Using this code it is possible to extract data using Modbus TCP/IP from a ZX 300 lidar. This data gets interpreted and may be used for different purposes such as the digital twin (IoT application). The register numbers are based on the Modbus Guide by the manufacturer ZXLidar. It is important to be aware that the implemented Modbus function is read only. Any changes to the Lidar unit must be made using the Waltz Software.
Additional information on how the lidar is used in project WindIO is available in the private lidar guide (nextcloud/499_LIDAR_und_Stromversorgung/020_documentation/Lidar_Guide).
After receiving the data (using lidar_data.py) the dataset is published internally on Motion Sensor Box using ZeroMQ. The dataset needs to contain specific parameters to be compatible with other services running on the Motion Sensor Box e.g. the fusionlog and the lidar_mqtt.py script.
lidar_mqtt.py is used to publish the lidar data to a server using the MQTT protocol.
These variables correspond to the variables names used in lidar_data.py (when read from modbus). For some variables, different names are used when sent via MQTT (see lidar_mqtt.py).
timestamp_data_received = this time stamp is generated by the motion sensor box when a new data set is received
uptime = the system uptime (of motion sensor box)
reference_of_dataset = each data set has its own reference number, the reference number may be used to check whether any data might be lost
timestamp_data_generated = this timestamp is generated by the lidar unit and corresponds to the reference number
set_height_x = the height of a wind speed measurement that is configured in the Waltz Software (symbol x = {1, 2, ... n}) (important: set_height_11 is currently incorrectly set to 0 m)
individual_reference_x = reference number of a wind speed measurement height dataset. Since the lidar unit is only able to measure each height at every full iteration once, the individual_reference_x corresponds only to this sub data set. This reference is used for the calculation of the individual_timestamp_x
individual_timestamp_x = time stamp of a wind speed measurement height dataset. The individual_timestamp_x is not received from the lidar station. It is calculated as followed: The time it takes to measure one height (usually 1 second) is multiplied by the place in the order of heights (measured from highest to lowest) and then added to the timestamp received by the lidar unit.
horizontal_windspeed_x = the horizontal wind speed at the given height measured in m/s
vertical_windspeed_x = the vertical wind speed at the given height. A positive value corresponds to -gravity direction (upwards), a negative value corresponds to +gravity direction (downwards).
wind_direction_x = the wind direction at the given height. 0° = wind is coming from North and blowing towards South, 90° = wind is coming from East.
temperature = the air temperature measured by the met station
battery = the voltage of the battery
air pressure = the air pressure measured by the met station
windspeed (met-station) = the windspeed measured by the met station (approx. 1.5 m above the ground)
tilt = tilt measured by the met station (if there is nonzero tilt the lidar measures in a line that has an angular offeset to the g acceleration vector)
humidity = air humidity
wind_direction = wind direction measured by the met station (approx. 1.5 m above the ground)
pod_upper_temperature = internal pod temperature as measured by the internal sensor located near the window
pod_lower_temperature = internal pod temperature as measured by the internal sensor located near the external heat sink
pod_humidity = internal pod relative humidity
scan_dwell_time = the time it takes to measure all wind speeds and parameters at one height
If the Lidar can not measure a specific value, the Modbus register contains error codes instead. Explanation is as followed:
9999 = it is not possible to measure wind speed data. Possible causes may include very low wind speed, partial obstruction on the scanner window or interference with the laser beam.
9998 = atmospheric conditions which affect the wind speed measurement. This may be fog or precipitation.
Since the data is streamed to a remote server which can only display the data properly if any extraordinary values are hidden, the value 9999 and 9998 are replaced with the value None
The Modbus registers may be found in the ModbusGuide by the manufacturer ZXLidar. Not all registers displayed in the ModbusGuide are polled in the script lidar_data.py. Some other registers were found by reverse engineering, these include the register numbers of the vertical wind speeds and the wind direction for each height. These register numbers may be looked up in the lidar_data.py script.
