Centroid CNC12 software supports a wide variety of Home and Limit input combinations and configuration. Centroid has provided the tools so that the integrator can configure and use the machine’s home and limit switches any way he likes. These tools are: The CNC12 PLC program, CNC12 Macro programming, CNC12 configuration menus.
The Acorn and AcornSix Wizard provides the user with home and limit switch configuration with auto home program generation so the users does not need to learn how to Edit a PLC program, CNC Macro programming or use the CNC12 configuration menus.
The Wizard provides easy drag and drop and click method to configure a wide variety of Home and Limit combinations.
Centroid’s #1 recommended home switch configuration is to use one home switch per each axis of the machine tool. Wire all Home switches into one input defined as “HomeAll” and use Software Travel Limits to control the travel limits of the machine tool on both the plus and minus side of each axis.
Centroid’s #2 recommended home switch configuration is to use one home switch per each axis of the machine tool. Wire all Home switches into one input defined as “HomeAll” and use Software Travel Limits to control the travel limits of the machine tool on both the plus and minus side of each axis. And use Limit switches on each side of each axis of the machine tool, wire them all into one input “LimitAll” as seen in schematic #14954
https://www.centroidcnc.com/dealersuppo ... 954.r5.pdf
I will describe here how to use Home and Limit switches with the Acorn/AcornSix Wizard.
So, lets start and define what a Home Switch is and what it does and what a Limit Switch is and what it does.
Home Switch:
- A switch used to define the location of the machine home position.
- Home switches are used to ‘remember’ the machine home position, machine work envelope and part zero position after a machine tool power cycle.
- Home switches are only active during the home program (cncm.hom, cnct.hom).
- Home switches are not limit switches.
- Home switches are ignored during regular CNC control operation
- The more accurate the home switches the more accurate the remembering of the part zero position is after a power cycle.
- Home switches should be “Normally Closed”. “Normally Closed” = Closed when NOT tripped.
- Home switches can be wired to individual inputs or all switches wired into one input called “HomeAll” (to save on inputs)
- Home switches should be mounted so that an axis over travel will not crush the switch.
- Home switches should be of the type designed for CNC use. Do not use long throw general automation / farm equipment limit switches.
Home Program: A G&M code program (aka macro) that commands the machine tool axes to move and seek the home switch trigger event, stop, back off to clear the switch and set machine home at that location.
Limit Switch:
- A switch use to indicate to CNC12 that an over travel event has occurred.
- A limit switch trigger causes a CNC12 Emergency Stop event.
- Typically used as last resort machine tool axis over travel crash protection
- Limit switches should be “Normally Closed”. “Normally Closed” = Closed when NOT tripped.
- Limit switches can be wired to individual inputs or all switches wired into one input called “LimitAll” (to save on inputs)
- Limit switches should be mounted so that an axis over travel will not crush the switch.
- Limit switches should be of the type designed for CNC use. Do not use long throw general automation / farm equipment limit switches.
- Limit switches are active all the time but can be ignored by CNC12 when commanded to.
- CNC12 can be commanded to ignore a limit switch, this is used to allow jogging if a limit has been tripped (or other special cases where it is necessary to ignore a limit for a short period of time)
- Typically mounted so the Limit switch triggers some distance before the actual end of travel of the axis and used in conjunction with a polyurethane bumper to adsorb the axis coasting to a stop after the limit event has been triggered.
- When ever possible mount switch so that the switch does not move with the machine so the cable going to the switch doesn’t get fatigued over time. When not possible pay attention to limit cable wire routing and strain relief to minimize fatigued and use good quality cable designed for movement.
Cons of using limit switches.
- Expense in the components and labor to wire
- Issues cause by durability especially in large format machine tools. Such as false limit trip message due to improper grounding, power supply issues, limit wire breakage due to fatigue in track ways etc.
Pros of installing Limit Switches
- Insurance to prevent unexpected damage to the machine tool. A good idea for machine tools that can not withstand an axis over travel event.
Limit Switch Discussion:
CNC12 Software Travel Limits works so well that it has largely eliminated the need for axis limit switches on many machine tools. Software Travel Limits work in the case where either the G code part program or the user commands the machine to move beyond its useful working envelope. It is however, up to the integrator to decide whether or not to use limit switches and how to best configure them for his particular application.
A case where the Software Travel feature will fail is when there is a catastrophic axis motor drive failure and the drive failed in the full power mode (aka axis motor runaway). In this case, a proper Limit Switch (and cnc control estop contactor wiring and a bump stop) would protect the machine tool from axis over travel crashing. Luckily at this day and age axis motor drive runaway is quite rare on modern drives. But, for the best machine tool damage protection use Limit Switches on all axes on both plus and minus ends of the axes and use an Estop contactor so the Axis motor drive power is disconnected from the axis motor automatically when a limit switch is tripped.