Gantry Limit switches & homing question.

Yes, I stayed within the LinuxCNC machine paradigm and wired my limits back to software inputs. I think to include them in a hardware circuit would be creating a rod for your own back becasue you loose the ability to recover if you do hit a limit stop. You will have to manually move the gantry to clear the error rather than ignore limits and jog off.

I think also it is wise to have separate home and limit switches on a machine with a lightweight high speed gantry (21 m/min in my case) as the machine coasts to a stop while homing over say 50-70mm. A combined home/limit sensor would reduce your travel by at least this much. My limit switch sensors are designed as Andy has described for mid travel just I'm only in the field of travel far enough to account for the coasting.

I think your servo drives can be considered as part of the hardware and you only need to protect from a software (eg LinuxCNC) fault. You seemed to want to include them in your fail safe. I think to implement your additional failsafe, you just need to add separate overtravel switches that disable the servo drives. You probably only need one switch at each end of the table to achieve this instead of on each side.

Somewhere you need to trust your motion controller and I think if you can't trust LinuxCNC perhaps you should use another motion controller. I saw a badly twisted plasma gantry end made from 20mm aluminium during the week where the machine must have racked the gantry. Which begs the question. How do you protect LinuxCNC from a hardware fault? I don't know the circumstances of the crash but I think one servo must have failed (it was a commercial machine). You can't twist a gantry by hitting limit switches!

BTW, 10 ft is not a large gantry. Some I've seen are 90 metres long!

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There&#;s probably not a day that goes by where you don&#;t use or encounter a limit switch at home, or at your workplace.

Types of limit switches

There are 4 general types of limit switches:

1. Whisker

2. Roller

3. Lever

4. Plunger

Depending on the application, a limit switch may be a combination of 2 of the general types such as roller-lever.

What are limit switches?

A limit switch is an electromechanical device operated by a physical force applied to it by an object.

Limit switches are used to detect the presence or absence of an object.

These switches were originally used to define the limit of travel of an object, and as a result, they were named Limit Switch.

Limit switches applications

When you open the fridge door, a light comes on inside. How does that happen? Yes&#;. you guessed it! A limit switch is used to detect if the fridge door is open or closed.

Let&#;s look at another application of a limit switch that you may encounter at home. On many overhead garage doors, there is a limit switch that stops the movement of the door when it reaches its fully opened position.

How Do Limit Switches Work?

Alright&#;.now that we&#;ve looked at a couple of limit switch applications where you might see them in action at home, let&#;s have a closer look at the device itself.

Limit switches are electromechanical devices consisting of an actuator mechanically linked to an electrical switch.

When an object contacts the actuator, the switch will operate causing an electrical connection to make or break.

Configurations of limit switches

Limit switches are available in several switch configurations: Normally Open, Normally Closed, or one of each.

Symbols of limit switches

Depending on the origin of the electrical schematic, you may see limit switches drawn in different ways.

The International Electrotechnical Commission (IEC) and the National Electrical Manufacturers Association (NEMA) have slightly different symbols.

Microswitch

Let&#;s have a look inside a microswitch that is a type of limit switch.

A microswitch has 2 limit switches operating together and sharing a common terminal. One limit switch is normally open and the other is normally closed.

To be technically correct, the switch configuration is Single Pole Double Throw, or commonly referred to as SPDT.

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The dashed line indicates that both switches are mechanically connected and will operate at the same time.

Microswitch simple circuit

Alright, let&#;s connect the microswitch to a lamp circuit. In the inactive state, the Red lamp is on as the device is not being operated by an object pushing on the trigger.

When the Trigger is pushed the device will activate, and the Green lamp will come on.

Limit switches in action

Now that you&#;ve seen the limit switch in action you are probably thinking about some of the applications where you have seen them in action.

For example, you might see limit switches operated by a container on an assembly line, or operated by a rotating machine part or by any number of other moving mechanical objects.

Limit switches could be used to count passing objects, or determining the position of a hydraulic cylinder.

Proximity sensor vs. limit switch

Limit switches are slowly starting to disappear from many industrial applications. They are being replaced by proximity sensors.

Unlike a limit switch, a proximity sensor has no mechanical moving parts.

A proximity sensor performs the switching action with electronic switches.

Limit switches will not completely disappear any time soon as they outshine their proximity switch counterpart in their ruggedness and reliable operation in difficult environments.

Generally speaking, limit switches are capable of handling much higher current values than proximity sensors.

You might want to review one of our other articles:

What is a Sensor? Different Types of Sensors, Applications

Summary

OK, let&#;s review&#;

&#; There are 4 general types of limit switches: whisker, roller, lever, and plunger.

&#; Limit switches are electromechanical devices operated by a physical force applied to it by an object.

&#; A limit switch is an electromechanical device consisting of an actuator mechanically linked to an electrical switch.

&#; Limit switches are available in several switch configurations: Normally Open, Normally Closed, or one of each.

&#; Depending on the origin of the electrical schematic, you may see limit switches drawn in different ways.
&#; Limit switches are being replaced by proximity sensors in many applications.

If you have any questions about using Limit switches, add them in the comments below and we will get back to you in less than 24 hours.

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The RealPars Team

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