My first incursions into the world of CNC machine control using stepper motors were long before 'Micro stepping' had been thought of and axis motions were in those days, to say the least, jerky. Direct drive, between the stepper and lead screw (when possible) led to low speeds and sometimes poor resolutions and the mass of the moving parts was all important in the machine design. Many advancements, over the years, have seen big improvements in the design of the control electronics as well as the stepper motors themselves and the biggest advance, in my opinion, is the advent of 'Micro Stepping'. This technique allows for much smoother motion together with less stepper noise, greater resolution and improved performance with regard to acceleration and deceleration characteristics.

 

There are a number of different ways in which micro stepping can be implemented and here I shall describe just one method for use with 'bipolar motors', which is used in my CNC controller, that of Sine / Cosine current switching.

Fig.2 shows a graphical representation of the current flow through each of the coils and it can be imagined as a crude sine wave having two distinct levels (plus zero) with coil A leading coil B by 90 degrees (or trailing by 90 degrees, depending on the direction or rotation).

 

If you now consider the rotor positions if the current through each of the coils was not equal but rather at a defined ratio one to another, then we have the basis for Micro stepping.

 

Referring to Fig.3 now consider an improvement in the resolution so that there are 8 distinct levels (plus zero) of current flowing through the two coils - now the waveform starts to look a lot more like a sine wave and the steps per revolution increase to sixteen. It can be seen that to keep each of the 16 steps at a constant 22.5 degrees the current variations between steps are not linear, they in fact follow the sine / cosine rule of the angle of rotation.

 

As an example, consider a basic four coil bipolar stepper motor operating in 'half step' mode to give eight individual steps per revolution.

 

Opposite pole windings are connected in series, effectively giving a motor with two coils and four lead-out wires. The rotor is polarised and contains a permanent magnet (sometimes it is just the shaft that is magnetised but some form of magnetic polarisation is necessary for correct operation).

 

In order to drive such an arrangement current must be switched on and off and also reversed in direction through the two coils and this must be done in a defined sequence.

 

Fig.1 shows a diagrammatic representation of such a motor performing a complete revolution. The direction of current flow through the coils is indicated by the red arrows.

 

This example assumes that when both coils A and B are energised the current flow through them is equal thus the rotor adopts a position exactly midway between the poles.

 

FIG.1

FIG.2

FIG.3

It would not, I think, be very practical to use a stepper motor that just had 4 physical coils to operate at 16 or more steps per revolution as the available torque would vary far too much between the various rotational positions in addition I doubt that the 22.5 degrees would be very accurate either.

 

In order to reduce the flux gap and improve performance multi-tooth rotors combined with multi-toothed pole pieces, together with many more individual windings (coils) have been incorporated into real world stepper motors. The multitude of these internal windings may be connected in series or parallel, depending on the performance requirements, but as far as driving the motor is concerned they still only amount to two coils with four lead-out wires.

 

For Tweakie I have used 200 step per revolution motors in eighth step mode, direct driving 5mm pitch ball screws. The theoretical resolution can be calculated at (200x8)/5 steps per mm and this is more than enough precision for all the tasks I intend to perform. The ball screws and ball slides that I have used are low friction devices and I have determined that the maximum drive current required for reliable operation is 2 Amps per phase. (see also design considerations).

 

You and you alone are responsible for the consequences of your actions.

Not only does this apply to CNC machining it also applies to life in general.

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