The function of a DC servo motor speed control unit is to convert speed command signals into armature voltage values to achieve speed regulation. Modern DC motor speed control units often employ thyristor (silicon controlled rectifier, SCR) speed control systems and transistor pulse width modulation (PWM) speed control systems.
The concept of speed regulation has two aspects:
(1) Change the motor speed: When the command speed changes, the motor speed changes accordingly, and it is desirable to reach the new command speed value by accelerating or decelerating as quickly as possible;
(2) When the command speed does not change, the speed of the motor remains stable.
To adjust the speed and direction of the motor, the magnitude and direction of the DC voltage need to be controlled. How can this be done?
The function of the DC servo motor speed control unit is to convert the speed command signal into the armature voltage value to achieve the purpose of speed regulation.
Common speed control methods used in DC motor speed control units include: thyristor (silicon controlled rectifier) speed control system and transistor pulse width modulation (PWM) speed control system.
1. Thyristor speed control system
With the AC power supply voltage constant, changing the control voltage Un* alters the armature voltage Ud of the DC motor via the control circuit and thyristor main circuit, thus obtaining the motor speed required by the control voltage Un*. The actual motor voltage Un is used as feedback and compared with Un* to form a speed loop, thereby improving the mechanical characteristics of the motor during operation.
The main circuit of the thyristor speed control system uses high-power thyristors. The function of high-power thyristors is:
(1) Rectification. Converting AC power from the grid into DC power; amplifying the control power of the regulating circuit to obtain higher voltage and larger current to drive the motor.
(2) Inversion. In a reversible control circuit, when the motor brakes, the inertia of the motor is converted into electrical energy and fed back to the AC power grid, thus realizing inversion.
To control a thyristor, a trigger pulse generator is required to produce a suitable trigger pulse. This pulse must be synchronized with the power supply frequency and phase to ensure correct triggering of the thyristor.
The main circuit is a three-phase fully controlled bridge anti-parallel reversible circuit composed of high-power thyristors, divided into two parts (I and II). Each part is connected in a three-phase bridge manner, with two sets of anti-parallel connections to achieve forward and reverse rotation respectively.
Each thyristor is simultaneously turned on, forming a circuit. To ensure that the two series-connected thyristors can be turned on simultaneously after the circuit is closed, or turned on again after the current is cut off, trigger pulses must be sent simultaneously to one thyristor in the common anode group and one thyristor in the common cathode group.
The thyristor speed control system uses high-power thyristors, which serve two purposes: first, as rectification, converting AC power from the grid into DC power; second, in the reversible control circuit, converting the motor's inertia into electrical energy during braking and feeding it back to the AC grid, thus achieving inversion. To control the thyristors, a trigger pulse generator is necessary to produce appropriate trigger pulses. There are many types of thyristor rectifier circuits; the most commonly used in CNC machine tools is the three-phase bridge anti-parallel reversible circuit.
The diagram shows a three-phase bridge anti-parallel reversible circuit. It consists of 12 high-power thyristors, divided into two groups: S11-S16 in one group and S21-S26 in the other. Each group is connected in a three-phase bridge configuration, with the two groups connected in anti-parallel to achieve forward and reverse rotation respectively. Anti-parallel connection means that the two converter bridges are connected in parallel with opposite polarities and powered by a single AC power supply. Each group of thyristors has two operating states: rectification and inversion. When one group is in rectification mode, the other group is in inversion mode. The inverter group operates when the motor slows down.
The voltage waveform of the three-phase fully controlled bridge circuit is shown in the figure. The thyristor firing angle α marked in the figure is π/3. The thyristors are turned on sequentially at intervals of π/3, and the motor rotates once every 6 pulses. Because the thyristors are triggered at a relatively fast rate, the current flowing through the motor is almost continuous.
The working process is as follows: When ωt = π/6 + α, S11 is turned on, and S16 has already been turned on before this. Therefore, when the voltage waveform of phase A is in the range of π/6 + α < ωt < π/6 + α + π/3, thyristors S11 and S16 are turned on, and the motor terminals are connected to phases A and B, so Ud = UAB. When ωt = α + π/3 + π/6, thyristor S12 is turned on, and current flows through S12, while S16 is turned off due to reverse bias (natural or grid commutation). At this time, S11 and S12 are turned on, and the voltage across the motor is Ud = UAC. In this way, another thyristor is turned on every π/3, and then the above process is repeated.
As can be seen from the waveform diagram, by changing the value of the firing angle α, the input value of the motor voltage can be changed, thereby adjusting the current value of the DC motor armature and achieving the purpose of adjusting the speed of the DC motor.
In the diagram, RW1 is the speed positioner U+n, which is the speed deviation voltage, Un is the speed feedback voltage, ΔUn is the feedback deviation voltage, A is the proportional amplifier, Uct is the trigger control voltage, and GT is the thyristor trigger control device.
The system's operation and automatic speed adjustment process are as follows:
When the system starts under a relatively small given speed voltage U+n, the motor does not rotate at the beginning, so the speed feedback voltage Un=0, and the feedback deviation voltage ΔUn=U+n. After passing through the amplifier, a larger output Uct is generated. The trigger angle α of the trigger output will decrease from 90° at the initial state, and the rectifier output voltage will also rise from Ud=0 to a larger value. Under this voltage (when the current does not exceed the allowable value), the motor starts running. As the speed increases, the feedback voltage Un increases, the speed deviation voltage ΔUn decreases, Uct decreases accordingly, α increases, the rectifier output voltage Ud also decreases, and the motor slip also decreases until the speed n approaches the given speed, that is, the feedback voltage Un approaches, and the motor runs smoothly. As mentioned earlier, the motor speed can only approach the given speed, and the magnitude of the deviation is closely related to the amplification factor. However, this system is inherently biased in principle, hence it is called a speed control system with deviation.
Thyristor-powered dual closed-loop DC speed control system
The aforementioned single-loop speed control system with negative feedback of rotational speed is actually unsuitable for CNC machine tool feed systems. For high-performance speed control systems on CNC machine tools, which require rapid start-up and braking, and minimal dynamic speed drop, a dual-loop speed and current system is typically used.
The dual closed-loop speed control system based on both speed and current is shown in the figure. To ensure that the speed and current feedback functions independently, the system uses two regulators to regulate the speed and current respectively, and these two regulators are cascaded together.
2. Transistor DC Pulse Width Modulation (PWM) Speed Control System
(1) Principle: The DC voltage is converted into a square wave voltage of a certain frequency by using the switching action of a high-power transistor and applied to the armature of the DC motor; the average voltage of the armature is changed by adjusting the control square wave pulse width, thereby adjusting the speed of the motor.
Average value of DC motor voltage
Where T is the pulse period and Ton is the conduction time.
Features: Simple control circuit, no need for additional shutdown circuit, good switching characteristics. Widely used in medium and low power DC servo systems.
(2) Composition of PWM system
USr — DC voltage converted from speed command;
U△—Triangular wave;
USC – Output of the pulse width modulator (USr + U△);
Ub—The pulse voltage output by the modulator, which is pulse-distributed and converted by the base drive.
The control loop consists of a speed regulator, a current regulator, a fixed-frequency oscillator and a triangular wave generator, a pulse width modulator, and a base drive circuit.
Difference: Compared to thyristor speed control systems, the speed regulator and current regulator operate on the same principle. The difference lies in the pulse width modulator and power amplifier.
Pulse Width Modulator of PWM System
Function: Converts voltage into rectangular pulses that can be adjusted by control signals, providing a pulse width voltage at the base of the power transistor with a width adjustable by speed command signals.
Composition: Modulation signal generator (two types: triangular wave and sawtooth wave) and comparator amplifier.
Transistor speed control system main circuit
The switching power amplifier is the main circuit of the pulse width modulation speed unit. It has two structural forms: H-type (also known as bridge type) and T-type. Each type of circuit has unipolar and bipolar operating modes, and the different operating modes can be used to form reversible and irreversible switching amplifier circuits.
The diagram illustrates the working principle of a widely used H-type switching circuit, a bridge circuit composed of four diodes and four power transistors. The DC power supply +Ed is powered by three sets of full-wave rectified power supplies. The pulse waves u1, u2, u3, and u4 output by the pulse width modulator are converted into pulse signals U1, U2, U3, and U4 with the same phase and polarity as the pulses themselves via an opto-isolator, and applied to the bases of the switching power transistors VT1 to VT4. When the motor is operating normally, during the time interval 0 ≤ t ≤ t1, U2 and U3 are at high levels, and power transistors VT2 and VT3 are turned on. At this time, the power supply +Ed is applied to both ends of the armature, supplying power to the motor. The current direction is from the power supply +Ed through VT3 → motor → VT2 → back to the power supply. During the time interval t1 ≤ t ≤ t2, U1 and U3 are both at low levels, VT1 and VT3 are cut off, and +Ed is turned off. Meanwhile, U2 remains at a high level. Due to the armature inductance, current continues to flow through VT2 and the freewheeling diode VD4. When t2 ≤ t < t3, U2 and U3 are both positive, and +Ed is applied to the motor terminals through VT2 and VT3, allowing current to continue flowing. When t3 ≤ t < T, U2 and U4 are both negative, and the power supply is cut off. U3 is positive, so the armature current continues flowing through VT3 and VD1, repeating this cycle. The voltage UAB obtained in the main circuit is a pulse voltage that varies between +Ed and 0.
The circuit diagrams for bipolar and unipolar circuits are the same; the only difference is the driving signals for the two transistors on the right.
Transistor DC Pulse Width Modulation (PWM) Speed Control System
(1) Working principle of DC PWM servo drive device
A PWM drive device uses the switching characteristics of a high-power transistor to modulate a fixed-voltage DC power supply, switching it on and off at a fixed frequency. The on and off time within a cycle can be changed as needed. By changing the "duty cycle" of the voltage on the armature of the DC servo motor, the average voltage is changed, thereby controlling the speed of the motor.
The schematic diagram of PWM control is shown in the figure. The controllable switch S is repeatedly turned on and off at certain time intervals. When S is on, the power supply U is applied to the two ends of the motor through the switch S, and the power supply provides energy to the motor, which stores energy. When the switch S is off, the energy supply to the motor is interrupted. During the period when the switch S is off, the energy stored in the armature inductance allows the motor current to continue to flow through the freewheeling diode VD.
If the voltage waveform applied across the motor is as shown in the figure, then the average voltage obtained by the motor is:
It is known from the formula that changing ton and toff can change the rotational speed, but this requires a corresponding device to achieve. The figure shown is a block diagram of a PWM drive device system.
As shown in the diagram, the control structure of a PWM drive device can be divided into two main parts: a power conversion circuit that transfers energy from the main power supply to the motor, and a control circuit. The power conversion circuit can be an H-type or T-type power amplifier circuit; the control circuit typically consists of basic circuits such as a constant frequency waveform generator, a pulse width modulation circuit, a base drive circuit, and a protection circuit.
When the triangular wave voltage UΔ and the DC voltage Uk are fed into the amplifier, if the triangular wave is higher than the control voltage, the output is "empty"; otherwise, the output is "duty". Changing the control voltage Uk can change the duty cycle. The output waveform is shown in the figure.
The pulse distribution circuit performs appropriate logic transformation on the V/W conversion signal according to the operating mode of the power conversion circuit, and distributes it to the base drive circuit to meet the voltage requirements of the on and off timing pulses when the power conversion circuit is working.
3. Fully digital DC speed control system
In a fully digital DC speed control system, only the input and output signals of the power conversion component and the execution component are analog signals, while the rest of the signals are digital signals, which are implemented by the computer through algorithms.
Computers have extremely high computing speeds, capable of calculating the input and output values of the current and speed loops within milliseconds, generating control square wave data to control the motor's speed and torque. A key characteristic of fully digital speed control is its discretization; that is, control data is provided only once per sampling period.
Within one sampling period, the computer needs to complete the calculation and output of control data for the current loop and speed loop once, and control the speed and torque of the motor once.
