This is a list of common issues when setting up a GPS to work with the Velodyne family of sensors.
The common symptom is the absence of GPS coordinates within the web interface. PPS lock may also be absent. See Figure 1 below.
Figure 1 Missing GPS Position
All Velodyne sensor models accept the following NMEA sentences:
These sentences may be sent through the serial port or through Ethernet.
The serial parameters are 9600 baud, 8-N-1. Acceptable voltage levels may be RS-232 or inverted UART.
If using Ethernet, the network port is 10110. The packets may be UDP or TCP. The destination IP may be the direct broadcast address within the same IP block as the sensor's IP address (e.g., sensor IP: 192.168.1.xxx, direct broadcast IP: 192.168.1.255); the IP address of the sensor (e.g., 192.168.1.201); or the broadcast address 255.255.255.255.
All NMEA sentences must be properly formatted. The GPS time must increment every second or the sensor will not synchronize its internal top-of-hour counter with the minutes and seconds portion of the GPS time. The checksum must be valid. Finally, the sentence must terminate with either a '\r' (carriage return character), '\n' (linefeed), or '\r\n' (carriage return and linefeed).
If both GPRMC and GPGGA sentences are sent to the sensor, the sensor will use whichever is the most recent.
A common mistake when using GPS is using it indoors. GPS receivers require line-of-sight with the sky in order to receive the L-band signal. Indoors, GPS reception is spotty at best. You may also encounter multi-path issues, meaning your true location cannot be ascertained as the signal from the GPS satellites have bounced off walls, floors, and ceilings on its way to the receiver.
The Garmin 18LVC, an optional unit from Velodyne, is configured to transmit GPRMC sentences. The pulse-per-second (PPS) signal is sent once satellite lock is achieved. If satellite lock is lost at this point, the GPS receiver will keep transmitting the PPS signal. Velodyne sensors are configured by default to continue processing this GPRMC sentence and PPS signal, even if satellite lock is lost.
Note: If on initial startup no satellite lock is achieved, then the PPS signal will never be sent.
The sensor accepts CMOS level inputs. Voltages 0.8 V and below are low and 1.4 V or above are high.
You may send the sensor either RS-232 or inverted UART. A common inverter that the Technical Solutions Group has tested is the 7404.
The RS-232 voltage level may swing from ground to supply voltage. The interface box is just a pass through to the sensor. Reverse voltage protection is built into the sensor.
Once you have determined your GPS receiver is configured properly (correct voltage levels, line-of-sight with the sky, properly-formatted NMEA sentences with the correct checksum, and carriage return and/or newline-terminated), then it's time to check the connections on the interface box.
One of the more common issues for the sensor not synchronizing its internal top-of-hour counter with GPS time is a bent PPS pin on the white socket molex connector in the interface box. Visually inspect to make sure all pins are fine.
Next, check the wires running from the sensor into the screw terminal in the interface box. Shown below is the interface box with the 8 wires of the LiDAR sensor cable coming from the left. They are attached by screws to the screw terminal.
Another common issue with the screw terminal is loose or even cut wires. Use care when pulling on these wires to make sure they are secured to the screw terminal. You may want to go as far as re-securing the PPS and GPS receiving wires by replacing them in the terminals.
If you still do not see the GPS coordinates in the web interface or PPS lock, then it's time to diagnose the issue with an oscilloscope. Probe the GPS receive pin for serial data. Remember that this is RS-232 levels, so look for negative and positive voltage swings. Check the PPS signal to make sure it's 50% duty cycle with 5 volts, peak-to-peak.
If you have any questions or comments, please direct them to the Technical Solutions Group. You may reach us through the Contact Us link at http://velodynelidar.com. Please click on Contact Technical Support.