Detecting if glass/cup is present

Hi guys.

I’m working on a drink dispensing machine that will start to dispense when a glass or a cup is present. The glass can be in plastic or glass, or it can be a cup. I’m looking for the best sensor to use and need some help.
Weight sensor is no option since it would not catch a plastic glass. Ultrasonic sensor could work but I haven’t found anyone that works on a small range ( < 3 cm). IR, TOF, LIDAR etc could also work but it could be a problem with the tranparancy of the objects, and the ones I have found does not work on such small distances. The sensor must also be waterproof.

Do you have any advice?

Looking at the things used commercially I recommend a sensing finger being pushed by the glas, plastic cup, whatever when being in position under the tap. They are used by Mc Donalds and similar fast food sellers as well as restaurants.

Instead of sensing the cup or glass, why not sense the heat of the hand holding the cup. You would have to do a little analysis of where to put it so that it does respond to both left and right handed people (two sensors, maybe?), and you'd have to shield it from stray infrared in the environment that might spill a drink if no cup were present.

See what industry uses and try to duplicate it.

A starting point: Presence Sensing Devices | Allen-Bradley.

A polarized retroreflective photoeye would probably work.

Don't overdo things. An arm, a lever and a microswitch..... unless You have unused hardware being close to catch fire in the drawer...

If it will only used a limited number of cups/glasses you could glue an RFID on the bottom of them.
I used this method on my vending machine and so when the mug is placed in to the machine it not only knows there is a cup ready but also which drink is required.
Rather amusingly I later tried to warm a drink up in the microwave which had gone cold and was wondering where the burning electronics smell was coming from...

BTW - See it in operation here: drinks robot

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