Detect moving rail


Thread Starter

John Scarborough

Does anyone know of a type of sensor that can detect if a piece of metal (steel rail) is moving past the sensor? We don't want to just detect that the rail is there - it must detect if it is moving. We are looking for a non-contact, no moving parts system due to problems with heat and scale etc.

Steve Myres, PE

How about a vision system?

You might try using presence sensing at a number of points along the travel path. You could use photo sensors with an axis perpendicular to the path. If the presence is moving from one zone to the next, there is motion. Since you probably know the size of the rail, you could have a wheel that is turned by the rail as it passes, then sense the whell rotation (encoder on the shaft, prox on gear teeth on a gear on the same axle, etc.). If the wheel is solid steel, the heat and scale may not be a problem.

Steve Myres, PE
Automation Solutions
(480) 813-1145

Robert Scott

A combination of two sensors might do the trick. One sensor would be a standard inductive prox switch to detect the mere presence of steel. The other detector could be an optical motion detector based on the slight variations in surface appearance. Such sensors have been successfully implemented to detect road speed in automotive road testing, looking downward to the ordinary pavement below. It is also the basis of the optical mouse, which works on any surface except one that is absolutely clean and smooth (not my desktop, certainly!).

If the optical method is not practical, then I would suggest a custom sensor based on imposing a varying magetic field at one point on the rail and detecting that variation with a magnetic pickup at a point downstream. This would treat the rail as sort of a piece of magnetic recording tape. The magnetic domains are not as fine as in recording tape, but it is possible to impose a detectable magnetization pattern with a strong enough and close enough coil.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
Ypsilanti, Michigan
I guess the rail moves along its longitudinal axis.

I suggest you look into a Doppler - ultrasonic (or even microwave / radar) system at an angle as the physical principle. I am not aware of actual products.

This is a similar application to cable measurment. I have worked in the past with a company called Beta LaserMike to integrate non-contact cable measurement systems using laser technology. Their website is


Michael Hartnett

WADECO Ltd. has a microwave sensor perfectly suited to your application. MWS-DP-2 is a solif flow/motion detector which uses the Doppler effect to determine if an object is in motion. the adjustable sensitivity allows full controll.

Check out WADECO's website for more info:
See the microwave sensor page and note the MWS-DP-2 sensor.

Email me a [email protected] for more info.