Driveway Gate Obstruction Detection

[krypton_john]

Funny thing is - I was agonising over how to build a sensor that didn't require a cable to the far side of the gate... but I had forgotten that when I put the gates in I left a spare multicore cable under the driveway just incase!

I'll probably experiment with both break beam and PIR so I can catch a car breaking the beam at the gate post line, and a PIR to cover the swept area.

 

[wapo54001]

How about detectors mounted on the gates themselves, activated only when the gate is in motion, and detecting obstructions only within x feet of the moving gate? I don't know how ultrasonic proximity detectors work, but that would be ideal, would it not?

 

[Dippy]

U/sonic on the gates eh...
How many would you need on each gate as they have a limited 'field of view'?
What would be their response when at certain gate positions they would be facing each other?
Or would you use a PICAXE to strobe them?
Assuming there are two gates, might there be a time when the sensors on one gate simply detected the other gate?
Wouldn't you need sensors on both sides of gates?

"...but that would be ideal, would it not?" - I guess if you put together a sophisticated sytem with suitable sensors and wires all over the place it would be.

A microwave/doppler sensor could detect moving objects within range.

 

[wapo54001]

U/sonic on the gates eh...
How many would you need on each gate as they have a limited 'field of view'?

You need sensors primarily out on the end of the swinging part and perhaps covering one-third to one-half of the way in toward the hinge (assuming you only need to protect against dinging a car or trapping a child.


What would be their response when at certain gate positions they would be facing each other? Or would you use a PICAXE to strobe them?
Assuming there are two gates, might there be a time when the sensors on one gate simply detected the other gate?

They would only face each other when they are quite far apart. I suppose you could adjust sensitivity to adjust for this, even use a picaxe to reduce sensitivity at certain points of the swing.


Wouldn't you need sensors on both sides of gates?

Yes, unless you assume that a car to be damaged or a child to be trapped has to be more-or-less centered between the gate halves.


"...but that would be ideal, would it not?" - I guess if you put together a sophisticated sytem with suitable sensors

Of course it has to be somewhat sophisticated -- it's a swinging gate and an IR-based system is inadequate. And a Picaxe would make the sophisitication easy to implement.


and wires all over the place

I don't think the idea is totally outrageous, but perhaps I am mistaken.


A microwave/doppler sensor could detect moving objects within range.

For a gate that swings from both sides, it seems to me that the simplest approach is to put the sensors on the parts that swing in order to detect objects within a few feet of the swinging gate anywhere within the arc of the swing.

 

[westaust55]

Another option might be the Revolution SRF005 Ultrasonic module.
These have a stated range from 3cm to 3 metres.

I have found I can get good results down to 1cm but have not tried over spec range or outdoors as yet.

With this type of option, only need wiring at one side of the gateway.


wrt gates opening outwards. This is common where I live. Council Laws require the fence to deviate in typically at least 1.5 metres at a 45 degree angle at driveways so pedestrain can see a car coming out the gateway before they step into the path of the car. The gates then work in this set back area. Obviously the gate setback would need to be a little more like 1.8 or 2 metres for a 3.5m wide gate set.

 

[Dippy]

Wap, don't be oversensitive. I wasn't saying it was outrageous or taking the mick, I was just suggesting some things (at first glance) that may complicate the issue and don't know just how complicated people wanted to get.
In fact, I had the SRF005 in mind when thinking about it.
'Complicated' is usually AKA expensive and fiddly - and I don't know how you would do this without wires all over the place??

Of course you can make it work. And you'll probably think I'm taking the P again but to be 90% effective just how many sensors would you require? Would the distance-between-sensors-on-the-gate-each-side have to be a child-width-standing-just-behind-the-gate apart? Look at the angle-of-dangle of the SRF module. And if the driven gates had loads of overshoot after motor power-off would they stop in time to prevent hitting the corner of your car? I guess theres a fair bit of inertia there.

So, without sophistication (and brakes), would it not be best, as originally planned to sense for objects in the swept area before driving the gates.

But maybe the ultrasonic idea mounted on the gateposts has some mileage ... reasonable angle of view, a reasonable range, pretty accurate range-finding and, unlike PIR, will detect stationary objects... and if it could sense large piles of envelopes it would also be able to detect stationery objects too. It could just work.

 

[gengis]

Outside the box a bit - what you want is called a curtain sensor (mutiple emitters and receivers) but that doesn't do anything for the sweep of the gate(s)

You could use a sliding gate to get around that. If money is no concern a video system works great - one or more horizontal lines are monitored to see if the scene is changing from what is in memory.

My OM wanted something similar - wanted to push a button and open the gates for his car. After a lot of hassle we came up with a somewhat workable solution by incorporating large springs in the gate drives - the spring was enough to decouple the gear motor from the gate in the event something blocked the gate - these were chain link and steel tube gates, and relatively lightweight.

Plan B, about a year later was much more elegant and worked much better. We got the idea to use an air cylinder - worked like a champ, but has to be mounted at right angles to the gate to get enough swing to open it fully. So a slide and total of four cylinders would needed to move the attachment point of the push/pull cylinder. - getting expensive

Plan B2 was to use a pair of tiny air motors (hacked air wrenches) coupled to gearboxes. Too expensive also. BUT there's a pneumatic actuator used in HVAC that turned out to be ideal - all the benefits of a cylinder but 120 degree rotary motion - designed to actuate dampers to open and close ducts in the system. The thing we got was a crescent shaped plastic housing with a vane inside that is double acting - similar to the vacuum operated windshield wipers used in cars in the 40's, but larger. There's a small click as the latch cylinder releases then the gates silently and majestically open and close.

The actuators are designed to swing some heavy unbalanced dampers and can generate a lot of torque with just 15 PSI of pressure - standard "instrument air" in the US - a small diaphragm airbrush compressor is more than enough to work one. They are UV resistant plastic with stainless screws and a stainless shaft so can stay outside. They cost hundreds but it was no problem finding a pair of old ones for free by asking at an industrial HVAC shop. They last nearly forever and require little or no maintenance. The gates themselves only have a few pounds of force on them so there's no concern of hurting anyone. A moving car might get scratched or have a mirror knocked off but we used a radio control system to actuate the gate from the car or switch in the house.