# 12. Output devices¶

For this group assignment, we evaluate the power consumption of output devices. We will focus on the stepper motor used in Quentin’s week assignment.

## Stepper motor¶

The stepper motor is driven by an H-bridge which inverts the polarity of the voltage. While the voltage value should remain approximately 5V, the current direction gets reversed. For that reason, we decide to make a differential measurement with an oscilloscope. We want to measure the voltage drop across a $4.7~\Omega$ resistor that we added in series with the motor’s winding. This will disturb the current’s value a bit, as the winding’s total resistor is now about $10\%$ higher. We can compensate for this when computing the power, but we will neglect this effect for now.

A closer look shows that the oscilloscope’s two channels have the same ground reference, but measure each side of the resistor.

On the Picoscope software, we can see channels A and B, and the slight difference between them. When the motor turns, the voltage applied looks like a square waves with overshoots.

We add a differential channel between A and B.

The new differential channel has a mean of 0, which is expected because the direction of the current is reversed every half-cycle.

The best way to estimate the overall power consumption is the RMS (displayed at the bottom).

Knowing the probe resistor’s value, we can compute the RMS current as:

I = \frac{V}{R} = \frac{285~{\rm mV}}{4.7~\Omega} \approx 60~{\rm mA}

Assuming a RMS voltage $V\approx 4V$, we get to a power of $P=VI\approx 0.24~{\rm W}$. We can also check the power consumption when the motor does not turn:

Which leads to $I\approx 67.7~{\rm mA}$ and $P \approx 0.27~{\rm W}$. We can replace the oscilloscope with a current meter to confirm the value of the current. The value is not 100% stable but is close to what we estimated.