Why the need for an IC?
August 26, 2008 11:19AM
I have a question about the RepRap design, understand I have no electrical/engineering experience.

Why is there a need for a circuit board on the RepRap at all? Shouldn't it be possible to control the motor and sensors almost directly from a computer in software?

You could just use your main computer and skip the circuit board, in addition to that complete small computer systems are now hitting the $50 mark, they might end up becoming cheaper than specialised embedded systems due to mass production and its much more flexible than relying on a specific board that everyone needs to track down if they want to build an unmodified RepRap.

I understand you need a basic circuit board to distribute power to the motor/heater, although USB 3.0 is supposed to allow devices to draw as much power as they want, so maybe it might even be possible to power the heater/motor with that depending on level of control USB will allow and assuming the RepRap doesn't use more power than the computers power supply.
Re: Why the need for an IC?
August 26, 2008 11:59AM
Type3Singularity Wrote:
-------------------------------------------------------
>
> Why is there a need for a circuit board on the
> RepRap at all? Shouldn't it be possible to control
> the motor and sensors almost directly from a
> computer in software?
>
If either Windows or Apple's OS X or Linux were Real Time Operating Systems (RTOS) it would be possible. Problem is that they aren't.

If you want to do that you need something like RTLinux

[en.wikipedia.org]

or iRMX or the like to do that job.
Re: Why the need for an IC?
August 26, 2008 01:23PM
Type3Singularity Wrote:
-------------------------------------------------------
> Why is there a need for a circuit board on the
> RepRap at all? Shouldn't it be possible to control
> the motor and sensors almost directly from a
> computer in software?

To control the hardware you need driver electronics and communication. If you want the hardware to work with Window, Mac, and Linux you need to communicate with it via USB or a serial port (which can be converted to/from UScool smiley. In order to interpret the serial communication you need at least a microcontroller such as the PIC or the ATmega in the Arduino.

To eliminate the need for a microcontroller the host computer would need direct access to the driver electronics. This could be done by designing a special PCI card and installing it in a desktop computer. Then you would need drivers for the three major operating systems. You would also be shutting out host computers that do not have an available PCI slot such as laptops and small footprint desktops.

> You could just use your main computer and skip the
> circuit board, in addition to that complete small
> computer systems are now hitting the $50 mark,

Really? I have not seen any home computers for anything close to $50. I have seen $400 systems but none much lower then that. Where do you find a $50 computer? What special inputs and outputs does it have that would let it control the hardware without a microcontroller?

> Its much more flexible than relying
> on a specific board that everyone needs to track
> down if they want to build an unmodified RepRap.

Getting a general-purpose computer, even one capable of running Linux, does not remove the need for interface electronics.

>
> I understand you need a basic circuit board to
> distribute power to the motor/heater, although USB
> 3.0 is supposed to allow devices to draw as much
> power as they want, so maybe it might even be
> possible to power the heater/motor with that
> depending on level of control USB will allow and
> assuming the RepRap doesn't use more power than
> the computers power supply.

Do you think USB 3.0 will provide the 12 volts needed for the stepper motors? USB 2.0 only supplies 5 volts.
sid
Re: Why the need for an IC?
August 26, 2008 01:33PM
Just because I've just read realtime operating system:
check out [www.menuetos.net]

very small, very nice (very free for personal use winking smiley)

Now we need to get the repraphost in pure assembler...
anyone?

'sid
Ru
Re: Why the need for an IC?
August 26, 2008 03:20PM
Quote

I understand you need a basic circuit board to distribute power to the motor/heater, although USB 3.0 is supposed to allow devices to draw as much power as they want

You don't need any such board. Reprappers generally use a bog standard ATX power supply, and connect boards directly to that to draw on the 5v and 12v power lines.

USB 3 may be able to do all sorts of things, but what I doubt it is gonna do is deliver the power levels that, say, steppers require. 3 steppers, 2 phases per stepper, 2 amps per phase, 12v. That's a fair amount of juice, and I don't imagine that any computer is ever going to deliver that over a generic IO port.

Plus, I don't wanna think about having unisolated lines between a bunch of motors and my PCs oh-so-fryable motherboard.

Quote

If either Windows or Apple's OS X or Linux were Real Time Operating Systems (RTOS) it would be possible. Problem is that they aren't.

True... but then it all goes and gets stuffed up again by using Java to handle comms winking smiley Having said that, there are far fewer complaints about pauses and blobs in the build nowadays, which is probably a good sign. Roll on comms buffers for the standard darwin electronics...

You could of course use EMC2 which uses real time linux extensions... but personally I hate having to use one PC for multiple operating systems (I seldom have the patience to use VMWare on my desktop, for example) and the hassle of getting a second PC is, well, a hassle.

PCs are getting silly cheap now though. I just got a second hand Dell optiplex with 2.8ghz HT P4, a nice big harddrive and the all important XP Pro license for
>Really? I have not seen any home computers for anything close to $50. I have seen $400 systems but none much lower then that. Where do you find a $50 computer? What special inputs and outputs does it have that would let it control the hardware without a microcontroller?
There are some for $100 at: [www.ewayco.com] I don't know if they are powerful enough since they seem to be <1ghz range and I don't know what it takes to drive a RepRap, there is also some embedded systems for $70 there.

Nvidia/VIA recently released an mini-itx mobo/processor combo claiming its better and cheaper than the Intel atom alternative, I remember seeing a price listed as $50 although I don't know how accurate it is. This also doesn't include PSU/case

There is an $80 Intel mobo here, add $30 for the processors and some more for ram. Once again no PSU/case sad smiley [www.linuxdevices.com]

There is an $80 300mhz, 128mb ram system here, and some other cheap systems shown here (with an annoying automated slideshow): [www.forbes.com] those prices are a little dated, the Dectop is now only $100 with mouse+keyboard [lifehacker.com] and for bulk orders its $66 [www.my-tech-deals.com]

These of course many of these don't include monitors or keyboards.

So we are still a some way off $50 but not too far now, within a few years is likely, there has been a massive mount of market demand for cheap computer systems for things like the subnotebook explosion and iPhones, still defiantly less than $400. There are also sub notebooks going for around $200 and they where $500 at the start of the year.

But there is also second hand systems to take into consideration, there are enough old computers lying around that for many people the cost is free.

I can't really comment on controlling motors since as I said its not my area, it seems sucky to have a full fledged processor that should be to do anything a microchip does (including emulate it) but not be able to have it drive the motor but maybe that's just how it is. I don't know what kind level of control the motor needs, if its just an on or off state then perhaps basic serial would be enough and just have a simple circuit that switches on the main juice when the serial has power. Maybe there is some $5 converter available or cheap off the shelf USB controllers that are widely available.

There are various embedded systems around that have special connections, maybe they can do things without needing microchip, once again not my area. If there is some simple interface such as a USB one that will let a computer talk to a system directly you could even find people driving the thing with an iPod or mobile phone depending on the software requirements smiling smiley

>Do you think USB 3.0 will provide the 12 volts needed for the stepper motors? USB 2.0 only supplies 5 volts.
USB 3.0 is defiantly supposed to support more power for recharging devices and such than 2.0, but I haven't found any information on exactly how much, by brain seems to remember vaguely that it would allow as much to be drawn as required if the system can handle it but my googlefu is lacking so I can't confirm. Wikipedia say "USB 3.0 is designed to be backwards-compatible with USB 2.0 and USB 1.1 and employs more efficient protocols to conserve power[26] while increasing the maximum power available for connected devices.", I would guess it would probably meet the 12 volt requirement, the heating element would be another issue since it would really need to be able to draw power but if you could get a really basic circuit that can toggle the power on from the main via computer control.

>If either Windows or Apple's OS X or Linux were Real Time Operating Systems (RTOS) it would be possible. Problem is that they aren't.
I understand the vanilla Linux kernel does have support for real-time, I don't know if it requires kernel recompiling or much setup (from memory its just an echo into the /proc but I never really looked to closely).
[www.linuxdevices.com]

But the other systems aren't sad smiley

Although if Linux is capable of it, it could be a good alternative since there would be a lot of people here using it and a lot of people willing to use a specialized OS for their RepRap.

I understand your not going to get rid of all the electronics, but I figure the less specialised components the better, just having a series of switches that draw power when the computer sends a basic signal done through USB or even out the audio jacks.

In the end I just figured you could either have a dedicated budget/secondhand PC running a real-time OS driving the device directly which would be useful to hobbyist or specialist but cut out the random Windows/Mac user crowd however saving the need for a lot of the IC stuff, or replace the specialised IC with a generic embedded PC system, more complex but more widely available (in 5 or so years it could be cheaper than a specialised one and a lot more flexible) and talk to it from any other system, or just continue to go the specialised IC route.
Ru
Re: Why the need for an IC?
August 26, 2008 03:32PM
Quote

I can't really comment on controlling motors since as I said its not my area, it seems sucky to have a full fledged processor that should be to do anything a microchip does (including emulate it) but not be able to have it drive the motor but maybe that's just how it is

The key phrase is 'hard real time interrupt guarantees', or somesuch.

A dedicated microcontroller can be programmed such that you can be absolutely certain that within n nanoseconds of receiving an interrupt (say, an endstop trigger, or a motor encoder tick, or a heater value change) you can read it, handle it, and carry on going.

General purpose operating systems absolutely cannot make this sort of guarantee. And delays in responding to important signals from your electronics are going to hurt the precision of your part at best, and break something at worst.
Re: Why the need for an IC?
August 29, 2008 02:30PM
As someone who has built PCI cards for PCs and even built cards to plug into the parallel port (using the 74367 chip) to control motors, I can tell you first hand it is easier to use a microcontroller to control the motors and heater. You screw up a circuit for a microcontroller you are out maybe $10, you screw up a circuit on a PCI card you are out a MOBO or more.

I've connect up motors to parallel ports, serial ports, and microcontrollers. Given the choice, I'll take a microcontroller and a motor chip any day - it's easier, cheaper, and safer.
Ru
Re: Why the need for an IC?
August 30, 2008 06:29AM
Quote

you screw up a circuit on a PCI card you are out a MOBO or more.

Making your own PCI cards seems like overkill here, when there are plenty of off-the-shelf IO cards available. As for frazzling your PC, I did mention optoisolators already winking smiley
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