see the operation of the motor and potentiometer in the 2nd video I posted above in the opening thread.

 

[vestaviascott]

A faulty feedback potentiometer will result in the symptoms you are seeing, rather than a faulty motor controller as the motor IS actually moving.

I agree. The issue appears to be the potentiometer circuitry.

As a test to eliminate the servo's mechanical potentiometer as the issue, I have replaced the defective console with another known good one. The exercise bike's resistance works as expected when using this replacement console with the existing servo motor and potentiometer assembly.

It appears that the problem is with the "potentiometer reading circuit" or IC inside the controller console. I just thought that was contained as part of that IC chip, so now I need to figure out which IC on the board is responsible for reading the potentiometer of the servo assembly. Because, as you indicate, the motor does move forward and backward, just not at the prescribed increments relative to the user inputs at the console.

 

[kellys_eye]

The signal (feedback) from the position sensor will be read by the microprocessor - probably using a built-in analog-to-digital converter. There may be some form of signal filtering (R-C filter) between the potentiometer and the uP so tracing the wires from the pot, to the board, and there-on to the uP might reveal something in that component path.

Since the pot is 'geared' have you checked the meshing of the cogs? If they are (inevitably) plastic then either stripped gears or even improperly meshing gears might cause a problem.

What is the IC that is socketed? Op-amp perhaps? This could be the signal conditioning and given they went to the trouble of socketing it, it might be because it is quite prone to developing a fault and therefore needs to be easily replaced i.e. a socket.

 

[kellys_eye]

Nope - scrap that idea. The socketed chip (when seen in close-up) is the EEPROM used to store switch-off paramaters etc.

Still, trace the feedback path and/or show it in a pic.

 

[vestaviascott]

Since the pot is 'geared' have you checked the meshing of the cogs? If they are (inevitably) plastic then either stripped gears or even improperly meshing gears might cause a problem.

Since the geared pot and servo motor work as expected when the known good console is swapped with the suspect console, I've eliminated the motor/pot assembly as a point of failure and I'm isolating on the suspect console.

 

[vestaviascott]

Still, trace the feedback path and/or show it in a pic.

OK, I'm not really skilled or knowledgable in tracing paths on boards. I'm learning though. Can you tell me what I can isolate on to help with tracing the path in a pic?

 

[vestaviascott]

Hopefully this helps? Its a wide shot from my last closeup pic above:

And here is the closeup again for easier reference:

 

[vestaviascott]

FWIW, someone made an instructable for a hack to bypass the resistance circuit when you have the above error condition where the console no longer communicates properly (or at all) to the resistance servo motor unit:
https://www.instructables.com/Fix-your-NordicTrack-Exercise-Bike/

I certainly don't want to resort to that hack, but I mention this thread because the interesting thing is that if you scroll down to the comments section, search for "BD6210", you'll find a comment by a user RaK6, who found that replacing that chip (very similar to the one mentioned above that I found on my board and suspect to be the motor controller chip - WT7901) fixed his issues with the resistance circuit. All these exercise bikes and ellipticals appear to use the same 6 volt DC motor + 5K potentiometer servo assembly, I suspect that the chips should be interchangeable? Perhaps I could swap in a BD6210 chip for the existing WT7901?

The question though, is, are you certain the chip cannot handle the feedback from the potentiometer? There appears to be a PWM circuit in the datasheet for the BD6210, could that potentially facilitate this function?

 

[vestaviascott]

Here's what I get from ohm test between the IC driver chip (WT7901) and the 12 pin harness:

Left columns 1-2 above black line are the 7901 chip pinouts, column 3 is the ohm matches between the chip pin and the 12 wire harness, and I found that capacitor C18 near that chip ohms to the VCC pin of the chip, for what that's worth.

 

[vestaviascott]

Update: To match the 12 harness wires with the servo motor assembly connections, here's a pic:

And here's the other end of that wire bundle at the connector plug where it plugs into the upper display console:

There is a semi-hidden wire at the left end between the PINK and the PURPLE wire. That wire is LIGHT BLUE and is part of the 3 wire white connector you can see near my ring finger in the top photo. That is the connector that attaches to the barrel plug on the front of the bike that takes the 9volt 2amp DC wall adapter.

Next to those are the BROWN and WHITE wires that are attached to the connector to the right of the WHITE one. That's the speed sensor wires attached to a hall effect sensor that measures crank rotation for the RPMs readout.

Next to those are the PALE YELLOW and GRAY wires. Those appear to lead to the Potentiometer fine-tune adjustment screw. You can see it as a white plastic screw head inside a small blue square between the motor and the pot.

Following those are the GREEN, BLUE and GOLD wires that are attached to the servo potentiometer. The BLUE (center) pin wire is the pot wiper feedback (in the video, that wire is white).

Finally, we have the BLACK and RED wires which lead directly to the DC motor.

 

[kellys_eye]

Clean the potentiometer track (squirt contact cleaner in or use a syringe and get some petroleum jelly in there).

If the system has a servo motor that isn't 'tracking' properly then you either have a bad drive to the motor or a bad feedback to the controller. Since the motor actually moves I'd go for a 'missing' (or intermittent) feedback signal.

It's also possible that the gearing between motor and feedback pot is missing tooth or two - this will also result in 'hunting'.