DE2536637A1 - PROCEDURE FOR CONTROLLING STEPPER MOTORS - Google Patents

Description

Applicant: VEB Kombinat ZENTfiONIK Research Center Patent Department (EP) ODR-523 Sömmerda Weißenseer Strasse

'Procedure for controlling stepper motors "

669811/0263

The invention relates to a method for controlling of stepper motors. Stepper motors are used for numerical control of machine tools, as servomotors in control and regulation technology and recently in data processing systems, in particular for setting type bodies in printing units, for transporting printing carriages or print heads Serial printing units, for the transport of magnetic heads used in disk storage and for transporting recording media.

Stepper motors are not connected to a constant voltage network, but to a special control device that uses control pulses to cause the motor to rotate gradually, usually by switching and reversing the polarity of the field coils. Due to the peculiarity of the M (ψ) - characteristic of the stepper motor, the inertia of the rotor, as well as the existing load moments of inertia, each stepper motor drive represents a vibratory structure. The rotation of the rotor by a defined construction-related angular step thus follows swinging processes. This overshoot leads, in accordance with the natural resonance of the rotor system, to a periodic decay of the torsional vibration that occurs until the end position of the rotor is reached. This physically determined operating behavior occurs in particular when the stepper motor is only lightly loaded. In the numerical control of machine tools, a positioning process usually consists of a very large number of angular steps, the settling time required after each positioning process being negligible compared to the time for the positioning process itself. In the case of a large number of drive tasks in data processing systems, on the contrary, movements over smaller distances or angles are required that have to be carried out in a short time with a defined end position.

60 9811/0263

In addition, with such drives, the stepper motor is usually only driven by the inertia of the Mechanism loaded. This relatively low load supports the periodic commuting of the Systems.

They are therefore used for drive tasks in data processing systems Corresponding measures have become known that the undesirable operating behavior of the stepper motor should be reduced to an acceptable level. It is known to dampen the movement of stepper motors an additional frictional torque proportional to the speed of rotation of the motor due to the viscosity using a suitable liquid (DL-WP 59 311). The disadvantage is that the damping Natural frequency of the motor reduced.

is known to decay through a chain of To prevent delay elements with different delay times (DT-OS 1 563 019).

The disadvantage is the high cost of components and the need to set the delay elements individually, since these are not effective independently of one another.

It is also known to interrupt the series of motor supply current pulses before reaching the desired holding position and to trigger a holding current pulse when the rotor has swung one step beyond the last position (DT-OS 1 613 333). It is also known, to delay the motor movement, to move the field of the motor so excessively fast that it is then in a position which decelerates the movement of the motor. The disadvantage is that both methods require a steady state, ie a constant speed. However, this state can only be achieved with a larger number of steps.

The purpose of the invention is to eliminate the deficiencies mentioned at a reasonable cost.

609811/0263

The invention is based on the object of a method specify for controlling stepper motors, which avoids periodic oscillation of the rotor and is advantageous is applicable when an object to be driven in intervals of any width and in any Sequence is to be transported or if movement intervals with constant speed are required

The object is achieved with the field windings are excited by control units for each movement interval of the rotor with a sequence of steps and the rotating field rotates at a constant angular velocity that, according to the invention, the rotating field as long as starting up is switched at a switching frequency until the angular velocity of the rotor becomes a first Time has reached its maximum that at this time the switching frequency doubles to a second value is, the angular velocity of the rotor remains constant, that to any stop a second time the switching frequency is switched back to a third value, the angular velocity of the rotor functionally decreases and when the switching of the rotating field is interrupted to a third Point in time the rotor stops vibrating, this point in time by half an oscillation period is shifted compared to the second point in time and that the associated control unit is known per se electrical means to compensate for the attenuation -is provided.

In an advantageous embodiment of the invention Solution, the switching frequency is large in relation to the Natural frequency of the stepper motor, but smaller than the starting frequency of the stepper motor, whereby the switching frequency is chosen so that the periodic step number of the stepper motor is twice as long as the first time is run through.

In a further embodiment of the solution according to the invention to compensate for the damping, either the

609811/0263

Speaking field windings are fed with excessive current after switching on or switching over a winding or the beginning of each step sequence is controlled with a switching frequency that is greater than half the second value of the switching frequency and during the braking process the switching frequency is set to a third value that is smaller than the Half of the second value of the switching frequency is reduced.

In contrast to the known prior art, the solution according to the invention offers the advantage of drives by means of Stepper motors, especially for data processing systems, with little effort but high effectiveness to realize.

The invention is intended below using an exemplary embodiment are explained in more detail.

In the accompanying drawing show:

Fig. 1: the angular velocity time curve for a Control cycle in the ideal state without taking into account the damping or with its compensation according to claim 5,

2: the angular velocity time curve for a control cycle with consideration of the damping and the compensation of the damping according to claims 6 and 7.

The curve shown in Fig. 1 z ^r IGT the time course of the angular velocity ψ for one control cycle in the ideal state without taking into account of the damping. The same curve shape can be achieved by compensating for the damping according to claim 5 by means of an excessive current supply of the corresponding field windings after switching on or switching over a winding. By exciting the corresponding field

603811/0263

a sequence of steps is initiated at the point in time to. The angular velocity w> increases according to FIG. 1 and reaches its maximum at time ti. fsl, and this state of the synchronous phase can be maintained as long as desired. The course of the angular velocity ip is shown in dashed lines in FIG. 1 for the case that the switching frequency fsl is not doubled at the time ti, ie, for example

fs2 = fsl. The angular velocity γ drops again towards O in a mirror-inverted manner to the start-up curve and a constant angular velocity ψ is not achieved. At fs2 = 2fsl, the braking of the rotor can be initiated at any point in time t2 within the synchronous phase, strongly drawn out in FIG. The angular velocity

t
efficiency γ * decreases functionally. If the switching of the rotating field is interrupted at time t3, the rotor stops vibrating. The point in time t2 must be selected accordingly, since the point in time t3 provided for the standstill of the rotor is shifted by half an oscillation period compared to the point in time t2. This course of a control cycle is shown in FIG. 1, with the time interval

ti - to = —ί , the time interval t2 - ti = be

liebig and the time interval t3 - t2 =. {} , are.

Ca)

The maximum angular velocity reached at time ti corresponds to the value 2fsl. fs, where ψ s is the step angle and 2fsl is twice the starting frequency.

At time t3 the angular velocity ψ = 0. A further periodic decay of the overshoots Üs of known solutions no longer occurs or has decayed to a negligible minimum «

609811/0263

For a better understanding of the mode of operation of the control, in particular the prevention of decay processes in rotors. Stepper motors should mathematically derive the relationships. Assuming idealized control (constant currents in the respective energized windings, no reaction of the mechanical system to the electric) can be described by the following equation aer drive:

(1) (& +0.>. U> "+ Μ, *. (V * ν *») = Μ (ψ-η · ψζ ^v ' ^v Mot load) j load ^v | J ^v / /

In equation (1), the reference symbols have the following meaning:
^ = Moment of inertia of the motor

(O, 4. = moment of inertia of the load ^v Las ι

ü> = angular velocity
M = moment of loading

, ο = rotor angle

η = integer variables.

The argument shift η. Ψ s of the M ( γ) characteristic, the gradual shift of this characteristic is expressed with each (electrical) switching step.

The M (vP) characteristic can be used for most stepper motors by

(2) M (W>) = - M _K. sin Z. f

can be approximated, where M _(< denotes the breakdown torque or the max. static torque of the motor and Z denotes the number of step cycles / revolution. To obtain a first approximate solution of equation (L), γ ^ '<

further in

(3) M (γ-) = - M _K

6098 11/0263

be simplified. If the rotating field angle ψ is also introduced into equation (3), this is given the form

(3a) M (γ) "" * ·. M _K (Z γ -y).

If the stepper motor is excited with a sequence of steps, especially in motors with a large periodic Step number the time course of '-Tp' (t) through (4) Y (t) -Ä (t)

can be approximated when the switching frequency fs is large compared to the natural frequency 00 of the motor. In equation (4), Sh denotes the angular velocity of the rotating field and * ψ denotes the rotating field angle. If equation (4) is used in equation (3a), the following solutions result with the initial conditions ^ = O and γ:

(5) v> = & t - j2L_ sincJ t and

(S) op = | S & (1 - costOt).

From these equations, if for t = - * ■ * - is set will recognize:

for t = -2L-. is Lf (t) = jöt = y and

If the switching frequency fsl is now doubled at this point in time ti (according to FIG. 1), ie'V (t) = 2 Sb (t), the motor continues to run at a constant angular speed * f s = 2J2} (synchronous phase according to FIG ). Stopping can take place at this point in time by first switching back to a switching frequency fs3 which, idealized, is equal to the starting frequency fsl. From this point in time t2 (according to FIG. 1) the second half cycle of equation (5) or (6) is run through, so that when the switching of the rotating field is interrupted at point in time t3 = t2 +? V, the motor stops vibrating. ^ This effect can be proven mathematically by

_ 27

into equations (5) and (6) for t =. ¹ S is set.

6098 11/0263

rHp is i / (t) = Sit

For t =

that is to say j vie η π at this point in time tS (according to FIG. 1) the switching of the rotating field is interrupted, when sout = V = 0, the motor stops without a decay process.

In the above derivation, the angular velocity ^ and the Sc: stop frequency fsi, fs2, fs3 used, The relationship between these quantities can be expressed by the following equation:

(7) & = </ s. fs (1,2,3).

Here, from equation (7), the dependence of the angular velocity Sh on the step angle Φ s and öer respective switching frequencies fsl, fs2, fs3, which according to the invention are effective in the times ti-to, t2-ti and t3-t2 according to FIG. 1, can be seen is.

Lie limits for this procedure are given by that the quotient of the angular velocity of the rotating field «Si» and the step angle * / s during the start-up not greater than the starting frequency fg of the stepper motor may be.

or equation (7) inserted into (8): (Sa) fsl <fg

ie « _f the switching frequency fsl must be kept lower than the starting frequency.

In most cases it proves to be advantageous to choose the angular velocity of the rotating field £ L so that in the time _p7 _

^τ U) '

where ts0 is the natural frequency of the motor, the periodic number of steps m of the motor and thus a control cycle is run through just once, because then the switching is always interrupted in a certain switching state of the control circuit.

609 8 11/0263

The above math section has been made for the sake of simplicity the attenuation z. B. by the drive object or by the fact that there is not exactly the freedom from interaction between exciter current and the mechanical values are not taken into account.

One possibility to compensate for the attenuation has already been given in connection with the explanations for FIG. As a result of this r-.assnahrne, a curve of the angular velocity y? According to FIG. 1 can be achieved with relatively low attenuation. In the case of greater damping, a method specified in claims 6 and 7 for compensation can be used in addition or modification according to FIG. 2, in that the beginning of each step sequence is controlled with an increased step frequency. First of all, the curve shape of the angular velocity y shown in FIG. 2, influenced by the damping, will be explained in more detail. As already described with reference to FIG. 1, a sequence of steps is initiated at time to by exciting the corresponding field windings. The angular velocity ψ of the lotor increases, but the maximum value ψ max · reached at time ti is, due to the damping, at a distance ^ (J below the value 2fsl. ^ S.

The course of the angular velocity t / is again drawn in dashed lines for the case that the switching frequency is not increased at the time ti but, for example, would be maintained. The minimum value of the dashed curve is correspondingly iui distance from the time axis t. Furthermore, the dashed line in Fig. 2 shows the speed curve from time t2 for the case that the start-up frequency fsl would be doubled, ^ie fs2. 2. fsl.

Due to the speed difference d / to this

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Point in time, this results in an oscillation process to speed 2 for the angular speed ψ. fsl. ^ s.

Thus, between times ti and t2, a constant IVinkelgeschwindigkeit t / can be achieved, the frequency fs2 during cor synchronous phase must be less than twice aie starting frequency fsl be, so

fs2 <2 fsl,

or the basic frequency fo must according to claim G by one Amount Af can be chosen larger so

f <zO

fsl;> - ~ - will.

If the switching frequency was switched back to a value fs3 to stop at a point in time t2, which corresponds to the start-up switching frequency fsl, that is to say fs3 = fsl, the curve shape drawn in dashed lines in FIG. 2 from point in time t2 would result. An overshoot Us occurs. At time t3, the rotor does not come to a standstill. The angular velocity γ at time t3 is not equal to O ₁ but has the value A * f , so the rotor swings over. The braking frequency fs3 must therefore be less than half the switching frequency fs2 during the synchronous phase, that is

so that the rotor comes to a standstill at time t3. This course is shown in strong lines in FIG. 2 between times t2 and t3. The deceleration frequency fs3 must therefore be reduced by an amount Δ f according to claim 7 in order to compensate for the disadvantageous influence of the damping, the absolute value of Δ f is the damping caused by the respective motor-load relationship, dependent and must be determined by measurement in the special case.

6U98 1 1/0263

Claims (7)

Claims 1. A method for controlling stepper motors, the field windings of which are excited by control devices for each movement interval of the rotor with a sequence of steps, the rotating field revolving at a constant angular velocity, characterized in that the rotating field is switched at a switching frequency (fsl) for as long as the start-up , until the angular velocity ( iP) of the rotor has reached its maximum at a point in time (ti), that at this point in time (ti) the switching frequency is doubled (fs2), the angular velocity (γ) of the rotor remaining constant that at any Stopping at time (t2) the switching frequency is switched back to a value (fs3), the angles 1- speed (^) of the rotor functionally decreases and when the switching of the rotating field is interrupted at time (t3), the rotor does not vibrate stops, where the point in time (t3) is half an oscillation period compared to the point in time (t2) is shifted and that the associated control unit with electrical means known per se for compensation the damping is provided. 2, method according to claim 1, characterized in, that the switching frequency (fsl) is large in relation to the natural frequency (^) of the stepper motor. ' 3. The method according to claim 1, characterized in that the switching frequency (fsl) is smaller than the start frequency (fg) of the stepper motor. 4. The method according to claim 1, characterized in that the switching frequency (fsl) is chosen so that in the time (2tl) the periodic number of steps (m) of the stepper motor is run through. 6098 11/0263 5. The method according to claim 1, characterized in that that to compensate for the damping the corresponding field windings after switching on or switching a winding are fed with excessive current. 6. The method according to /-.nspruch 1, characterized in that to compensate for the damping of the beginning (to) of each step sequence with a S ~ stop frequency f s2
(fs: r> ■ ) is controlled. 7. The method according to /-.nspruch 1, characterized in that that to compensate for the damping during braking d
is decreased. f cO Braking process set the switching frequency to fs3 ^ ——- 60 9 611/0263