Electric drives with stepper motors
Relate to the class of discrete electric drive, make metered movements with fixation of the position at the end of the movement. They are well combined with digital control machines and software devices. Widely used in CNC machines, for robots and manipulators.
Stepper motor (SHD) - synchronous motor, however, the magnetic field rotates in the air gap discretely (in steps) due to impulse excitation of the windings using an electronic switch. Rotor, as a rule, a permanent magnet (bipolar) is called active. In Fig. 5.65 is a diagram of a stepper motor with an active rotor.
When a voltage pulse is applied to the winding 1Н-1К. the rotor is vertical when the winding 2H-2K is feeding the horizontal position, providing a step equal to 90 °. The position will be stable, because if it deviates from it, the synchronizing rotational moment acts on the rotor ', where in is the angle between the axes magnetic fields of the stator and the rotor. If voltage pulses are applied to both windings at the same time, the axis of the stator magnetic field will be located at 45 °.
The rotor will rotate by 45 ° so that its maximum field crosses. If the voltage is removed from the winding 1Н-1 К, the rotor will become horizontal (the next step), then, changing the polarity of the voltage, the axis of the magnetic field will move by another 45 °, etc. (this circuit is called asymmetric).
The angular movement of the stepping motor is determined
where - the number of pairs of poles of the rotor; n is the number of cycles (cycles) in a cycle equal to the number of phases of the SD for symmetric commutation and twice the number of phases for asymmetric switching.
Stepwise motion of the rotor is carried out by applying pulses with switching windings (one switching step - one step of the rotor). The total rotation angle of the SD is proportional to the number of pulses, and its speed to the pulse frequency, the amplitude and shape of the pulses can be different. To reverse the SD, it is necessary to turn on the reverse polarity of the winding, which is currently off, then the rotor will make a step in the opposite direction.
The main mode of operation of the SD is dynamic. The SD enters into synchronism from the state of rest and self-braking. Therefore, start, braking, reverse and switching from one frequency of the control pulses to the other are provided in the SD. Start of the SD is effected by a stepwise or gradual increase in the frequency of the input signal to the operating one, braking by decreasing it to zero, reversing by changing the sequence of commutation of the windings of the SD.
According to the design, SDs can be single-phase, two-phase, multiphase, with active or passive rotor. The active rotor is made in the form of permanent magnets or with an excitation winding, as in SD (magnetoelectric stepper motors). Such SDs have a large rotor pitch of 90 to 15 °. To reduce the step, increase the number of phases and switching cycles, and also use a two-motor or two-rotor design.
The maximum frequency of the control pulses, at which it is possible to start the SD from a stationary state without falling out of synchronism (skip steps) is called the frequency of pick-up. The higher the electromagnetic and mechanical inertia of the SD and the greater the load torque, the lower the frequency of acceleration.
The maximum speed of an SD with an active rotor is 208-314 r/s, the pick-up frequency is from 70 to 500 Hz, the nominal torque is from 1010 6 to 1010 'N m. For high pickup frequencies, a SD with a passive rotor is used, which are divided into reactive and inductor. The rotor is made of ferromagnetic material, has no windings (passive). On the stator, the teeth (clearly pronounced poles) with windings, on the rotor, the teeth without windings.
If , with each switching of the stator windings the rotor makes a turn (step) a equal to
By decreasing the difference , you can reduce the pitch of the rotor. The SD with the passive rotor (Ш, ШДР, ШР, РШД) have a step from 1.5 to 9 °, the moment>
Consider the control schemes of a discrete electric drive.
A certain sequence of voltage pulses is applied to the stator windings of the SD. The average speed of the SD depends on the switching frequency of the windings :
which is issued by an electronic switch and varies widely. The switch is a frequency converter (IF), and a discrete electric drive is a system with frequency control of the SD.
In Fig. 5.66 is a block diagram of the electric drive with SD, where HEAD is the electronic stepping block; БПРТ - the block of smooth acceleration and braking; PSU - power supply; CHIRN - frequency-pulse voltage regulator; FI - impulse driver; РИ - the distributor of impulses; PU - intermediate amplifier; K - commutator (inverter); The UOS is a feedback amplifier; DP - position and speed sensor; CR - digital controller.
The control signal in the form of voltage pulses goes to the FI input, which generates pulses by duration and amplitude, RI - converts the generated pulses into a four-phase system of unipolar voltage pulses corresponding to the number of phases of the windings engine.
The pulses amplified with the help of a pulse are fed to the switchboard to supply the windings of the SD. Usually the switch is powered by a constant current source. In Fig. 5.67 is a diagram of the thyristor switch.
The thyristors VSI-VS4 provide pair switching of the windings of the SD, two windings of 4, (four-phase circuit) are included at each moment; VSI and KS3; VS2 and VS4 form two trigger circuits in which switching of thyristors is performed by oscillating circuits
Suppose, in the initial position, the thyristor KS1 is open and current flows through the OU winding, and the thyristor KS3 is closed. If you send a control pulse to KS3, it will open, and a current will flow through the OU5 winding. At the same time, the capacitor recharge begins and the thyristor VS closes, as the potential of the cathode becomes more positive than the anode potential. Thyristors VS and KS3 work alternately in the trigger scheme.
Vibrational contour and diodes VD 1, VD2 switch thyristors in trigger circuits.
Similarly, the trigger on thyristors VS2 and KS4 works. To remove the overvoltage, a chain is used, R-VD3.
The closed circuit (Figure 5.66) with negative current feedback automatically regulates the current in the windings, the signal is removed from the resistor . The difference in the signals and the setting U, forms a control signal that is fed to the input of the feedback amplifier.< center>
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