Angular position measurement


Thread Starter

Alireza Fatehi


I'd like to measure an angular displacment position with 0.1 degree accuracy. The machine has a shaft encoder but I'd like to measure in another more accurate one so to check the accuracy of the shaft encoder too.

To reach it, an idea is to use a laser beam where the gun is connected to the machine and then measure the displacement of the beam on a screen on, say, 18 meters from the source (roughly 3 cm displacement is equal to 0.1
degree). But we have not any laser instrument and it is expensicve. Any other idea?



Steve Bailey

Why not just replace the existing shaft encoder with a more accurate one? 0.1 degree resolution is 3600 PPR. There are more accurate encoders available.
Hi Alireza, I think a shaft encoder will do. It has to be an absolute type or incremental type. With an incremental type you get only
relative displacement unless combined with an index switch. How is your existing shaft-encoder coupled to the machine, directly or with a
transmission? What is the resolution of the encoder? What measure area (how many degrees of displasement) are we talking abot?
I had a similar problem where I needed to check for safety the over speed of an overhead crane that picked up airplane sections. On the motor, I put an encoder that was capable of measuring motor shaft position (and indirectly RPM), connected to a PLC via a high speed counter. The encoder is compared with an additional tachometer installed on the hoist's drum. The tach. came with its own controller. The motor and hoist are mechanically connected through a gear box and the two are used as a redundant over speed and safety check.

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Magnetostrictive is an accurate way. Sometimes used in level sensors (we don't offer one yet) but check with the hydraulic industry. The
product is used in robotics and more for accurate piston movement (.0001)

A chip is also available the gives a reading in proportion to a local magnet.

Thanks Bob Hogg

Curt Wuollet

Get a $10.00 (or equivalent in local currancy) laser pointer and two aircraft style hose clamps.(est cost $1.00) Use a paper clip to hold the button on. Clamp the pointer to the other clamp, perpendicular to the shaft. Problem solved. Or super glue a small mirror to the shaft and reflect the stationary laser pointer. If your budget won't allow this, borrow the laser pointer from the guy in your company who's in love with Power Point. Every company has one of these and they all have a laser pointer.



Gerald Beaudoin

You did not talk about mechanical limitations in your application, but I have seen some creative uses for the laser sources which are to be
clipped onto a carpenter's level. Even cheaper again are the dime store variety to be used as pointers. Certainly not "industrial", but very
cheap and dirty.

Gerald Beaudoin