Match Motor Type to Drive Electronics
The driver electronics and stepper motors have to be compatible. For two phase stepper motors that breaks down into two basic categories, Unipolar and Bipolar. Two phase motors come is several wired configurations which can simply determined by the number of wires coming from the motors dictating how they can be used:
|Motor||Unipolar||Bipolar||Bipolar Series||Bipolar 1/2 Coil||Bipolar Parallel|
Verify Driver Current Rating for Motor
The driver has to be capable of adequately powering the motor. As an example, driver electronics that have a maximum current rating of 1A would not be a good match with a motor that is rated at 3.2A. A significant percentage of the time, maximum current rating is the determining factor only second to motor type. 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 next piece of important information to consider, does the drive have current limiting, and if so what type. Here are some of the most popular:
|None 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 off board with current limiting via either a power resistor or external circuit. Some older chips such as the Allegro 5804 have no current limiting.|
|On board power resistor How to calculate of values|
|Off-board circuit Example of typical circuit, possibly a schematic|
|Chopping The most popular form of current limiting is referred to in many circles as chopped. Chopping simply means when the drive senses you have reached the maximum current level it electronically switches 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. (Diagram of on/off ratios possibly a simple schematic of a circuit)|
|Linear current limiting Simple, but the downside of linear current limiting is the heat dissipated by the driver. For example if you have a motor that has a 2A rating at 2.6V and you have a 45V motor power supply, the driver has to dissipate approximately 85 Watts. Consequently, linear current limiting has practical limits on how much power it can dissipate, not just the maximum current rating.|
Next, what voltage rating can the drive electronics handle in conjunction with the motor power supply. Stepper motors performance are enhanced by having motor power supplies much higher than the coil voltage rating. If you want to understand that more, there is an excellent paper on Geckodrives website. (Which I might add Gecko's are excellent drives, and no I don't have any financial interest in Gecko)
Step Size Capabilities
The simplest is Full Steps, followed by Half Steps. A method of Half Step that is more complicated is equal torque half step.
|Simple half step is accomplished by inserting an off state between transitioning phases. i.e. 0% and 100% This cuts a stepper’s full step angle in half. For example, a 90° stepping motor would move 45° on each half step. Since one of the windings is not energized during each alternating half step there is less electromagnetic force exerted on the rotor resulting in a net loss of torque. Equal torque half step, requires a driver that can have an additional current limit point of 70.7% for each coil.|
Then comes microstepping, i.e. quarter step, eighth, tenth and so on. Step size is a debatable point and depends on application. As you go down in step size, your available power goes down also. But to counter that as you go down in step size the action becomes smoother and the problem of motor resonance goes down also, which is a speed limiting factor.
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. delivers 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, usually a DAC of some form.
|Chip||Type||Current Limiting||Max A - Max Vdc||Step Sequence|
|Allegro 3977||Bipolar||Chopped Current Limiting||2.5A - 35V||Full-Half-Quarter-Eighth|
|Sanken SLA7060/1/2||Unipolar||Chopped Current Limiting||1/2/3A - 45V||Full thru Sixteenth|
|Sanken SAL7051/2||Unipolar||Chopped Current Limiting||2/3A - 45V||Full-Half|
|ST L297||Sequence logic & Chopper||No Driver||Full-Half|
|ST L298||Full Bridge Driver||2A - 45V||No Step Sequence logic|
|National LMD18245||Half Bridge(Two Required)||Chopped Current Limiting||3A - 55V||No Step Sequence logic|
|TMC239/A||Bipolar/Unipolar||Chopped Current Limiting||2.5A - 34V||Full- Sixteenth|
|L6201/2/3||Full Bridge Driver||Chopped Current Limiting||1-5A||No Step Sequence logic|
|L6505||Sequence logic & Chopper||No Driver||Full-Half|
The chips above have a limitation, they are bound to lower voltage and current ratings than can be accomplished by utilizing discrete MOSFET's which are the last stage of the driver electronics. For example the Gecko G201 is capable of handling 80V power supplies at 7A, significantly more than the above chips, thus significantly more capable.
So who uses what:
Picstep is based on the LMD18245
Linistepper is discrete components
Xylotex Bipoar Allegro 3977
PMinMO: Allegro 3977, L297/L298 paired, L297 Unipolar uses discrete drivers, SLA7051, Discrete Boards use no high level chips.
Match motor to drive type unipolar vs bipolar, make sure the driver can supply the current your motor is rated at, rule of thumb to get the most out of your motor 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 typically have less torque the smaller the step goes, but some times you can get more speed with smaller step sizes due to resonance.Updated 4/16/2009