Relay for switching small signals (serial and I2C)?
max72
Posts: 1,155
I have a friend with a drone and he's doing the necessary certifications.
Italian norm requires the possibility to switch off the drone, and to do that independently (separated power supply, switch transmission...).
His drone main board sends the signal control to the ESC board using I2C and serial 5V signals.
I would like to be able to cut the I2C and serial signals. I started considering an I2C repeater or a similar solution for the serial (I asked about that in the forum).
Due to the separated power supply requirements the solution is not optimal. I'm wondering if relays could work in this case and allow a correct signal trasmission when closed.
Obviously your insight (including other possible solutions) would be greatly appreciated.
Thanks in advance,
Massimo
Italian norm requires the possibility to switch off the drone, and to do that independently (separated power supply, switch transmission...).
His drone main board sends the signal control to the ESC board using I2C and serial 5V signals.
I would like to be able to cut the I2C and serial signals. I started considering an I2C repeater or a similar solution for the serial (I asked about that in the forum).
Due to the separated power supply requirements the solution is not optimal. I'm wondering if relays could work in this case and allow a correct signal trasmission when closed.
Obviously your insight (including other possible solutions) would be greatly appreciated.
Thanks in advance,
Massimo
Comments
A latching relay accepts a pulse to change state, and doesn't require constant power to the coil to hold position. You may find that a conventional relay would create excessive drain of power on the battery. After all, even a small 5VDC relay coil might demand 30ma or so.
Jameco has some small ones that will give you an idea of what is available. http://www.jameco.com/1/3/5v-latching-relay
I wonder why one would bother cutting the I2C signals when cutting power to microcontroller or the I2C chip would do the same thing. You might consider just selectively cutting power.
There are chips that could put the I2C bus into a high-impedance mode without bothering with relays or cutting power. And a flip-flop could toggle that state on and off.
So there are alternatives that you may have overlooked.
I was thinking of using latching relays, to increase safety.
Accessing directly the board is very difficult, while cutting trasmission would require a board to be placed in the middle of an ongoing trasmission between two separate boards.
The I2C tranceivers are very nice chip, but I have to power it, either with the main power or with my separated battery.
In the first case there could be problems with the certifications (they consider a copter and a plane the same, forgetting that with no power a multi copter flies like a stone...).
In the second case a failure of the secondary power would shut down the drone, and I want to reduce the danger to have the safety system causing mayhem: in case of a failure of the secondary system the thing must fly.
Another alternative is cutting the main power (tested and worked fine), but the currents involved are very high, so I was looking for alternatives operating on other systems.
There also is the possibility to overload the switching regulator of the main board, but it could damage the microcontroller, and while the system is expected to be used in extreme cases, I cannot exlude that it could be tested, with no damage.
Massimo
What are the "legal" requirements?, does drone need to drop out of the sky if radio signal is lost?
Here is the difference with drone vs chopper/plane, with drones it's better that it stays airborne and try to return close to home base.
Do you need to have a kill-switch radio signal button? signal going in to the main board LDO-Enable and it proves that the motors shuts down if main mcu board is not on.
Of course that would require a GPS unit.
The requirement is an independent system. Consider it a last resort, in case everything goes crazy, and possibly never used.
It must use a different radio and receiver, and a separate battery. It must kill the system and let the drone fall down. The requirements are the same both for a plane and a multirotor, so something looks silly because of that.
For instance the separated battery makes sense in case of a plane that can plane and go a long distance even with no main battery. In a multi rotor a problem in the main battery means falling immediately.
So it must be triggered from land. The flight zone is less than 300 m, so little issues with loss of signal, but in case of signal loss it must not automatically activate.
In case of a failure of the emergency system it should not be triggered.
Massimo
-Phil
Like the guy on the ground perhaps?
I found there are latching relays suitable for high vibration environments, so it there is not adverse effect on the signal quality that could be worth a test.
A possible solution using the I2C tranceiver and being still compliant, would be to have a backup battery operated from the main power supply, but it would add complexity to the system.
Thanks Phil for the PhotoFET link! I searched for suitable solutions but found none... this one looks very nice. I have to provide a normally closed solution, but this wouldn't be too difficult. I'll prepare a test circuit with this one.
Massimo
in Italy we had a free for all approach until recently. Now things are changing, and they are discriminating between drones to be used in non critical areas and critical areas.
One the discriminats is the presence of people below. In this case the drone must comply to other requirements, including maximum mass and impact force, propeller protection, and so on.
Massimo
Still, if that is what they really want / spec, then you do not need to switch serial and i2c, you need to remove the total system power (A failure could be a power-device failure, or a zapped controller, so link switch is not enough)
- for that you could try a * Photovoltaic Output Optocoupler and a suitably rated Power MOSFET, wired to power ON when there is no current from the optoCoupler & Optocoupler over-rules the gate resistor when activated.. That gives total power removal, so covers all failure modes, and also gives you a brief re-boot option as well.
It also ensures no driven props on falling, which should be safest impact wise.
* Photovoltaic Output Optocoupler - typically source 5-15uA when driven, and behave like a chain of small solar-cells.
eg TLP190B specs 8V clamping (Io = 0uA) and 12uA Iout, (shorted)
In theory parachutes are a great idea, but in practice you may need to have a small explosive charge deploy an emergency parachute. That's a bugaboo. What happens in the parachute deploys accidentally while someone is handling the aircraft during pre-flight?
I guess the idea is to have a backup safety system.
So you should reasonably avoid single point failures (in this case my safety system). The basic system has a lot of redundancy, and the idea is not of break the redunded system by adding a "safety" feature. For what I understand the system has double flight control, extra propeller, multiple power supply, go home and various safety functions.
The extra system that we must add is to be used if something extremely unexpected happens. And in this case you simply drop the extremely expensive drone. So in principle the failure of the extra safety system alone should not drop the drone, because we have many other options.
While I like the idea of explosive bolts,I think the only viable solution for a parachute is a servo driven lid. Let's say if we are lucky we'll reduce the damage.
I have another more general option that cuts all the power, but currents involved are quite big, so I was investigating a small signal/voltage alternative...
Massimo
The problem is that you are deploying an emergency parachute close to the ground. Just waiting for it to slide out of a tube and catch air might be too late.
Frankly, I think the best safety mode is to attempt to go into a hover and a slow decend before you run out of power. I can't grasp the concept of you need to comply with. I do understand that it is Italian, and that may just be a legal requirement that isn't well concieved.
I would first comply minimally to get certification and then see if the local authorities change their minds. The world has a lot of rule makers that really don't understand what they are doing or asking.
constantly to enable the flight system.
The power required to operate a relay during flight is tiny compared to the power to the motors (100W is typical
of a mid-sized drone). I would suggest a dual reed relay if you can find as the lightest option. is there any
reason against opto-isolation? An opto-FET might be possible.
Can't see a parachute being reliable from a tumbling craft BTW, too likely to wrap round things, though
it could be deployed from a vertical mast to increase its distance from CoM.
[ I am presuming the reason for the cut-out is when control signal is lost or a separate manual override is
selected - you want to prevent it moving outside the operating zone should this happen - the cut-out itself
has to be high reliability/certified, then the rest of the craft can be experimental ]
I have opto FETs on order. That would be the best solution. I only have to invert the logic.
The requirements are rather strange, but they grouped rc airplanes and multicopters, so things are messed up and sometimes illogical.
Anyway this is what me must comply with....
Massimo