Ultra Low Speed Radar

[JoshJack]

At the moment I am working on reliable train speed indication in our coal rail receival. I am having issues in finding an appropriate instrument in deriving such readings. The first issue is in the resolution of the instruments I have found since our trains vary in speed between 0 and 2km/h most low speed radars will reach around 0.4km/h minimum and if possible I am chasing one that is able to reach as close to 0km/h as possible. The second issue I have encountered is the output signals of these instruments. Majority of our instruments are a 4-20ma and therefore our electricians are familiar with such. The radars I have found are either PWM or RS232, we are able to use converters but it would be ideal to stick with what is familiar to the electricians for simplicity. Is there any such instrument that would suit such requirements, or at least one of these?

 

[Compositepro]

Why radar? Is the sensor on the vehicle or off?

 

[itsmoked]

Presuming it is off the train as a check about delivery speed, is this past a particular speed check spot or at the actual parking spot?

You should be considering ultrasonic instead of radar. Ultrasonic is often designed for 0 to a low speed.

Also laser range finders work to 120 meters and would be read and read again to see the position change in that time.

You can also use a cheap PLC and two simple retro-reflective proximity sensors or beam interruption sensors. The PLC sits there looking for the first prox switch to be active then times the time for the car to activate the second prox switch. Distance/time = speed. From there you can have the prox sensor display any number of indicators to help you. You can have multiple remote displays, a tower light RED/YELLOW/GREEN, a giant display saying "Slow down", or a giant "your speed" display. Even a cheap PLC could notify your local network about the speed of every car, for keeping a data base of the reckless drivers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Rather than looking for a speed sensor, have you considered a distance sensor and using your automation platform (PLC? DCS?) to calculate the speed from consecutive readings? You should be able to achieve a 100ms update frequency quite easily, perhaps faster depending on your hardware. Rail cars don't accelerate or decelerate all that quickly so my guess is that a 100ms interval would be OK. You may find other uses for the distance information too.

For example, this one from Sick is protected to IP67, laser-based and can measure out to 1200m, although in an outdoor dirty environment I'd want to be well inside that figure. I haven't used this particular sensor, but other Sick products I have used were well-built and reliable.