Mother Earth vs. ground
A common misconception holds that ground (mother Earth) is a body that will suck up any electricity thrown at it. This is simply not the case. It is merely one conductor in the electrical power grid because the power industry chose to connect one side of their alternators to mother Earth many years ago.
I often ask my students, “What would happen if you took a metal stake, drove it into the ground about two or three feet, and then connected a jumper cable from your car battery’s positive terminal to the stake?” The answer: Nothing! A complete loop is not formed from the positive battery terminal, through the blue jumper cable, through the stake, and through Earth. The tires, being electrical insulators, prevent any current from flowing through the jumper cable and mother Earth.
The electrical circuit of Figure 2 has no mother Earth connection at all, so it has a floating ground. That is, whatever point we designate as ground, however arbitrary, has a voltage relative to mother Earth that is undetermined and unimportant. Billions of circuits are functioning right now that have no connection to ground whatsoever. They are battery-powered toys, flashlights, cell phones, even airplanes. The nearest analogy we have in hydraulics to a floating ground is in the case of a sealed and pressurized reservoir. In such a system, the lowest pressure point floats above atmospheric pressure by an amount equal to the reservoir pressurization value.
Some electronic control devices are designed with a kind of local ground, or common point — an electronic reservoir that is not associated with mother Earth. I prefer to call these points common, but some circuit manufacturers persist in calling these ground, which leads to confusion because they do not mean mother Earth.
There is also confusion over positive and negative, probably because our cars are all built with negative ground (chassis) systems. There is nothing special about negative or positive, and some cars were built with positive chassis connections.
Before that, it was arbitrarily decided that electrons would be labelled as being negative. They could just as well have been labelled positive. The problem is that many electronic designers use + to designate a signal or terminal, and – to designate ground, or common. Not only can this lead to misinterpretation, but also to erroneous connections and even to danger for people and machines. I prefer to use the terms signal and ground, or common, to distinguish the two terminals.
Electronic control system integration includes the connecting together of many electronic devices, including amplifiers, signal conditioners, computers, motion controllers, transducers, and on and on. It is absolutely necessary when building these control chains to connect the common of every device to the common of another. All of them must share the same common point.
Consider a hydraulic analogy, Figure 4. If a circuit has two pumps, each supplied by separate reservoirs that are not interconnected, eventually all the fluid will be drawn from one reservoir and deposited into the other. Clearly, this will not work. If we fail to connect the common terminals in the electrical circuit, the consequence is more subtle, but the result is noisy (electronically speaking) operation — causing machine axes to drift, jitter, or jump around if they function at all. In the hydraulic system, all the reservoirs need to be interconnected, so do the common terminals on all the electronic devices.