I am doing research on impact of proximity based safety sensors in collaborative robots. The papers I read indicate capacitive sensors are the most common in industry. However, in service robots there is a vast variety being used. Please comment. Thanks  

Thanks @SJQ_Mirza!

@matthewd92, @Ryan_Weaver What do you guys mainly use?

Safety sensors we use light curtains and safety mats. 
For proximity sensors we use a lot of fiber optics both diffuse and through beam. This is due to a lot of the parts we work with aren't conducive to using standard inductive or capcitive prox sensors.   We do use some however, especially with pneumatic grippers. We also have a number of magnetic reed switches on air cylinders and the like for knowing cylinder positions. 

Thanks @matthewd92 . Unfortunately I can't find much research papers on optical proximity sensors. Can you share why do you prefer primarliy capacitive or fiber optic based sensors in collaborative robots. I have seen in alot of service robots infrared sensors are used where as in robotic  arms intelligent skin via capacitance technology is used.

Your help is very much appreciated.

Thanks

@SJQ_Mirza the primary reason we use a lot of fiber optic sensors is we deal with a lot of aluminum parts and that fact cuts the sensing distance of inductive sensors way down as they are not as inductive as say a steel part.  So we find they are more reliable and we are able to place them more out of the way of the robot and humans.

Here is an interesting article on the different types of proximity sensors with some advantages and disadvantages.

http://machinedesign.com/sensors/proximity-sensors-compared-inductive-capacitive-photoelectric-and-ultrasonic

I guess one point of clarification, are you referring to proximity of the human to the robot or sensors in the cell that help the robot to "see and sense" its work environment?

@SJQ_Mirza so the only thing that we use separate from what is built-in to the UR for sensing proximity are safety devices such as pressure mats and light curtains.  These are usually used to vastly reduce force/speed or to stop the robot motion when an operator enters an area where there is potential for risk from the robot system.