I have a question regarding the purpose of \'grounding\' something. I recently h
ID: 1813461 • Letter: I
Question
I have a question regarding the purpose of 'grounding' something. I recently had to connect a flow probe to a transmitter which had 3 connections on it (V+, data, shield). I guess for now, you could consider V+ to be 'hot', and data to be 'common' but the shield connection is what I'm having troubles with. There was a terminal on the transmitter for the shield wire but in this case, the shield must have been connected to the ground terminal in order for the transmitter to read the flow. Why is it that a device must be grounded for it to work. My basic understanding of ground is for faulty current and isn't necessarily required for the device to work. Why is that? My example may be hard to understand and if so, just explain why grouding something is necessary?
Explanation / Answer
ArduCopter supports the MaxSonar line of sonars for low level terrain following and in the future collision avoidance. Sonar is NOT necessary for altitude hold and will not work above 10m. Even if you have a sonar sensor, you should disable it in the Mission Planner hardware setup page unless you specifically need it for terrain following on a particular mission. Sonar tends to be a somewhat noisy data stream and will not do as a good a job in altitude hold as the built-in baro sensor on APM 2.5.
When in Alt-Hold or Loiter flight modes the sonar is used to adjust the absolute target altitude to keep a constant distance from objects sensed below.
To avoid spikes in the sonar data you should connect with a shielded cable and include a capd resistor to filter the power to the sonar as on techniques to reduce noise and improve sensor performance.
The sensor's GND, V+ and "AN" or "3" pins should be connected to APM 2's A0 pins as shown in the diagram below:
The sensor's GND, V+ and "AN" or "3" pins should be connected to the IMU shield's Pitot tube GND, +5V and IN pins as shown in the diagram below:
It's important that the sonar sensor be mounted at least 10cm (3 inches) away from the body, to avoid electrical noise from the ESCs and other interference. There are sonar mounts available for the 3dr quad, hexa-a and hexa-b frames. Below is another example in which a user used a strip of plastic attached to the body:
You can enable the sonar sensor and set the model from the Mission Planner's Configuration screen under the Hardware Options as shown here:
Alternatively you can enter the CLI mode using the Mission Planner's Terminal tab. Type "setup", "sonar on" and then set the model by entering "sonar 0", "sonar 1" or "sonar 2" for the XL-EZ0, LV-EZ0 or XL-EZL) respectively.
You can enable/disable the sonar in flight using your ch7 switch by setting the CH7 Opt field to "Sonar" and then pressing "Write Params" button on the Mission Planner's Arducopter Config screen. This is useful to avoid situations where the sonar senses the tops of trees and attempts to follow them leading to erratic alt-hold.
The Mission Planner's terminal screen can be used to test the sonar. After ensuring that your COM port is selected, press the Terminal button. When the prompt appears type "test" and "sonar". Note that the minimum distance displayed by the sonar will be about 20cm.
You can also test your in-flight sonar performance by checking your dataflash logs's CTUN message and comparing the Sonar Alt vs Baro Alt. In general they should move together except when you go out of sonar range (see how sonar alt in red below flat lines as copter climbs over 7m). General Instructions for downloading and analysing dataflash logs are here and here
The MaxBotix XL-EZ0 sonar (and other variations of their indoor sonar) may not function for about 10 minutes after an extreme drop in temperature (i.e. a drop from a warm 27C/80F house to the chilly 10C/50F outdoors) because of condensation building up on the sensor. This includes one member's (Geir engibekken) solution (initially suggested by the developer Olivier ADLER) involving attaching 4x10Ohm resistors to the base of the sonar powered from a 5V source.
If you are having problems with excessive noise on your sonar, there are a number of possible causes which should be investigated:
Some users have reported that these issues can be overcome by switching to the Maxbotix-EZ4 sonar although this sensor may have a shorter effective sensing range.
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