OpenTX - Application Note 34
RPM & Temperature Sensor
This application sets up the RPM sensor. This telemetry sensor also has two temperature sensors. It is compatible with any of the X4R, X6R X8R or S6R receivers via the smart port.
Using these makes it easy to move the RPM sensor from one model to the next
To scale the number of pulses detected by the RPM sensor to motor RPM, the sensor needs to know how many poles the motor has. The RPM sensor uses four LEDs and a pushbutton to set the number of poles.
To place the sensor in configuration mode, press and hold the pushbutton until one or more of the LEDs turns on and then release it. The number of configured poles is displayed in binary by the four LEDs. Press and release the button until the required number is displayed on the LEDs. Turn the power off to retain the selected configuration.
The temperature sensors range is -20 to 250°C and does not require any configuration. I placed temperature sensor 1 on the heat sink of the ESC.
I placed temperature sensor 2 on the base of the motor. (The sensor is held in place but is not squashed by the washer.)
Turn power on the receiver. On the Taranis telemetry set up screen, select “Discover new sensors” and press Enter. The following three telemetry variables are defined, “Tmp 1”, “Tmp 2” and “RPM”. Press Exit to end the new sensor discovery.
To enable logging of these variables, edit each one and tick their “Logs” boxes.
To log data on the SD card, I created the following special function to enable data logging when switch SC is in the middle position.
I set up telemetry screen 2 to display the current values and the maximum values.
To monitor the temperatures during flight, I created the following special functions to announce the value of temperature sensors 1 and 2 every 5 seconds when switch SC is in the middle position. (Same as data logging.)
The motor has a spinner, which inhibits airflow through the motor, but it has good airflow over it from an air scoop that directs air from the inlet vents around the motor to exhaust vents on top of the cowl.
I found during a flight that the ESC temperature rose from 19°C ambient to 40°C, a temperature rise of 21°C. The ESC has very good airflow over its heat sink. This is well within the ability of the ESC to continuously operate with.
The motor temperature rose from 19°C to a maximum of 55°C, a rise of 36°C.
On days when the ambient temperature is 30°C, the ESC temperature would get to 51°C and the motor temperature would get to 66°C. Some motor manufacturers have said that motors should not operate above 80°C or the magnets can become demagnetised.
Based on these graphs, I chose to set an ESC temperature warning alarm at 50°C and a critical alarm at 55°C. Also, I chose to set a motor warning alarm at 65°C and a critical alarm at 70°C. With the alarms in place, the temperature announcements described above are changed so that the temperature is only announced if it is above the warning alarm.
“t1walm” is an audio file which says “ESC temperature warning”.
“t1calm” is an audio file which says “ESC temperature critical”.
“t2walm” is an audio file which says “Motor temperature warning”.
“t2calm” is an audio file which says “Motor temperature critical”.
These audio files were created and placed in the folder [SOUNDS]/[EN] of the SD card.
On hot days or if something goes wrong, the alarms will tell me that something is hotter than it should be and to land before something fails.