Crank torque page, 1/2 power?

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30 December 2017 11:30 AM

ibooij Newbie

Total Posts: 17

Joined 2013-02-23

Hi,

I'm implementing a slave for, amongst others, bike power meters. It works well for power meters that send standard power page (0x10).

There's an issue with the values from crank torque power meters (that don't send data page 0x10, only 0x12, like older Quarqs). As I don't have a crank torque power meter I thought I'd test my implementation with SimulANT+.

I'm running into the issue that the power reported in this way is roughly half of what it should be. I'm using the calculations from the "ANT+ Device Profile Bicycle Power" documentation.

Below is the C++ code I'm using. This calculation is from section 11.3 of the Bicycle power document.
The calculation is only done if event counts and period for current and previous message are not the same

const quint16 accumatedTorqueDelta = currentMessage.accumulatedTorque() - previousMessage.accumulatedTorque();
const quint16 periodDelta = currentMessage.period() - previousMessage.period();

const qreal torqueDeltaAsReal = static_cast<qreal>(accumatedTorqueDelta);
const qreal periodDeltaAsReal = static_cast<qreal>(periodDelta);

return static_cast<int>(128 * M_PI * torqueDeltaAsReal / periodDeltaAsReal); 

What could I be missing here? Something silly probably..

Thank you for reading,
Ilja Booij
BigRingVR.com

08 January 2018 05:21 PM #1

heather.honecker Moderator

Total Posts: 1

Joined 2017-05-04

PM

Hi Ilja,

The calculation in your code looks correct. I think SimulANT+ is the source of the mismatched power values you're seeing.

In the simulated crank torque sensor, the configuration section includes both crank torque and instantaneous power. The crank torque simulator only transmits the configured crank torque value, which does not correspond to the configured instantaneous power. This means that the power value calculated from the transmitted crank torque may be different than the instantaneous power shown in the simulator.

To change the power value calculated from page 0x12, you can modify the crank torque and/or cadence values in the configuration.

Feel free to ask if you have any further questions!

Heather

10 January 2018 02:24 AM #2

ibooij Newbie

Total Posts: 17

Joined 2013-02-23

PM

Hi Heather,

thank you for your answer.

I was under the impression that the simulator would automatically match crank torque & instantaneous power values. I was clearly mistaken!

Thanks for clearing it up!

Ilja Booij
BigRingVR.com

13 August 2018 11:51 AM #3

Russell Brown Newbie

Total Posts: 24

Joined 2015-12-30

PM

ibooij - 10 January 2018 02:24 AM
Hi Heather,

thank you for your answer.

I was under the impression that the simulator would automatically match crank torque & instantaneous power values. I was clearly mistaken!

Thanks for clearing it up!

Ilja Booij
BigRingVR.com

How does one set up the simulator for a certain power, then? What values for rpm, cranklength, Nm do I need for example, 320w?

13 August 2018 12:14 PM #4

Russell Brown Newbie

Total Posts: 24

Joined 2015-12-30

PM

Russell Brown - 13 August 2018 11:51 AM

How does one set up the simulator for a certain power, then? What values for rpm, cranklength, Nm do I need for example, 320w?

If I might answer myself: In the version of simulant+ I have(2.2.0.0) the CTF simulator _DOES_ show the correct computed value in the Instantaneous Power field of the config properties for a given Cadence and Nm. SOOOOO you can then copy those values into the Crank Torque config and use that to verify your outputs match your inputs.

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Crank torque page, 1/2 power?