Sensor Type Suggestions/Ideas? - Page 1
Forum section: electronics / general

author: DeezNANDs
author_rank: Newbie
timestamp: March 08, 2012, 07:29:57 pm
content: Hi all - first post here (well, aside from the intro thread) - glad to join the communityI'm in the planning stages of a new hobby project, which will ultimately be a pretty simple sensor controlled mechanical system - probably PIC based (will probably try 16F628A/877 since I have a handful of these in the bin). In it's simplest form, picture a system where a motor or an actuator is pushing multiple items towards a discharge chute (maybe 2.5-3" in diameter), and a sensor of some type in the discharge chute senses the falling object and cuts off the motor/actuator.Where I'm looking for ideas on this one is the best type of sensor to be used. For my design, the objects that would be falling through would be relatively large (smallest probably would be half the size of a deck of playing cards), and a precision count of the falling objects isn't very important (really doesn't matter if 1 or 5 fall at once). The most important part is that the sensor definitely picks up the objects at some point.What's giving me second thoughts here, is that the sensor used will have to put up with a lot of nasty slop, such as oil, saltwater, etc., which I assume will make normal sensors (like IR) useless once they get mucked up.Had thought of trying some type of ultrasonic sensor, but really have no experience with using these, or if they're even appropriate for this type of application.Just looking to get pointed in the right direction - any thoughts or input would be greatly appreciated. Thanks!

author: sacherjj
author_rank: Frequent Contributor
timestamp: March 08, 2012, 09:09:25 pm
content: Well, sometimes the simplest method is the best. Are these object heavy enough to trigger a switch? You could use a simple flap on the bottom that triggers a snap-action switch:http://www.allelectronics.com/make-a-store/category/727/Switches-Snap-Action/1.htmlIR may work, even with muck. An IR remote works by modulating a signal with some carrier (40 kHz or so). With this, you can still detect a signal, even if the ambient light changes in the room, or oil "mucks things up". When you detect a carrier, nothing has broken the beam. When the carrier is dropped, you either had something go by and too much muck.I don't think ultrasonic will work that well, because the size difference is small. Others can comment on this. I've only used ultra sonic for fairly large differences (4" increments or more.)

author: mtkaalund
author_rank: Contributor
timestamp: March 08, 2012, 09:48:52 pm
content: Quote from: sacherjj on March 08, 2012, 09:09:25 pmUltrasonic, will probably work down to 1 - 4 cm (4" is about 10 cm), but again, for detecting the object I will probably use an IR sensor instead, to trigger when an object is cutting the IR "beam" .DeezNANDs, can you post a picture of the system you thing about building?

author: G7PSK
author_rank: Super Contributor
timestamp: March 08, 2012, 09:53:22 pm
content: What are the objects are they metallic or conductive, if so an inductive loop or sensor.

author: DeezNANDs
author_rank: Newbie
timestamp: March 08, 2012, 10:14:06 pm
content: Thanks for the feedback - unfortunately this stuff won't be metallic or conductive (though if they were the inductive loop is a great idea). It sounds like IR may be still be an option, though I have thought of the flap activated switch - and I agree that it's is a good, simple backup solution.Like most things, I suppose the best way to confirm would be to knock together a crude prototype and see how the badly the IR solution would be affected...

author: SeanB
author_rank: Super Contributor
timestamp: March 10, 2012, 09:39:50 am
content: An old sensor, but a very good one, is to use a small incandescent lamp (12V lamp run on 9V with a series resistor to limit switch on surge) and a phototransistor to detect the light. AC coupled to an amplifier ( only amplify the change, the DC will vary slowly with time and dirt on the sensor) and detect the change to drive the load. The lamp and the sensor are in small boxes with a glass front ( and a mask inside to limit the light into a beam across the channel) and you have the right sensor. I have them at work as counters, very reliable, very robust and these use small solar panels ( or a very large area photodiode if you prefer the description) as sensors behind a glass cover. Works in dust, with windows almost covered with a thin film.

author: T4P
author_rank: Super Contributor
timestamp: March 10, 2012, 03:31:47 pm
content: Quote from: SeanB on March 10, 2012, 09:39:50 amThat's a fresnel lens .

author: Kilroy
author_rank: Regular Contributor
timestamp: March 10, 2012, 04:45:55 pm
content: If it has to work in spectacularly fouled and corrupted environments, where copious amounts of unimaginable filth, muck and decay are flung about recklessly without any common regard for cleanliness of any description, then you might consider something like an eddy-current sensor.Otherwise, some sort of capacitive sensor would work a treat.

author: Lawsen
author_rank: Frequent Contributor
timestamp: March 11, 2012, 09:39:35 pm
content: Falling objects sensing are best done with an accelerometer and electric altimeters like the type airplane ejection seats. Altimeter is important because the rocket motor needs to keep working to allow a safe height for parachute deployment and oxygen levels for the occupant to breathe. Accelerometers are available at Radio Shack here in the U.S.A. and mail order. Accelerometers are good inputs to a micro-controller computer.