(c) copyright 2004-2007 www.stappenmotor.nl
These guidelines are a simple way of putting things, but they are correct, and can be used by a beginner..
There are two kinds of drivers ( remember these are simple guidelines ), constant voltage drivers and constant current drivers. These can by subdivided in unipolair and bipolair drivers. And then again in fullstepdrivers, halfstepdrivers and microstepdrivers.
Constant voltage drivers
Constant voltage drivers work with a voltage that is just enough to put the nominal current throug the coil of the steppingmotor. If for example the steppingmotor has a coil with a resistance of 5 ohm and a nominal current of 1 Amp. the voltage acros the coil needs to be 5 V.To activate or de-activate the coil, this voltage will be turned-on and off by means of a transistor or fet. This kind of driver is very cheap, but has the disadvantage of not being able to perform well at high rotations. This is because the steppingmotor wenn it starts turning generates a counter-voltage which is very quickly as large as the voltage of the driver. This driver will function correctly, as long as the steppingmotor doesn't turn to fast. Fast means in this case 1 a 3 turns a second. The torque drops as fast as the motor turns.
Constant current steppingmotordrivers
These steppingmotor-drivers use a much higher voltage than normaly would be posible with a particular steppingmotor. In the above mentioned steppingmotor the driver could be use a voltage of ( for example ) 50 V. Normaly there would then run a current of 10 Amp., destroying the steppermotor, but the driver limits the current to the correct amount of 1 Amp. How the driver does this depends on the way the driver is build. One way is putting a large resistor in series with the coil of the steppingmotor ( socalled RL-driver ), and another way is turning the voltage quickly on and off ( PWM ).
Constant current steppingmotor-driver with resistor ( RL )
This driver is by itself very simple, the only disadvantage is the heat the resistor has to dissipate. If we look at the above steppingmotor with a current of 1 Amp and a powersupply of 50 Volt this would be: 50 Volt ( power supply ) - 5 Volt ( voltage across the coil ) = 45 Volt. The power the resistance needs to disipate is 45 Volt * 1 Amp = 45 Watt ! The steppingmotor itself only gets 5 Volt * 1 Amp = 5 Watt. To be quite honest this is not striktly speaking a constant current driver, as the current does change as the motor turns.
Constant current steppingmotordriver with PWM
This driver is much more complicated as the current through the steppingmotor needs to be constantly monitored. And this current needs to be adjusted if it is to high or to low. The advantage is the low disipation of this driver. Compared with the the above steppingmotordriver it is almost 45 Watt!
This driver alway sends ( or tries to send ) the nominal current through all the coils. This will give the highest torque the steppingmotor can give. Disadvantage is the fact that the steppingmotor each time takes a big step ( with the steppingmotors of stappenmotor.nl ) of 1.8 degree. This by itself induces a rather large shockwave in the system, often resulting in bolts and alike unfasten. Furthermore these fibrations let to the steppingmotor starting to resonance. And this often results in the steppingmotor not being able to maintain sync with the pulses.
A halfstep driver turns the coils alternating one or two coils ( fase ) on and off. This results in doubling the number of steps the steppermotor makes compared to a fullstepdriver. The disadvantage is the torque the steppermotor outputs is not constant. If one coil ( fase ) is activated the torque of the steppingmotor is half that of the torque of the steppermotor with all coils activated. Advantage is of course the higher resolution and hence the less vibration. There is also less resonance.
This kind of driver doesn't turn the coils on or off, but sends a current that is adjusted gradually. The resolution is of course much higher and can reach up to 10.000 or more steps a rotation. This is usually more than the bearings and spindels have. Of course the disadvantage is the higher cost, as the construction of these drivers are much more complicated. Advantage is the ( almost ) lack of vibrations and resonances. Therefore a microstepdriver is better to use than a half or fullstep driver, as the steppingmotor doesn't have to turn as much. And a steppermotor-driver with a higher voltage is better than one with a low voltage. The torque at higher speeds is much greater.
Optocoupler inputs have the advantage of not have problems with the earthing. A second advantage is the fact that the driver doesn't breaks down if the computer does and viceversa. The system can made in such a way that there is no connection between the computer and the driver.
Power supply of the steppermotor-driver
If a driver has a common DC-powersupply for both the logic and the powerstage there must be payed very good attention to the capacitors on the powerlines. Otherwise the Z-ax will start to turn if the X-ax accellerats due to a spike introduced on the powerline to the Z-ax by the X-ax. Better are the driver with a separate supply for both the driver and the powerstage, or is the powersupply is ac.
(c) copyright 2004-2007 www.stappenmotor.nl