Rule 6.3 The Link

[mark_m]

Theres a couple of things you could do.
Servos can be fitted with return springs so that they move to the off position on loss of power. This is very difficult to get right though with standard servos as they have lots of friction, but if higher quality servos are used, like coreless ones, they are relatively easy to back-drive. Most serious nitro RC car drivers use them (throttle return springs).
The other way would be to have a main power contactor. this could be fed from a smaller relay that was powered by the reciever battery. That way if the reciever lost power, everything would loose power.
Admittedly these are all one more thing that can go wrong, but if you already had a main power contactor, as a remote off switch, like some people have, it wouldnt be that much more complicated to implement this idea.

Mark

[the__kraken]

Jim, I see what you€™re saying, but as you said, you could loose any power sources to the servo and end up having a micro switch stuck in the €˜on€™ position. As you say, it could get you around the tech check, but I feel there would be a false sense of security about doing this. It wouldn€™t make it safer.

Mark, again I see where you€™re coming from with the return spring, and I think we may have had this discussion on the old RW forum.

On your idea of one main power contactor, One Relay to cut all power to everything. This was how my Robot Victor worked way back in Series 2!
We had all Drive and Weapon power running through a Relay which was activated by a servo/ micro switch set-up. This servo was connected to a failsafe, a G M Services one back in those days, that would cut main power if signal was lost. And just to show how things have moved on, that was the one and only failsafe we had in the whole robot! We stopped using this set up when the removable links were brought in.
This idea could be updated, but my question would be, what would you replace the servo/ micro switch set up with to activate the Relay, which would work if the Rx lost power.

At the moment I have three types of failsafe in my robot, GWS FS-1 units, and the ones built into the Team Delta RCE210 Relay Switch, and the Delbots weapon interface. I€™ve dug out the instructions for all these and all say failsafe mode will activate with loss of signal, None mention anything about loss of Power.

Looking at it, I€™d imagine that the RCE210 would failsafe with loss of power. The FS-1 would fail to de-activate what ever it was connected to with loss of power, and I€™m not sure about the Delbots Board.

Any help, advice or comments would be greatly appreciated.

Guy

[jim]

With regards to the main power contactor, you could run the power for the contactors coil through a small 6V relay powered by the receiver power supply, then through the microswitch which the servo activates. Arrange for the 6V relay to be downstream of the receiver to minimise the chances of a cut wire leaving the relay powered while the receiver is not.

Regarding servo power - I wouldnt recommend any way of getting around tech checks, it was more to point out a possible flaw in the rule. There are also legitimate reasons for running your servos on a different power supply to the receiver.

[the__kraken]

Jim when I said getting around the tech check, I didnt mean it in as breaking rules. I was just trying to echo you point about and any power source standing the chance of being cut/lost.
I see where you coming from with the relay running off the receiver power. You could probably do something with out using the servo and micro switch. I still use a switch to switch on my Receiver. You could take a feed after this switch to power a small relay, which in turn powers up a larger relay, which would be inline with the link, which has all power running through it for drive and weapons. Loss of Rx power would shut the relays down, shutting down the Robot.
I guess keeping in the servo and micro switch would give you a remote kill option.

Next to this, any views on the built-in failsafes I mentioned?

Guy

[roger_plant]

If you use a relay, powered by the receiver power, to supply to the micro switches you will get the same effect as cutting the main power without having to use a heavy duty relay in line with the power link.
i.e. if the receiver power fails, the small relay drops out, no power to the micro switches, no power to whatever the micro switches were controlling, even when the servo operating them loses power and stops where it is.

If you are using interfaces that do not always failsafe by removing the drive to your speed controllers, then power them via the same small relay as well.
That will remove the drive to the speedos.

I have experimented with heavy duty relays to cut the main power and found two main problems with them.
1) Their contacts can bounce open with severe shock.
2) They are not cheap.

Using a 12 volt car starter relay via a resistor from 24 volts is hopeless, they over heat long before the end of a five minute bout.

Reliable 24 volt heavy current relays cost about £30.
They are not easy to find second hand either.

[jim]

I wish Id bought one of the 180A relays that Maplin used to sell - they were selling them for £9.99. I believe they were sold under the Shark brand name who make lots of cheap car audio accessories. If anyone sees them on sale anywhere please let us know.

180A may not be enough for some modern robots but would do fine for me. As for contacts bouncing open - I have heard several different stories about how likely this is. The biggest problem I expect would be them opening while the controllers are trying to dump energy back into the battery - could cause welding of the contacts and damage to the speed controller.

I agree with Roger about car starter relays or solenoids as theyre often called - theyre not meant to conduct for more than a few seconds.

[ukrobotics]

http://www.albright.co.uk/http://www.albright.co.uk/ sell a wide range of heavy duty DC contactors at reasonable prices. Everything from 80A to 1800A!


Dominic

http://www.ukrobotics.comhttp://www.ukrobotics.com

[roger_plant]

You seem to be mising the point. You do not need to buy a power relay if you use a small relay powered from your receiver power to control the supply to your interfaces or micro switch driven devices.
If the receiver power is missing, the power to the microswitches or interfaces is removed thus removing the power to whatever they were controlling.

[jim]

At some point you need to have a device capable of switching or regulating the power from your batteries to your motors. Normally this is a MOSFET speed controller or weapon-firing relay. Both types are capable of failing in such a way that they will conduct power without any voltage supply to their control inputs, leaving you stuck in forward drive or with a flywheel stuck on.

You may find this risk acceptable, indeed it is fine by the current FRA rules, however, a separate power relay reduces this risk significantly.