Notifications
Clear all

Proximity sensor precision  

  RSS

hayttom
(@hayttom)
Active Member
Joined: 2 months ago
Posts: 14
2020-10-13 6:28 pm  

Dear community,

 

I hope to discover my own answer to this question by experimentation, but in the meantime:

I am building a 'transfer table' for my model railroad(railway).  This is an apparatus with parallel tracks that aligns a selected track to an approach and departure track so that trains can be parked or introduced into the layout.  (There are real examples in maintenance depots but mine will be in my non-scenic area just for train storage.  I have seven parallel tracks.) 

My transfer table will be moved by a stepper motor driving a 3D-printed rack and pinion and controlled by an Arduino.  I need sensors for homing and will need about 0.5 mm or ideally better precision to align the transfer table well enough to avoid derailing.  I can get this positional precision with micro stepping but I don't yet know if I can match that precision for homing.

Do microswitches work well enough?  I have noticed that my 3D printer seems to approach its microswitches quickly, then back up, and then approach again slowly.  Is this because they are precise but don't respond quickly enough?  3D printers have the benefit of being able to being printing at somewhat arbitrary coordinates, while I have to lock into my real world precisely.  In my imagination hall effect sensors will not provide the precision I need.  I think that IR 'speed' sensors viewing a narrow slit might be best ... am I right?

Best wishes from Kyiv,

 

Tom

Tom


Quote
sj_h1
(@sj_h1)
Eminent Member
Joined: 2 months ago
Posts: 27
2020-10-13 6:48 pm  

A video would be helpful

 


ReplyQuote
DroneBot Workshop
(@dronebot-workshop)
Workshop Guru Admin
Joined: 1 year ago
Posts: 652
2020-10-13 7:10 pm  
Posted by: @hayttom

I think that IR 'speed' sensors viewing a narrow slit might be best ... am I right?

If you're speaking of optical source-sensors then I think you're on the right track (pardon the pun LOL).

0.5mm is very precise, I think a microswitch arm would risk bending out of shape and wouldn't be able to maintain that accuracy.  The optical solution would be more precise IMHO, although aligning ANY solution is going to be tedious work.

😎

Bill

"Never trust a computer you can’t throw out a window." — Steve Wozniak


ReplyQuote
hayttom
(@hayttom)
Active Member
Joined: 2 months ago
Posts: 14
2020-10-22 6:11 pm  

Today (while enjoying my first ever 'staycation') I experimented with infrared sensors, the type that Bill has shown for use with slotted disks for robot speed sensing.  (I'm developing a model railway turntable and train lift and traverser table - all moved by stepper motors.) My goal was to establish their positional precision - if a project traverses a mask through the sensor and then a slit comes along, how precisely and accurately can the sensor lock onto the slit coordinates?

My apparatus was a 3D printed mask with a 0.2 mm slit.  The real slit dimensions are probably fuzzy but I'll say the nominal gap is 0.2 mm.  My visual estimate with manual manipulation was that with my 0.2 mm slit, and the mask filling most (4 mm) of the 6 mm gap in the sensor, the sensor went high during at least 1.0 mm of traverse.  I'm thinking this is either due to diffraction or the fact that the IR emitter and actual sensor seem to be at least 1 mm wide, but I was a bit disappointed that it wasn't finer.  HOWEVER I noticed that the points in the traverse where the signal turned on and off were very consistent, repeatable, and predictable. 

Though I might 3D print a jig to incorporate my micrometer for more accurate investigation I think I have already discovered the way I will use these sensors - instead of trying to pinpoint the stepper motor step that lines up with a narrow slit I will home in on the position at the edge of a wider slit where the sensor goes low.

Best wishes from Kyiv!

 

Tom

Tom


ReplyQuote