Wireless emergency stop

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I need to be able to wirelessly enable a relay to turn off an autonomous lawnmower. In the past, we used a "mechanical engineer" solution of using an RC transmitter (like the kind for RC cars) which would remotely trigger a servo which turned until it hit a switch which triggered the relay.

There are multiple problems with this solution, one being the amount of power the servo used, and two was with system instability (sometimes it would receive a stray signal and trigger the servo). Another problem was there was a second or two delay from when the servo was initially trigger until it closed the switch.

I have looked at a few different wireless transmissions methods such as using a wireless radio modem or something of that sort, but those tend to be rather expensive (a few hundred at least).

We also thought about possible buying a remote locking system for a car and hacking into that to use it for our purposes.

Let me know if you guys have any suggestions for cheap devices we could possibly use.

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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Well, www.aurel.it makes a few modules, cheap enough. And the ads around here also link to radio modules. I'd give you the adresses, but I blocked all the ads from here.

They've got afaik also digital modules, and at prices quite low.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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If this is a safety system you should not rely on receiving a signal from the transmitter. Instead the transmitted should be used to allow the lawn mower to keep going, and the receiver should stop the lawn mower if it receives a stop signal, or if there is persistant loss of signal, or data corruption. The time out period is dependant on your safety requirements.

If you don't have these fail safes in place you are playing russian roulette with your safety. What happens is there is interference, or the transmitter battery is flat?

Tim

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Be sure to carefully think through what happens when your autonomous lawn mower runs over the neighbor's dog. And you tried to stop it but the wireless transmission didn't get through.

I haven't been directly on that side of the fence (pun intended) but have rubbed shoulders with this in other parts of companies for many years. There are guidelins (rules?) that should/must be followed.

That said, it becomes a bit easier if your remote device has a power source and can be listening all the time. then, it can expect a poll (say) every second, and pause/stop if none received. The response to the poll will be the "green light" on your master console/ESTOP. Then you have good assurance that when you send the "stop now" message instead of the normal "all OK" message that it will get through and be acted upon.

You should have quite a few modules to work with nowadays, compared to a couple of years ago. Many in 2.4GHz. You have to decide how to avoid interference--manual or automatic "hopping" channel selection.

A module may be able to trigger an output that can drive a relay circuit. If you attach a micro to your receiver you can do the more intelligent "still OK" stuff.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I was definitely planning on attaching a micro to the E-stop system so that a emergency stop could be triggered via RS 232, or via the manual stop button as well.

Also, we were already using a system where if the RC went out of bounds, then the servo would go out of it's place (which was holding a switch in place, and when it went out of bounds, it would release and the switch would shut off the mower; thus the reason why it was drawing so much power because the servo was essentially always being used). So I'll definitely make sure to implement a system using a micro where it randomly polls to make sure it's still talking to the remote stop.

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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Also, I should note that we have no intentions to make the lawn mower a commercial product, it's for an autonomous lawn mower competition ( www.automow.com ).

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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I would put something in the mower that would stop it UNLESS there is a trigger radio signal every second or so. So if the transmission stops so does the mower.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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My suggestion: don't use RF signals unless you bought your own frequency (which is quite pricy). The open frequencies everyone may use are used in almost any household, so you will almost certainly get much interference. We got some RF controlled wall power outlets here and if one of our cars is locked with the remote, one of these wall power outlets switches off. Safety and interference-robust my ass!

I suggest you use infrared signals to stop the mower. These can only travel in straight lines, which is a good and a bad thing. On the one hand there is very little chance you will get interference, on the other hand you can only stop the mower when it's in sight.

Did you also think about an ultrasonic approach? Sure, there are parking systems that use it and if your mower uses it to get distances you can't use it either, but think about it!

You might even use something within hearable range. If it's only for the contest, I'd get an FM radio receiver, turn it to a frequency that's not used there, build a cheap "test" radio transmitter (DIY kits available everywhere) or buy one of these $20 FM transmitters for MP3 players and attach an amplifier.

Markus

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Well, one of the guys I work with had the idea of hacking into a garage door opener and receiver and make the button so that it's "always on", thus always transmitting the garage door code. And then we could hack into the receiver and try to find the relay in there and use that.

So, essentially whenever the receiver wasn't getting the signal, it would turn the mower off. My idea for dealing with interference problems (i.e. if the signal would to accidently be lost for a second), then I would have the avr start a timer, and if that timer elapsed without reacquiring the signal, then I would have it stop the mower.

Do you think this would be enough to deal with losing a signal or interference?

Also, I'm not too worried about the amount of power used since the overwhelming majority of the power is being used for turning the blade, powering the motors, and the lydar system.

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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I just realized this may not be the proper forum for this to have gone, if a moderator wants to move it, please feel free.

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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> I just realized this may not be the proper forum for this to have
> gone, if a moderator wants to move it, please feel free.

I moved it to the General Electronics forum.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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Quote:

hacking into a garage door opener and receiver and make the button so that it's "always on", thus always transmitting the garage door code. And then we could hack into the receiver and try to find the relay in there and use that.

Might be a good quick approach.

I can only relate my experience with a commercial 2.4GHz module that we designed into a product a few years back. The channel (sub-band) can be selected to stay away from those commonly used by WLAN and such. It has the concept of PLAN and address, so in total there are millions of combinations of "address code" to choose from. The module does n retries automatically of packet transmission.

Nowadays, you have lots more canned modules to choose from such as those that Sparkfun sells that others have had good luck with. I'd think the advantage to you with a micro at both ends is that besides the e-stop feature you can have the response to the "are you alive" poll be status info, so you can report what your 'bot is "thinking".

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I would just use a normal closed limit switch and a rope. Its quick, its simple, its cheap and it reliable.

I would also get in touch with the event staff see what safety is REQUIRED, The dune buggy race a few of my buddies were in required special type of fuel cutoff valve that had to be mounted in a specific place as well as hand break.

Quick legal fact on E-stops:
An E-stop has to shut off the power to the machine and must be hardwired in a redundant/failsafe way.

A "pause" does not have to shut off the power to the machine (is not considered a valid safety device). Can be hardwired or software controlled. Does not have to be hooked up in a redundant way. CANNOT be red or labeled as an "stop" or "e-stop"(should be yellow).

A nice example:
A conveyor has 5 workstations, 4 manned 1 automated robot cage.

Each of the maned work stations would have PAUSE buttons hooked up as normally open to a VFD. If one of the pause switches is hit (say one of the workers gets behind and needs a few more seconds to finish the job) the VFD stops the line over 3 seconds (ramps down the speed so that the parts dont fly off the conveyor due to an instant stop).

The robot cage however has to use a proper E-stop chain (E-stop buttons, safety mats, light curtains ... etc) Each of the devices on the e-stop chain must be hooked up as normally closed. If one of the stop switches is hit the motor power should be dropped out immediately, Motor Brake comes on, Things come to a stop as quickly as possible, Parts fly due to instant stop :wink: .

The robot Can however, also have a pause for situations that don't require it to stop instantly (like when its time for a smoke break).

Oh yeah and the guy in the station closest to a robot cage should have an e-stop as well simply due to proximity to the robot cage (probably not required due to the other safety devices that prevent access into the robot cage , but its $30 bucks that keeps the safety inspector/safety rep/manager and workers happy).

Consult your local electrical safety authority for any special rules. .... cant use black mat switches in this county *insert obscenities and threats of revenge here*

I'm not a cartoon character but I play one in real life.

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Just a thought. What is it that monster truck crews use to remotely kill the trucks? My kid was watching some T.V. show about the history of MT's and one thing the show mentioned is how they utilize a dead mans kill switch via RF. Designed so if the driver inside is knocked out and unable to control the truck, a ground crew can trigger a kill command to the engine cutting the gas. This is on top of the manual kill switch the driver has. I presume the kill switch does other things as well, maybe apply brakes or something, I'm not sure. Early versions used walkie-talkies, but I understand later designs became dedicated and far more reliable in the intense RF interference around the venues.

Clearly, when dealing with such a HUGE dangerous vehicle in a public venue, a few issues of safety are probably required. Not the least of which is reliability in wireless communication. What works for a big truck able to kill a dozen people at once might work for a lawn mower.

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Hello, beginner here!

A couple of 'ink-blot' stand outs:

1. A wireless dog fence with the collar hacked to kill your mower if 'runaway mower' should occur!

2. A tilt switch.

3. A perimeter 'bump' switch where the mower would halt if the bump switch was touched.

4. I do not think a rope would technically be 'autonomous' but, a derivation of the rope idea would be a pole mounted in the center of the yard with a cord reel attached to the top. The length would be the maximum allowed travel distance. Exceeding that distance would pull a power plug!

5. Photocells mounted on the sidewall of the wheels that would measure whether or not the mower was in the grass or on hard surface. (Grass would obscure photocell and increase ohmic value which could be controlled with the AVR and would shut the mower down if it traveled off the grass)

OK that is all I have :D (Lucky you!) I guess my point is that many of the above could be included to eliminate the need for constant radio contact. Trust me, if you lived next to me my Rangemax router (2.4Ghz) would cause your mower to do wacky things...and vice versa!!

John

Just some guy

Last Edited: Sat. Aug 18, 2007 - 01:51 PM
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Mr. anderson
Linx technologies makes an RF link with a 3000' outdoor range. I agree 100% with Lee that the signal needs to be as proprietary as possible.

Here is my suggestion:

The Linx modules are undet $10.00 each from Digikey. The receiver draws less than 30ma so current consumption is not a biggie. The transmitter can plug into the ac in the house through a wall transformer. Have an AVR at the transmitter send out a byte pattern through the USART to the transmitter Then at the reciever, another AVR that looks for the code. you can then have a retry counter that when the maximum number of missed codes(i would say two) has happened shut the mower off. You can unplug the transmitter or simply hold the resert line low to act as an emergency stop. The rf modules can take decent data rates up to 19.2k, but even at 2400, two missed transmissions in a row will trigger the stop condition in less than a quarter second. The mower will shut down slower than that.

This mower sounds like a project I read about in poopular electronics years ago.

All the best on your quest

Jim

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