Tachometer for watch hairspring

Projects General Guidance
1

Hi guys, hopefully somebody might be able to tell me whether or not this is feasible. So a couple of months ago I was working on this project and was given some advice as to how to approach it with another microcontroller. I am now picking up where I left off on the project and would like to see if it is possible to use an Arduino for the task at hand. I am trying to measure the frequencies of unknown hairsprings that are found in watches. I was thinking that I could use a reflective sensor and by painting the wheel spoke of the spring white I could then catch when the spoke breaks the plane and implement a counting program to figure out the number of oscillations in a minute. My problem is the spokes of these hairsprings are typically only about 1mm in width. For this reason I realize that a simple IR LED/sensor combo would not work because the sensor itself has to be smaller then the object.

I don't believe a Hall sensor would work in this case because of the constraints regarding the size of the hairsprings. I definitely would not be able to place a magnet on the hairspring itself.

The other problem I was having with the other microcontroller was that it didn't have enough power to simultaneously send a signal to power the IR LED and perform a counting operation. There was a way to get around this by creating a 555 timer circuit which would keep the LED set to high and I could then go to my counting routine but I could never get the circuit to work reliably. Would this be a problem as well with the Arduino?

At this point I don't really know how to go about tackling this project. If anybody has any recommendations I would appreciate it.

2

use a laser led and shine it on to a spoke, and use the reflection of it in a photodiode.
another one is by using a small lens to capture the reflection.
and yes if the current is too high for a port use a transistor.

3

In general, the Arduino should not suffer the problem you described.

However, it is important to understand the operational boundaries.

  • min/max freq of springs
  • surfaces, presumably all shiny metal
  • size ....<1mm
  • can you insert the spings into a custom test rig or must they stay within the existing watch housing?
  • upload a zoomed in photo with a scale for reference

On the face of it, it seems that any small physical changes would likely change the frequency.

4

I assume you want to know if a watch runs fast or slow by counting the number of ticks per minute.

Many years ago "Elektor" magazine had such a project.
I remember they used a common 1" piezo, mounted on an enclosure, with the watch on top of the piezo.
The piezo "listened" to the ticks.
AFAIK, time between ticks was measured, and a single LED on a LED bar would light up.
If the watch was running fast or slow, the LED did "walk" left or right.
Leo..

P.S.
Jan. 1990

5

Use your ear and an ordinary digital stopwatch.

Hold the watch up to your ear and count the ticks in 10 seconds or so. Then use arithmetic to get ticks per second or per minute.

If you are trying to regulate a watch, keep in mind that the rate is likely to change as the watch winds down.

6

I presume that you mean a complete watch escapement or complete watch. The resonant frequency of the hairspring on its own doesn't tell you anything useful.

You certainly cannot attach a magnet or even add a drop of white paint without affecting the timing.

Shooter's suggestion of a laser shone onto a spoke sounds feasible although everything will have to be aligned carefully. Listening for the ticks is the common method for commercial timing machines like this

Russell

7

russellz:
Listening for the ticks is the common method for commercial timing machines

Yes, Sir!

If the project is about "calibrating mechanical wristwatches", the method of "listening to the ticks" is not only the method that commercial devices use, but also homemade AVR solutions are doing it exactly the same way.