Driver board basics

[tante]

Hi!
I have few basic questioins about driver boards. What is the difference between driver boards, for example, linistepper and L297 from pminmo, beside one is for unipolar and one for bipolar motors? I know that one use chopper current limiting, and other limits current with resistor, but what is the difference between these desings in practice? And why some boards use motor driver chips (like L297, LMD18245 etc) and some don't (like linistepper, "discrete" driver from pmnimo etc)? Can these boards without these motor driver chips work like boards that have these chips?
THX!

[NC Cams]

Your questions could involve some long,detailed replies, depending on the specificity you're looking for.

Simply put, the driver IC needs to be matched to the current control requirements of the motor and vise versa. Try getting your hands on some of the chip manufacturer's data and/or application sheets. These often provide answers to some of the questions you ask. This is why a DC brush motor needs a brush type controller and a synchronous motor needs something different so as to create the commutation needed for the motor to run.

Regarding chopper vs resistive current limiting: resistors limit current flow via resistance. The current that doesn't get to the motor is turned into heat within the resistor.

Choppers simply "chop" the current off and on only real fast. By doing this, you limit current flow by varying the duty cycle on and off. THus, if you pull 10 amps at full load, a chopper that limits flow to a 50% duty cycle would allow an average of 5 amps to the device. Sounds neat as you're not wasting current turning it into heat.

BUT, as you chop current, you tend to build and collapse the electrical field. Each time you do so, you induce a current spike when the magnetic field collapses . This inducted current has to be dealt with and kept from feeding back into the drive circuitry and frying stuff. These spikes can get ugly.

For example a simple relay fed 12vdc can kick back a voltage spike of almost 60vdc when you deactivate the solenoid. An ignition coil in a car is charged with 14vdc but a 20,000vdc voltage is induced when the current flow in the primary side of the coil is switch off.

Imagine the mess you'd have to deal with if you're switching the current of an 80vdc servo motor at 20,000cycles/sec (20khz) which is a switching speed of some DC servo amps.

Regarding discrete versus dedicated IC's used to drive motors: in the early days of CNC, one had to make their own drivers via proper logic circuit matching to create motor drivers. As the technology got more popular, the IC makers started creating IC"s that would do the same job with less parts in in a smaller space, with more steps, microsteps, etc.

The desire to make smaller and lighter drives (computer, servo, aerospace, automotive, etc) led to development of smaller and smaller driver packages with more and more features having greater levels of sophistication.

Originally, man cooked food with a simple wood fire - now he has microwave ovens. Same process only more advanced technology provides more sophisticated cooking devices. Yet we still spend huge amounts for a barbeque to cook like our ancestors did. Go figure.

Motor control circuity evolved in a similar fashion.

[pminmo]

tante,
Well....this is probably more info than you want. 1st the driver has to be able to handle the motor. While we talk bipolar or unipolar it really is a little more flexable than that. The only motors that can't be run either way is a 4 or 5 wire motor. The 4 has to be run bipolar the 5 has to be run unipolar. 6 and 8 wire motors can be run either unipolar or bipolar, and 8 wire motors can be run bipolar parallel (more power).

Next step after settling on driver type is getting a driver that is capable of running the specific motor. 90% or the time, maximum current rating is the determining factor. i.e. a 1A drive trying to run a 3A motor is not a good choice. A little under powered drive is ok, but do it only for cost or already have reasons. i.e. a number of people run the 2.8A stepper motors with 2.5A drives.

The last piece of info is does the drive have current limiting, if so what type. This is where some of the differences come in.
1. The simplest/cheapest form of drive is no current limiting like the discrete 1 axis or 3axis board on my website. They work, but to get higher performance you have to go offboard with current limiting via either a power resistor or external circuit. Some older chips such as the 5804 have no current limiting.
2. The most popular form of current limiting is referred to in many circles is chopped. Chopping simply means when the current senses you have reached the maximin current level you electonically swtich the drive off for a very brief amount of time, then turn it back on until it reaches maximum again. This typically happens thousands of time a second. The real positive about chopping is it is the most power efficient, thus least heat dissipated.
3. The linistepper utilizes linear current limiting. The downside of linear current limiting is the heat disipated by the driver.
4. There are some other forms of drives, but you dont see them very much.

Chips -> Over the last decade many chips have come about for steppers, primarily for consumer products such as printers and scanners. Essentially there are several "blocks" necessary in a stepper motor driver the way they are used in the cnc world. You need power drivers (i.e. delievers the current capability of the drive), step sequencing full/half, current limiting, and for microstepping drivers you need additional step sequencing and a method to generate intermediate current limiting points other than full on or full off, usuall a DAC of some form. Running down a list of popular chips, Allegro 3977 is a bipolar all in one that has all the "blocks" in one package, the ST L297 is a block that has Full/Half step sequencing and current limiting circuitry in it, but cant drive a motor directly. Its most used companion is a L298 which is a full driver for bipolar motors 45v 2A. The Sanken/Allegro SLA 70XX series are mostly unipolar chips, the SLA 7051-52 are full "Block" chopped /2A/3A unipolar all in one chips that do full/half stepping. The SLA7060-62 are full "block" chopped 1/2/3A unipolar all in one chips with microstepping. Some of the more popular "blocks that are not a total solution is the LMD18245 which contains the DAC/current limiting and drivers 3A 45V for one coil of a stepper motor, but does not contain the step sequence information. To make a full 2 phase stepper motor driver you need two plus typically a microcontroller to make a step/direction driver. Beyond that there are
the LMD18200 a simplar part the the 18245, 5804 an old full/half step unipolar 1.5A non current limiting, TMC239/A Biploar all in one, L6208 Bipolar driver 2.8A 52V, and others.

Bottom line, match motor to drive type unipolar vs bipolar, make sure the driver can supply the current your motor is rated at, to get the most out of your motor rule of thumb the higher the voltage rating, the higher the faster it should be able to step, which means current limiting. Full vs half vs microstepping becomes specific to the application. You typicall have less torque the smaller the step goes, but some times you can get more speed with smaller step sizes due to resonance.

Something tells me I should include this and more in a webpage.

[tante]

Wow, thanks guys, this is what I wanted to hear
I asked this becouse I am looking for driver to build, and I was confused with these IC's, which are pretty expencive for my, and I didn't know could I make good driver without Ic's. I think I will make one linistepper and one board with L297/8 (which is not too expencive) form pminmo's site and see how that works.
One more thing: for fastes speeds I must have higher voltage rating? Isn't speed regulated with speed of driver (frequency of pulses)?

[greybeard]

Phil - your throwaway line of "a web page" would be a wonderful resource for all the newbies like myself.
Many, if not all the points you and NC cams have just covered have appeared many times in generous response to our questions.
There have been suggestions of 'glossaries' and the like before, and while they go so far, it seems to me that because they have relied on multi-person inputs(? sorry, there must be a better way to say that) they tend to be overlooked, or not added to because they're rarely quoted.
We say over this side " if you want something done, ask a busy person".
Your own pages are a well known reference point, so the sort of info above, possibly in tabular form would be a bonus.
John

[pminmo]

tante,
while speed is controlled by the speed of the step pulses, the motor will only turn a load as fast as its physical/electrical properties. The motor power supply voltage as it increases increases the torque of the motor at higher step rates.

greybeard,
Can you eloborate on tabular? I really need to get some instructional stuff on my website, in the end I think it would save me time.

[pminmo]

Ok first cut at a webpage on the subject:
http://www.pminmo.com/drivers.htm

[greybeard]

Phil - I've sent my ideas as an email to avoid a clog up on this page, but basically continuing the style of your own site.
John