DC ground vs zero volts

I had a problem at work a while back when I was trying to figure out why my robot wasn't working correctly. I ended up finding that somebody had disconnected a wire from the PLC, but I never fully understood what was going on. The robot had a relay where I could measure 24 volts to the zero volts, but I couldn't measure the 24 volts to ground.

A couple of days ago I had another robot where I suspected that a proximity sensor wasn't working correctly. The proximity sensor had one wire 24 volts, one wire for zero volts, and one wire for the output. Here again I could measure the 24 volt output signal with respect to zero volts, but not to ground.

Can somebody please explain to me the difference between this zero volts and ground.

Try this
https://www.google.co.uk/url?sa=t&r...I7gBrXDg0pSJgQodSCqZR2g&bvm=bv.44442042,d.ZWU

Normally zero volts is considered to be the ground (common) point. What is "zero volts" in your circuit?

I'm going to have to look into what exactly is the zero volts in these circuits. It confused me because ground should be the universal common and my multimeter reads zero volts when I use it for testing this circuit. I'm also going to have make sure I have a good ground that i'm using to test.

Not every circuit is referenced to ground, but it's very common that they are. The 0V point may just be an isolated power supply's reference point. A lot of times the schematic would reference different grounds with an inverted triangle and a number or letter. They may or may not be connected together and to earth at one point

"where I could measure 24 volts to the zero volts, but I couldn't measure the 24 volts to ground."

Most likely your circuit's zero or negative, is not tied down to ground....though in this case, being a robot, one should say tied to chassis.