With 17 years at Goodyear, I have used them all and by far the best at the time I left in 2006, the best bar none in all categories was:
STI Valuscan
Out of the box, you get perfectly linear voltage or current, easy alignment, rugged construction, full serial programmability.
I replaced ultrasonic of all types with these. I installed no less than 10 sets.
Every one of them bench tested out of the box before calibration:
0 inches blocked = -0.005vdc to +0.005vdc
6.5 inches blocked =4.995 vdc to 5.005 vdc
13.0 inches blocked = 9.995 to 10.005 vdc.
The linearity never faded, and there were high quality multi-turn pots to set the range wherever you want.
We chose 1vdc to 9vdc to avoid rollover in the PLC inputs, and scaled it to inches of loop height in most applications.
I never saw one fail even after 5 years of continuous operation.
I mounted them on a common frame for the beam shears, so they could moved depending on the product loops being controlled. The frame was a u-shape that could be positioned under the twisted or untwisted loop of various widths.
I routed the cables under the 3" channel iron that was the base, and don't recall any cable failures either. I know for sure the main controllers were bulletproof and trouble free.
You can program them serially to find holes too.
They can be set up to spit out a serial stream and not even connect their analog outputs if you wanted. The serial stream was used for hole detection in a test mode but I quit before I got to play with it further.
Can't go wrong with them if the height range fits your application.
Paul
http://www.stiscanners.com/measuring_light_curtains/valuscan_VS6500.html