Limit switches or sensors are not absolutely required on a machine, it will work just fine without, but they are a very good addition as they prevent potentially expensive and dangerous crashes. There are three levels of safety when it comes to limits:
- Take care to never program the machine to go outside the working envelope.
- Use a software specified limit.
- Use limit switches or sensors.
Option 1 is for the foolish, option two for the people on a very strict budget and option three for the rest of us. The main problem with the first option is that one day you will accidentally program a move that is outside the safe working envelope and the gantry will attempt to launch itself from the machine. On small machines crashes like this may just knock the machine out of alignment but on larger machines it’s quite possible strip threads and bend pieces.
The first thing to decide with the limit switch circuit is whether it will use normally open (NO) or normally closed (NC) switches1. From a safety point of view normally closed is generally considered better as it can detect a break in the circuit which would go undetected with normally open switches. If wiring with normally closed switches the circuit must have the switches in series and if wiring with normally open it must be in parallel. The reason for this should be obvious from looking at the circuit, if normally closed switches were wired in parallel the opening of one switch would be undetectable.
Typically, with stepper based machines, there will be at two switches fitted per axis, one at each end. One of the switches acts as a limit the other acts as home and limit. An alternative is to add a separate home switch per axis, this is allows for the use of high precision home switches and cheap limit switches. With servo driven systems it is often necessary to use this alternative, dedicated limit, set up.
1. Many switches support both NC and NO modes of operation but check the data sheet as not all do.