PWM Motor-Control Signal Verification

Materials and Setup

• XRP Robot
• Red Pitaya (configured for LV ±1V inputs)
• Oscilloscope probe (configured for 10x attenuation)
• Two male-to-male jumper wires (GPIO 34 and GND).

Measurement Configuration

1. Verify Zero-Power State: Ensure the XRP is completely disconnected from USB-C and the physical battery switch is OFF. Power off the Red Pitaya before changing its input jumpers or probe connections.
2. Ground Connection: Insert a male-to-male jumper wire into a XRP GND pin on the extension header.
3. Connect the Probe Ground: Attach the Red Pitaya Channel 1 probe ground clip to the exposed male end of the jumper connected to XRP GND. Connecting the ground clip first is a good practical habit, but make sure the system remains unpowered while making all connections.
4. Prepare the Signal Connection: Insert another jumper wire into the XRP expansion-header pin labelled GPIO 34 / Motor L Phase.
5. Connect the Probe Tip: Attach the Channel 1 probe hook to the exposed metal end of the jumper connected to GPIO 34. Make sure the hook contacts the metal conductor, not only the black plastic housing
Connection summary:
1. XRP GPIO 34 ── male jumper wire ── Channel 1 probe hook
2. XRP GND ── male jumper wire ── Channel 1 ground clip
3. Secure the jumper wires or probe hooks so that the connections do not need to be held by hand while the XRP is powered. Do not adjust or reposition the connections during the measurement.
6. . Configure the Probe and Input: Set the physical probe to 10×, set the Red Pitaya software probe attenuation to 10×, and verify that the Red Pitaya input hardware is configured for LV.
7. Vertical Scale: Set the vertical scale to 1 V/div. With a correctly configured 10× probe, this displays the 3.3 V logic waveform clearly while keeping the voltage at the Red Pitaya input within the LV range.
8. Timebase: Set the timebase to 10 ms/div to display the approximately 20 ms PWM period.
9. . Trigger: Set the trigger source to Channel 1, the edge to rising, the level to approximately 1.65 V, and the mode to Normal.
10. Enter Safe Testing State: Once the wiring and oscilloscope configuration are verified, connect the USB-C cable to the XRP and leave the battery switch OFF. In the tested setup, this powers the controller logic without battery-powered motor operation. Keep the robot secured during the test.

Exploration and Analysis

1. Signal Capture: Run the MicroPython script commanding the left motor to drive at 25%, 50%, and 75% effort. Use the Red Pitaya oscilloscope interface to capture the waveforms Click STOP in the Red Pitaya web interface during any stage to freeze the waveform for cursor measurements.

import time
from XRPLib.defaults import *

print("Starting Left Motor Verification Sequence...")

try:
    # 1. 25% Motor Effort for 7 seconds
    left_motor.set_effort(0.25)
    time.sleep(7)

    # 2. 50% Motor Effort for 7 seconds
    left_motor.set_effort(0.50)
    time.sleep(7)

    # 3. 75% Motor Effort for 7 seconds
    left_motor.set_effort(0.75)
    time.sleep(7)

except KeyboardInterrupt:
    print("\nExecution interrupted by user.")

finally:
    # 4. Safe automatic stop
    left_motor.set_effort(0)

2. Duty Cycle Verification:

• Test 1: 25% Effort
• Visual Target: $$T = 20\text{ ms}$$ (2 horizontal divisions at 10 ms/div)
• Task: Measure the duration the signal stays HIGH $$(\Delta t_{\text{HIGH}})$$.
Check expected values & waveform

• Expected $$\Delta t_{\text{HIGH}}:$$ 5.0 ms (0.5 horizontal divisions)
• Calculated duty cycle: $$\frac{5.0\text{ ms}}{20.0\text{ ms}} \times 100\% = 25\%$$
• Observed frequency: 50 Hz

• 50% Effort
• Visual Target: T = $$20\text{ ms}$$ (2 horizontal divisions at 10 ms/div)
• Task: Measure the duration the signal stays HIGH $$(\Delta t_{\text{HIGH}})$$.
Check expected values & waveform

• Expected $$\Delta t_{\text{HIGH}}$$: 10 ms (1 horizontal division)
• Calculated duty cycle: $$\frac{10.0\text{ ms}}{20.0\text{ ms}} \times 100\% = 50\%$$
• Observed frequency: 50 Hz

• 75% Effort
• Visual Target: T = $$20\text{ ms}$$ (2 horizontal divisions at 10 ms/div)
• Task: Measure the duration the signal stays HIGH $$(\Delta t_{\text{HIGH}})$$.
Check expected values & waveform

• Expected $$\Delta t_{\text{HIGH}}:$$ 15.0 ms (1.5 horizontal divisions)
• Calculated duty cycle: $$\frac{15.0\text{ ms}}{20.0\text{ ms}} \times 100\% = 75\%$$
• Observed frequency: 50 Hz

3. Waveform Anomalies (Overshoot): Examine the sharp rising edge of each pulse. You may observe a transient voltage spike exceeding $$3.3\text{ V}$$ (often peaking near $$4\text{ V})$$ before settling. This is not true battery voltage leakage, but rather high-frequency inductive overshoot caused by jumper wire inductance, probe ground lead loop inductance, and the fast switching speed of the microcontroller GPIO.

Conclusion

Shutdown Procedure

1. Halt Execution: Stop the MicroPython script in XRPCode.
2. Verify Waveforms: Confirm via the Red Pitaya interface that all active PWM logic traces on GPIO 34 have ceased pulsing and flatlined.
3. Unplug Logic Power: Disconnect the USB-C cable from the XRP Controller.
4. Isolate Battery: Double-check that the physical battery switch is set to OFF.
5. Safe Modifications: Only alter, disconnect, or clean up your physical probe connections and jumper wires once the system is in this completely unpowered state.