PC 104 Boards in UAV

[Ceros]

Hi Guys - a PC 104 form factor question from a newbie, who has an airframe and mechanical background, but very knowledge of electronics.

Have a PC 104 digital PWM board, which some time in the future is going to be used to drive large Futaba digital servos.

The question is this: Futaba digital servos have a 3 wires (Postive, Negative and Pulse) but as far as I can see, both the channel inputs and outputs on just about every PC 104 PWM digital board I have seen, is set up with 2 pins per channel.

What becomes of the 3 wire?

My thinking about this is that one pin takes the Pulse wire, the 2nd pin is one or other of the Vdc power wires that supply voltage to the servo, and that the 3rd is a common - which along with the same wire from all servo's running off the same PC 104 board, are connected together somewhere else?????

......... I've go it all very wrong, haven't I???

Okay, Okay, as said computer electrics are not my strong point - can someone please explain to me how 3 wire digital servo's are correctly connected up to PC 104 boards/output channels which only appear to have 2 pins availiable - what am I missing, where am I going wrong??

Many thanks guys.

[Unterhausen]

You need to have some kind of intermediary board or cable between the servo and the digital output board. The middle wire on a servo is the power. Generally, digital i/o boards will not supply power.

[Ceros]

That's what I thought originally, but having looked online at about a dozen different university projects, none of them are driving hobby type servo's off PC 104 stacks via an intermediary board of any sort - the schemtics show direct connection between the servo's and the 104 boards, though not in sufficient detail to discern extacly how its done.

I'm pretty sure there is a straightforward answer to this - there are thosuands of univeristy UAV projects up and running with PC 104 avionics and Futaba or Hitec type servos being used ... and having also looked through 100's of online manufacturer data sheets of digital controller boards, there are many which offer programmable voltage outputs - often from 0Vdc - 30Vdc - and yes, this is on opticaly isolated digital outputs.

[Unterhausen]

You are probably right, it's probably done with wires and electrical tape.

Not sure what you are looking for here. the pulse input is relative to the negative supply voltage on the servo. Thus the servo power, the center wire, needs to be referenced to the same ground as the pulse input, but it doesn't have to come from your pc104 board.

I've never seen a pc104 board that was specifically designed to drive hobby servos -- doesn't mean you aren't going to find one and prove me wrong. But the reason nobody shows the connections on their schematic is that it's a fairly trivial. It's probably done with servo extensions and electrical tape.

[Ceros]

Okay - let me try to explain this better - a large part of this problem is my next to zero knowledge of electronics, which means I am proberbly also explaining myself very badly.

Hypothetical scenario: have 8 large Futaba digital servo's, which are going to be used to run a large fixed wing UAV - along with a GPS/INS, AHRS, engine sensors, camera controls ect ect ...... all the usual stuff you'd find in a meduim/large commerical type UAV.

The heart of the whole project is a suitable PC104 SBC, connected to which will be a number of periphiral boards(e.g. video processor and frame grabber - and that much is clear as cyrstal to me.

Having looked through a whole bunch of UAV projects on the internet that use PC 104 type boards I see that many so have micro-controller and I/O boards plugged in as well.

First question: could you explain to me just what the I/O board does in a typical UAV setup - what role does it play, where does it sit in the system and what components does it interface between?

I always thought an I/O board was something that took information from the mainboard in one type of "formatt" and turned it into something else, so that it could drive an actuator of sort e.g. a servo, but in all the examples I looked at on the net, I could'nt find one example of an I/O board driving a servo. Why do they have so many channels - some I have seen have 32 analoge or digital Inputs and Outputs - how are these normally used?

Things get more complicated when if comes to PC104 microcontroller boards. Isn't that what you program and then plug small dc sevo's into?

What about so-called "data acquisition boards" - just what the hell does that do, where does it sit in the chain and what role does it play?

Many Thanx

[psyrex]

As far as I know (and I've actually never worked on PC/104 boards, smallest is EBX boards for me), the I/O pins on a PC/104 mobo is usually digital in/out. For that, two pins is more than enough. Servos require PWM input, and a very specific format, too. You'll need to look into PWM capable i/o boards, and i think there are PC/104 daughtercards for that.

FYI, there are no "typical" UAV because everybody builds their own. You need digital i/o for stuff like turning on navigation lights or taking a picture. You need analog (pwm) i/o for driving servos and reading analog output sensors. Maybe TWI for sensors that output in that format, too. A UART or three for GPS for positioning, serial RF for telemetry, or microcontrollers for other controls. Your desired sensor and controls package will determine what kind of interfacing you'll need.

Oh, and one more thing, your board might not have enough power to supply several full size servos. Those guys can demand alot of power if you have a bunch of them. You may want to just look into a dedicated servo board anyways and save yourself the hassle.

[Unterhausen]

there aren't very many people here that have airplanes big enough for pc104 computers. People have certainly done it. Most of us use a microcontroller board that is specifically designed to drive servos. The reason you probably want to do this is that driving a servo using a digital i/o port on a general purpose computer requires your computer to stop what it is doing, do some calculations, and write to a port way too often for most computers. It may seem counterintuitive, but a microcontroller can handle the load better.

There is a group I am familiar with that uses PC104 which talks to a Cloudcap autopilot.

[reedchristiansen]

This is a bit off topic, but we were able to stuff a pc104, ip modem, and autopilot into a 60 inch flying wing. JHUAPL uses it for research.

Reed


there aren't very many people here that have airplanes big enough for pc104 computers. People have certainly done it. Most of us use a microcontroller board that is specifically designed to drive servos. The reason you probably want to do this is that driving a servo using a digital i/o port on a general purpose computer requires your computer to stop what it is doing, do some calculations, and write to a port way too often for most computers. It may seem counterintuitive, but a microcontroller can handle the load better.

There is a group I am familiar with that uses PC104 which talks to a Cloudcap autopilot.

[_helitron_]

The simplest way to control servos from a PC104 board is to use a little servo controller board connected to a serial line of the PC104. For example this here:

http://www.servocity.com/html/servo_controllers.html

Cheers,

//Erwin

[Unterhausen]

This is a bit off topic, but we were able to stuff a pc104, ip modem, and autopilot into a 60 inch flying wing. JHUAPL uses it for research.

Reed
Reed, that's nice, what motherboard is that? I'm always on the lookout for a better PC104 board since my robot controllers are getting old and cranky.