Some questions on the electronics???

Hi Group,

My first post here, so very excited to get into the RepRap community. I will be getting some bare boards soon and plan on starting my build next week most likely. But I am trying to build them with the bare minimum, so I have a few questions some of you may be able to help me with. Thanks in advance!

1. Do I need the I2C link? It almost looks like the RS485 does all the communication. Is the I2C like a future thing?
2. I imagine the SD card is not active yet, right? I may put this one later if so.
3. How much current do I need on the 12V power supply?
4. On the stepper boards, do I really need all those pull up resistors? How about the RC filter on the STEP pin? I thought these chips had internal pull ups and the AVR was a push pull. Or am I missing something vital here?
5. Is the quadrature encoder being used? I am thinking maybe the extruder, but have not been able to see where the shaft encoder is.
6. What are the PWM Drivers on the extruder controller used for? The heater element?

Thanks all for helping me during this enlightening process!

There are quite a few variations on the firmware. I think the makerbot versions do support the SD card and the RS485.

I have been lashing up an ASM mega 16 version of the firmware that supports SD. Recently I went ahead and bought one of the blank makerbot motherboards as I already had a mega644. Implementing SD support on such a large chip is not difficult. I am pretty sure I saw FAT32 libraries in one of the repositories.

I do no know the state of using I2C. I plan to use this in my own asm development as I already have code for supporting these comms.

Current required is defined by the stepper characteristics. This can be as low as a few milliamps per coil, To 3.5 Amps or so. Much here depends on how the coils are driven. The maximum supported by the current electronics is 2 Amps. More than this and the smoke leaks out.

Unlike a lot of engineering, motor control runs close to the upper limits. This is because it is an RC oscillator, There are pulse currents and transients, 100 of times the ratings, which last for a few microseconds.

Most stepper boards are designed to run from a parallel port through an opto isolator. I am currently in the build process of some v1.2 driver boards. I am not sure where the RC filter is on the Step pin. Having worked with AVR chips for nearly 10 years, you are correct on the push pull. Most of the designers here are still in school and correctly do things by the textbook.

Quadrature and PWM was used on the DC driven extruder. PWM is still used on the heater. Look in the Darwin 1.0 archives to see how this was used.

Makerbot is the commercial offshoot of one of the designers who was active here. They sell power supplies and stuff. Look at the ratings used and that should indicate what is needed.

-julie

Hi Julie,

Thank you so much for your input. Just to make sure, the I2C and the RS485 are mutually exclusive, right? You do not need to use both? I was looking at the firmware for the Mendel and it all seems to be RS485 driven, so I guess I2C is not being used at the moment (at least for that machine).

I think I understand the steppers. I am actually thinking of using 24V as this is a power supply I have widely available and it will make it easier to charge the inductors. You get better speed response this way. I do understand the drivers regulate the current, so it matters not what voltage we use as long as the drivers do not enter low voltage protection. However, I am finding it hard to believe you need 2A to move this carriage mechanism. Can it really get that heavy? I would have expected stepper currents as large as 1A to be more than enough. Specially if we are microstepping. I guess I'll find out soon...

There is still a lot I need to learn, so I will have a busy weekend... Thanks for your input!

My experience with steppers has been on my big (compared to reprap) leads crew based Desktop CNC. This has steppers the size of small coffee cans, These are rated at 3.5Amps. There is a network of resistors used for current limiting as the driver is over 20 years old. Eventually I would like to try micro stepping them. The best the old drivers can do is 1/2 step, which I may try this weekend. This thing works at less than 10 inches per minute, which is just too slow for any practical repstrapping.

The 2 amp limit is that of the driver chip. At least on the v1.2 boards. The v2.3 boards may be slightly higher rated. Makerbot has been out of stock on these for sometime. I got the v1.2 drivers 6 months or so ago as parts for these boards could be had locally.

I have been reading these forums for a bit over year, Sometimes it is like learning to hit a moving target. Hence the confusions over I2C and other comms. Perhaps someone else can answer that part.

-julie

RS485 is for driving separate head controllers, mounted on the head and is used to talk to the extruder controller on Makerbot. It is also used on Mendel, but the boards are next to each other I think and two extra wires are used for step and direction. These happen to be the I2C pins, but are not actually I2C. It is a crazy scheme to have both serial comms and step and direction, IMHO.

I2C is not good for this application because of ground noise, etc. It is designed for comms across a board, not across a machine. I use it on HydraRaptor, but it has separate PSUs and I still had issues with it, so I consider it a design mistake and will move to RS485 on my new electronics.

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[www.hydraraptor.blogspot.com]

Regarding the I2C lines with Adrian's 5D firmware - I've just made a few test prints with a Darwin running the 5D firmware, and there's a bit of a hack in there right now. Adrian is using the I2C lines to control the extruder stepper, and the RS485 for everything else. It's just step and direction signals, no actual I2C, so you could probably swap out some different pins. I think the I2C lines were just handy.

The Makerbot firmware leaves the I2C lines free, but it isn't as fast in the corners.

Wade

Excellent!

Can see better now. Anybody has an idea of how much current I need on the 12V rail power supply? I know the ATX Power Supply has quite the capability on the 12V output, but I wonder if I need it all. I will use 24V for the steppers, so I guess is 12V for the extruder heater. But how much current is this? Have not been able to find this parameter on the building pages.

heater takes a bit under 2 amps, but it's not on all the time

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Wooden Mendel
Teacup Firmware

You can run the heater off of 24 volts as easily as 12. Just use a longer (twice as long, since it's twice the voltage) piece of nichrome wire, and you'll get the same amps drawn, and twice the heat production and power consumption. (Watts=Amps^2*Resistance, and Amps=Voltage/Resistance)

This may even be better, as the nichrome's surface area will be larger, and it would run cooler? Needs research.

I'm running my heater off of 24 volts myself. According to my thermistor it works great, and achieves temperature control just fine. Haven't actually extruded anything due to problems with the polymer pump bit. My DC motor was much to large, blew up the controlling electronics, and I'm now in the process of making a stepper based extruder instead.

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I'm building it with Baling Wire