although typically the robot enthusiast groups deal with wimpy little robots. But, it's likely some have good control experience, analog experience, and motor control experience.

Try looking up PID control (Proportional Integral Derivative) - that's what is normally used for closed loop control, e.g. on all the servo systems we build (and for temperature control, etc) (there's also fuzzy logic). We use commercial motion controllers or PLC's to do the real time stuff.

You'll have to interface to the feedback system (analog front end plus A/D converter to get from resistance to voltage to bits) and jack screw (motor driver of some sort).

I'd recommend using something with a bit of computing horsepower so it's easier to write the NMEA code. There are plenty of little boards available; if you want to use something other than assembly, BASIC, or C/C++, get something that gcc runs on (e.g. x86, PowerPC, ARM) and you should be able to use Objective C via gcc.

Also, it's likely you can visit eBay or other surplus sites to get commercial grade stuff (e.g. brush or brushless motor amplifier) to make your life easier but still be affordable.

--Tony

There are many different ways of building such a system. Three examples are using a PLC (more industrial approach, but more costly), using a single board computer (ZWorld, New Micros, and many others), or using an industrial PC (e.g. PC104 system)

It might be good to see what the Seattle Robotics people like - then you hopefully can good feedback and help. Or pick a system with an active mailing list.

Protection from the elements is going to be a big concern. It may be better to build your own enclosure - you can get splash resistant systems (e.g. IP67 using sealed connectors), but they cost considerably more.

BTW, they're probably too pricey and I don't have any experience (not a good fit for our applications), but I've always found [link|http://www.exlar.com/|Exlar's] linear actuators interesting.

--Tony

We've used A-B, Panasonic, Automation Direct, Omron, and Siemens PLC's.

The PLC approach does have its advantages. They are industrial quality, with various input (24VDC, 120VAC, analog in, etc) and output (24V, relay, analog out, etc) options, while the C-programmable stuff is typically bare boards with TTL (5V) inputs and outputs. If you need 4-20mA, you'll have to check that the PLC has an option for it.

Some do have built-in PID; we've done PID temperature control. If you want to do PID, make sure it's built in.

Older PLC's are programmed solely in ladder logic, which is a fine fit for combinatorial logic ("when these inputs are in this state, do this"), but sucks for everything else (e.g. sequence of actions). Newer PLC's support the IEC61131 standard, which supports five langagues: ladder, function block (ladder written with AND/OR instead of wires), structured text (our favorite; a mix between Basic and Pascal), flow chart, and ladder assembly.

PLC's are horrible for any kind of data handling, so if you do use them, that recommended approach (use PLC as PID controller, don't do NMEA conversion) makes sense.

The price will be higher, and you'll have to pay for programming tools (A-B's are very pricey, and have copy protection; Siemens, Aromat, Automation Direct are more reasonable) or know someone who has them.

Some companies have starter deals which include a PLC and software. Aromat has run specials on PLC's and Operator Interfaces occasionally. Siemens runs them often (including both PLC and Operator Interface); you will need to contact your local distributor.

I recommend looking at [link|http://www.sea.siemens.com/automat/product/plc/auov.html|Siemens], [link|http://www.nais-e.com/plc/|Panasonic], and [link|http://www.automationdirect.com|Automation Direct] PLC's. Automation Direct has all their pricing on line, and it's competitive. (A-B does has on-line pricing, too, if you register, but it's quite high, even after a 30% discount). Then you can use resources such ebay and [link|http://www.plccenter.com|PLC Center] to see if you can get surplus or used equipment at much better prices.

--Tony