Best repeatable switch - mechanical v Optical v Inducative v Capacitive

[ftkalcevic]

What's the most accurate switch/sensor when it comes to repeatability?

Background...

I want to build a tool Z height sensor. I don't want to use the electrical contact type (tool completes the circuit to ground). I want it to be either a mechanical switch, or optical or other proximity sensor.

It will be a plunger type design, either hitting a mechanical switch, or moving past an optical sensor (reflective, or slot), or an inductive or capacitive sensor.

(I also plan on putting a limit switch at the bottom of the travel to stop the system - I have broken many expensive tools because of touch off mistakes).


I don't know what repeatability I need to aim for. I'm thinking 0.01mm or better.


Searching through digikey, I can't find much information on repeatability of optical sensors. This one, the omron EE-SX670, (datasheet here http://www.ia.omron.com/data_pdf/dat...eet_csm483.pdf) has a graph on page 4, "Repeated Sensing Position Characteristics" but I don't understand what each of the deltas represents. 0.002mm repeatability looks good, but 0.04mm is a bit much.


Digikey also has a precision limit switch, also an omrom D5A Series, (http://www.ia.omron.com/data_pdf/dat...eet_csm591.pdf) which has a repeatability of 0.001mm, but is $198 - way too much.



I've seen the DIY home/limit switch thread on this forum, but reading about the hysteresis of the magnets makes me question the accuracy.


Any thoughts?

Frank

[underthetire]

Most tool sensors I have seen, BIG, renishaw, metrol, etc, all use a mechanical type, however it's more like a pin that is spring loaded against some ball bearings. When the shaft of the pre setter is pushed down, it breaks the contact (normally closed) giving the skip signal to the control. Also, the break away stylus is a nice addition, it has a small link between the switch body and sensor head that breaks with too much force, but is cheap and easy to fix.

[Al_The_Man]

There are some fairly high precision probes such as heidenhain multi direction measuring probes that use a an optical method, the probe sits in a gimbal which is the fulcrum point so with the probe being around 3" in length the opto on the opposite short side of the gimbal, the measuring distance is magnified.
Other makes follow a similar system.
Al.

[underthetire]

Actually, our most accurate, down to micro inch, is the good old lvdt probes. You think 200 is expensive though, start pricing a lvdt set up! Think the last one with amp was ~6k. Ouch.

[RomanLini]

I designed a Z height tool setter with mechanical break contact, that offers very high repeatability.

It can be easily cut from some 12mm flat stock as a 2D cut.

You can see it in this thread, about half way through;
DIY hobby small plastics mill/router

It's repeatability is better than 0.01mm and it easily measures the thermal expansion in my spindle shaft and tools once they get warm.

[ftkalcevic]

I designed a Z height tool setter with mechanical break contact, that offers very high repeatability.

It can be easily cut from some 12mm flat stock as a 2D cut.

You can see it in this thread, about half way through;
DIY hobby small plastics mill/router

It's repeatability is better than 0.01mm and it easily measures the thermal expansion in my spindle shaft and tools once they get warm.

That's a great simple design. The only issue is it wont hand over-travel very well. I didn't really spell out my requirements, but I also want to be able to set fine PCB milling cutters and drills.

I've never seen LVDT sensors before. Googling suggests they are pretty simple (conceptually) - Macro Sensors - LVDT Basics - What Is An LVDT? - How Does An LVDT Work?. I wonder how DIY they are.

[mcarvey]

You should be able to get that level of repeatability with any of those 4 types of sensors, if they are implemented in a well thought out manner. The key is to understand the principles that govern their operation and then design accordingly. For what you're trying to do I think that either mechanical or photo-interrupter would be the easiest to implement since they are likely to use less external circuitry.
I also looked at the EE-SX670 datasheet and agree that the "Repeated Sensing Position Characteristics" seems to be missing some vital information. I was able to find a more detailed databook about photomicrosensors that sheds some light on what the graph means. Page 25 should be helpful, as well as searching for that phrase.

http://www.google.com/url?sa=t&rct=j...EcSEewOOg5Uwuw

I'm not an expert on this, but I'll share my guess as what the deltas could represent. First some terminology. So the photosensors have a light emitter and detector, seperated by some a distance (lets say 4mm) Suppose we had a perfectly opaque, wafer thin interrupter plate. If we place the this interrupting plate right next to the emitter, lets call it d_e+0mm. 1mm from the emitter would be d_e+1, etc. d_e+4mm would be right next to the detector.
The edge of the interrupter plate creates a shadow, and since the light source is not perfectly narrow the shadow will be crisper if the interrupter plate is closer to the detector, and fuzzier if it is closer to the emitter. Since the photodetectors work based upon a light threshold, it would make sense that the interrupter plate's position in the slot has some affect on the repeatability. I could be wrong though.
Then again, the graphs don't really show anything to support this, so it may just be temperature dependent. Also, It's also not clear what impact the no. of repetitions means. Is that the worse case? Average?

Hope this helps,
-Matt

[Karl_T]

I designed a Z height tool setter with mechanical break contact, that offers very high repeatability.

It can be easily cut from some 12mm flat stock as a 2D cut.

You can see it in this thread, about half way through;
DIY hobby small plastics mill/router

It's repeatability is better than 0.01mm and it easily measures the thermal expansion in my spindle shaft and tools once they get warm.

I went looking through your thread and couldn't find it. I did find a broken link to a Z height on page 7. Was that it?

Sure like to see what you did

Karl

[ftkalcevic]

I went looking through your thread and couldn't find it. I did find a broken link to a Z height on page 7. Was that it?

Sure like to see what you did

Karl

This is a direct link to it.

DIY hobby small plastics mill/router

[RomanLini]

That's a great simple design. The only issue is it wont hand over-travel very well. I didn't really spell out my requirements, but I also want to be able to set fine PCB milling cutters and drills.
...

It handles overtravel of 1mm or so, which is totally irrelevant as it detects the descending tool to 0.01mm accuracy and always stops the descent.

It actually stops the tool before you can see any visible deflection of the top plate, my friend says it's "magic".

Why do you need overtravel? Do you expect the sensor to fail? As a break sensor it can't fail on a descending tool, it is fail-safe in that direction.

I use it often for very fine tools like PCB milling tools and micro endmills etc. It's given 12 months service with zero defect and zero wear (well no wear I can detect).

Thanks for providing the direct link, the tool height setter is in post #18 of my build thread.

[ftkalcevic]

Why do you need overtravel? Do you expect the sensor to fail? As a break sensor it can't fail on a descending tool, it is fail-safe in that direction.

I have many years of inexperience when it comes to touch off probes. I've broken some expensive cutters and damaged a nice touch off gauge because of operator error, bad programming and faulty hardware. As a hobbiest, I need all the protection I can get.

[RomanLini]

Yep I respect that. I still don't understand what you think would fail?

The tool comes down, and when it kisses the sensor plate it breaks open the contacts, and the machine stops. Unless you think the software will fail? If the wiring fails open circuit (or the contacts are dirty) the tool would not start to descend so fails are in a "safe" manner where the tool does not descend.