[joecnc2006]

Pardon the PUN "Step it up a notch"

Has anyone built, building, planning, or thinking about a DIY 80v 6a-7a controllor? I would like to make them, but do not know ehere to start as far as drivers and layout etc. Would be nice to have a DIY gecko alternative people can make and easy to replace parts on them (don't get me wrong Gecko's are great drives) but you know diy is just plane fun....

Joe

[joecnc2006]

No Replies?

[mcarvey]

I am in the final stages of development of a DC servo controller using a 18F452, and am in the process of doing a writeup/webpage. Were you thinking of making a controller for a Servo Motor or a Stepper Motor?

-Matt

[sdantonio]

Joe,

Since we talked last I have done a couple of searches of the web and haven't found anything by way or a 80V stepper board schematic. For some reason people seem to be stopping at about 50V (Alan's PicStep for example). I did see a reference, and I don't remember where, that said something about not being able to go over 80V (80V being an absolute limit) due to back EMF. But I didn't read it carefully and don't remember enough of it to say for sure what the reasoning is.

Steven

[BCwanderer]

http://www.datasheetcatalog.com/data...1/IR2184.shtml
The ir2184 bridge driver is good to 600 volts.
A high voltage stepper driver should be buildable with high voltage FETs
Do you need Bi-polar or can you use unipolar stepper drivers?
Unipolar can use low side FET drivers

[NC Cams]

Check out the Servo amp/servo motor treatise on the Rutex site. Real good info.

From his treatise, in concert with some reverse engineering, here's my take:

The voltage rating is a function of several things:

1. Volatage rating of fets.

2. Whether they get power from power buss - on the regulated side of a PWM'd fet control, they seem to be essentially the same.

3. Back EMF.

Obviously, 60v fets won't work at 100 and 200v will be horrible overkill run at 60. Select accordingly and with enough capacity to handle surge current and voltage.

Back EMF can be absorbed by the epitaxial diodes inherent in the fets, if you have enough of them. It is better, however, to shunt with appropriate diodes. Make sure you have diodes w/enough voltage and current capability. Schottky's are preffered.

If the drive gets power from the same buss that feeds the motors, you usually have to double or tripple regulate to get from 180 to say 100 then to 20 and the to 12 or 5vdc that the drive circuit runs on.

A shunt regulator can do the grunt work to get from unreg to something liike 12-14vdc that an LM2940T5 will handle (these are MUCH better than 7805's - don't bypass with a diode!!!! for reverse insertion protection is in there already).

I tuned up some servo amps via using heavier versions of these very components and they seem to work fine - but you really have to dig into the controller to get to adjust current limits, over voltage, etc when you try such surgery - definitely NOT for the faint of heart nor electronically challenged...

[BCwanderer]

Here is an idea. Untested but interesting!
Has anyone tried making a high voltage bi-polar by using an allegro a3982, a3983 or a a3984 chip as a translator.
These allegro chips are available for less than $5.
If the outputs from the chip were loaded with a resistor, the current direction and step could be sensed with a fast opto-coupler.
There would need to be 4 opto-couplers, two for each "coil' connection.
The opto-couplers could drive ir2184s which in turn would drive a high power H bridge.
The allegro chip could be operated at 12 to 15 volts with low current while the H-Bridge could be operated from a high current-high voltage supply.

The chopping could be sensed by the Allegro chip by running the H bridge current through a sensing resistor which would be connected to the sense connection on the Allegro chip.
The allegro chip would be isolated from the H bridge by the optos. The common ground and the sense connection would be the only shared connections.

This could also work as a unipolar driver by having each of the optocouplers drive a low side driver FET to a unipolar motor.

The allegro chips have full, half, quarter, eighth, and sixteenth step sequencing, depending on the chip chosen.

Any ideas or comments please.

[joecnc2006]

I'm still searching, would like to stay with Bi-polar steppers, cheaper and easier for the diy. Or thats what most people understand and recognize.

Joe

[pminmo]

I presented essentially the same question to an Allegro support engineer, they won't say it can't be done, but they don't advise it either. (their bipolar parts) I'm heading down a different path as I believe there are performance gains that can be had vs. how the Allegro parts handle.
Unipolar I don't see as a problem, but again I'm going a different path there also.

[Arthur Clampitt]

Re: high voltage bipolar idea

Nice idea , but I think you might find it difficult in practice, I can't see how you would manage to feedback the current level in the extrernal transistors to the chip for control of the PWM chopping, From the simplified diagram on the data sheets , the sense resistor is needed to give the ground for the internal bridges and the sense connection is connected to the internal bridges , Trying to superimpose a voltage from sensing on the External FETs would be a problem

Bearing in mind that the block diagrams given on data sheets are highly simplified there may be a way of doing it , but you'd need more detailes circuit info

[phantomcow2]

A friend of mine made a stepper driver good for 8 amps and 100v. It was simple unipolar, good for 1/2 stepping only. Based off transistors and PIC chips. It sure worked well though

[spalm]

Couple of thoughts on higher voltage and current drivers.

You will find a cut-off of around 50 volts for the integrated type drivers (Allegro, SMC, National). These are easier to use but are usually DMOS devices. DMOS allows the chip designer to put low voltage and high(er) voltage devices on the same substrate and get a little more current ability for their output section with the requirement of using lower voltages to satisfy both types of FETs. Using P-MOS at high current also has the disadvantage that it is a bit slow, so it will run hotter.

Here’s DMOS (Dual-MOS) from Wikipedia:
----------------------
This type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the Single type switch. The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential (Vdd) and the body of the N-MOS is connected to the low potential (Gnd). To turn the switch on the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential. For voltages between (Vdd-Vtn) and (Gnd+Vtp) both FETs conduct the signal, for voltages less than (Gnd+Vtp) the N-MOS conducts alone and for voltages greated than (Vdd-Vtn) the P-MOS conducts alone.The only limits for this switch are the gate to source, gate to drain and source to drain voltages for both FETs, at which time the FETs are damaged.
------------------------

If you build with discrete N-MOSFETs and use a rated hi-side driver as BCwanderer mentioned, you can go as high as you want. It will cost you dollars every time you inch up voltage or current. Neat thing about discrete designs is you can pick your price/power point for every motor.
Steve

[sdantonio]

http://www.datasheetcatalog.com/data...1/IR2184.shtml
The ir2184 bridge driver is good to 600 volts.
A high voltage stepper driver should be buildable with high voltage FETs
Do you need Bi-polar or can you use unipolar stepper drivers?
Unipolar can use low side FET drivers

My personal preference is bipolar, but it is Joes thread

[sdantonio]

Also, would chopped be to much to ask for too? Trying to balast an 80V, 7A circuit would require huge resistors and generate a huge amount of heat

[pminmo]

The technology to do 80V and greater N-CHN discrete Mosfet H bridges is out there, NBD. The logic to implement a dual H bridge is NBD. Starting to insure timing and recircuilation starts to up the complexity. Physical PCB layout handling the FETs gate charge, current requirements ups the problem. Miscrostepping implementation ups the complexity. Before you know it it turns into more of a challenge than it looks like off the cuff. Not that it's impossible to do, but when you start getting into component costs, numer of components, it's an agressive DIY project. I'm currently playing with a simple implementation of a compromise which looks like it maybe a good fit for those who want the most bang for the buck. i.e. power, speed, accuracy and minimizing resonance. But just aint enough free hours and energy...............

[joecnc2006]

The technology to do 80V and greater N-CHN discrete Mosfet H bridges is out there, NBD. The logic to implement a dual H bridge is NBD. Starting to insure timing and recircuilation starts to up the complexity. Physical PCB layout handling the FETs gate charge, current requirements ups the problem. Miscrostepping implementation ups the complexity. Before you know it it turns into more of a challenge than it looks like off the cuff. Not that it's impossible to do, but when you start getting into component costs, numer of components, it's an agressive DIY project. I'm currently playing with a simple implementation of a compromise which looks like it maybe a good fit for those who want the most bang for the buck. i.e. power, speed, accuracy and minimizing resonance. But just aint enough free hours and energy...............

Sounds very good I am looking forward to seeing what comes about, Their are people that will help you out if you need it maybe a private section on your site/forums, to grant certain individuals access to that area only which are working with you to try to produce a Bi-polar drive above the 50v mark.

Joe

[NC Cams]

The ability/technology to switch the 80 volts exists. The challenge comes in when you try to any electronic swithing FAST.

One very serious challenge in switching any H bridge drive at high speeds is "shoot through" - the point in time when two conductors on the same side of the bridge are on/partially on. This is effectively a short to ground which always has a high potential for smoke emissions.

Inductive feedback is also possible but lets leave that asside for the moment.

The enemy is "time" as in time to swith the pass element (fet) on and off at the same time. Since the element can be partially conductive (hence semiconductor), this "gray" area is what causes problems.

Stray capacitance (size of traces, trace location, trace length,etc) affect the rate at which you can charge/discharge the gate. Hence even though you turned the drive OFF, the element may still be conducting enough to give rise to shoot through. Hence, you can't merely drive with an PNP/NPN totem pole - you need interlocking logic to prevent the phenomenon.

Once you get past this, you may have a suitable design as in IC's, logic, et al. It would work on a real, properly laid out PCB. Yet, it goes nuts on a breadboard.

Decision time.

It SHOULD work but doesn't - it will PROBABLY work if you had a PCB. Do you go ahead and spring for a PCB??? And the kid needs new shoes, the wife needs a new kitchen and the cable bill is late (can't lose the DSL connection).

Remember, its a DIY project and the board may cost several thousand to layout and prototype.

Imagine doing that several times before you get a production board to work.

Having been through the iteration a couple of times with very simple in comparison projects, I now see why a board with maybe $20 in parts costs $500.

Now the issue of inductive feedback....... another variation of what's transpired above...

To simplify: you can't see electrons, therefore you can't trust them. Easier than herding cats without a food dish but not by much...

[joecnc2006]

So are we saying It just not possible for DIY 80v Bi-polar Drivers? or just stating what has been read.

Joe

[sdantonio]

I figured if it were easy then it would have been done a long time ago and folks like Gecko wouldn't be able to charge what they do because there would be more competition in that voltage range and their products wouldn't be so proprietary.

This is why folks like me need engineers and machinists to keep us grounded in reality. I look at something like this and say, well all the parts are there for about $20 bucks, and I can make a 50V board, an 80V should be just a small step away. But you guys are the specialists who know the subtle little problems that I don't see looking at the grand overview of things.

So it seems servo's may be the way to go in that voltage range

Steven

[NC Cams]

The laws of physics are pretty hard to refute. To illustrate, think about this for a bit:

Take a 12 volt relay, engage it and then disngage. Do so with and without a protective diode. Watch the result with a recording oscilloscope. You should see a pretty decent voltage kick so be careful.

Now imagine doing that with the multiple windings of an 80 volt coil. Imagine doing so at 20khz (1.2million times in a minute). Can you spell "light show"???

The point is not in that it can or can't be done. They said you couldn't put a man on the moon - and we did using computers not as powerful as the one being used to generate this message.

What's being said is that viability for a DIY project may not be there. The essential point in the reply was merely a reality check - something that is often readily overlooked by even the best intended DIY'er.