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Microcontroller speed controller of the collector electric motor. Encyclopedia of radio electronics and electrical engineering

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In many drives, in particular household electrical appliances, collector motors with series excitation are widely used. There are numerous variants of speed controllers for such motors using controlled thyristor rectifiers (see, for example, the book "Thyristors. Technical Reference" / Translated from English by V. A. Labuntsova et al. - M .: Energy, 1971).

The use of microcontrollers (MC) in these devices with the implementation of the main functions of controlling the electric drive at the software level opens up qualitatively new opportunities. In this case, the regulator turns out to be quite universal with the ability to be configured to control various options for electric drives or other loads by changing the program recorded in the memory of the MK.

The article describes a version of such a regulator developed by the authors based on the PIC16F84 MK from Microchip Technology.

The proposed device uses a pulse method of voltage regulation in DC circuits, which is widely used, in particular, in the electric drive of vehicles [1].

The essence of the method lies in the fact that voltage is applied to the motor by pulses with a high repetition rate by means of a non-contact key element. During a pulse of duration t and (Fig. 1), the full voltage of the power source U is applied to the electric motor and the current in the motor circuit increases, and during a pause tn, the voltage is turned off, and the current gradually decreases under the action of the self-induction EMF, closing through the blocking diode circuit. The average value of the voltage Ucp at the terminals of the electric motor, and hence its speed, is regulated by changing the duty cycle K3, which is equal to the ratio of the pulse duration ti to the switching period T=ti + tn: UCP = K3U; K3 = ti /T. (1)

Microcontroller speed controller of the collector motor

To reduce the amplitude of the current ripples and expand the range of regulation, the key element is controlled by a frequency-width control with a simultaneous change in the duration of the switching period according to the relation T = Tmin/4K3(1-K3), (2) and speed of the microcontroller; in this case, Tmin is taken equal to 2,5 ms.

To demonstrate the capabilities of microcontroller control of an electric drive, the following set of functions is implemented in the proposed device:

- speed control by changing the filling factor K3 in the range of 0 ... 100% with a step of 2%. The mechanical characteristic of the electric drive (the dependence of the rotational speed on the torque on the shaft) is soft: with increasing load, the rotational speed decreases, which protects the electric motor and power source from overloads;

- maintaining the set speed with an accuracy of ±5% using the principle of closed-loop control by deviation: the actual value of the speed is compared with the set value, and if there is a deviation, K3 is programmatically changed until the deviation is eliminated;

- changing the direction of rotation of the shaft (reverse) of the electric motor;

- generating a signal to turn on the brake element when the drive stops;

- automatic shutdown of the electric motor according to the signals of the emergency mode sensors (if used), as well as in case of failures in the execution of the program;

- the ability to control two electric motors with a temporary shift of the supply voltage pulses;

- accounting and storage in the non-volatile memory of the MC information about the total operating time of the drive;

- visual indication of the selected control algorithm (with or without speed stabilization) and the direction of rotation, as well as the values of the duty cycle, set and actual speeds.

In specific applications, some of these functions may not be used.

A schematic diagram of the motor control device is shown in fig. 2. Its basis is MK DD1, operating at a clock frequency of 10 MHz. The controls are buttons SB1 ("Forward"), SB2 ("Stop") and SB3 ("Back"), connected to bits RB0 - RB2 of port B MK. In parallel with the SB2 button, if necessary, you can connect the output of the load current sensor, which, if the set current threshold is exceeded, will disconnect the drive from the power source.

(click to enlarge)

A powerful composite transistor KT834V (VT2) was used as a key element. Due to the large current transfer coefficient of the base, it is controlled directly by the voltage from the output RB4 of port B through the current-limiting resistor R5.

The control program provides for the possibility of simultaneous control of a second electric motor by connecting the input of a similar key element to the RB5 output. At the same time, in order to reduce current ripples in the source circuit, voltage pulses for the second motor are formed with a time shift equal to the pulse duration ti, as shown in Fig. 1, a and b.

Powerful field-effect or hybrid power transistors can be used as keys in the device with control circuits connected directly to the MC terminals [2], which allows the regulator to be used in power drives with a power of up to hundreds of kilowatts, for example, in electrified vehicles.

The reverse of the electric motor is carried out by changing the direction of the current in the excitation winding of the electric motor LM1 using the switching contacts of relay K1. Its winding is included in the collector circuit of the transistor VT1, controlled by the voltage from the output of the RB3 MK.

The regulator uses a REN18 relay (passport PX4.564.505) with four switching contacts (two contacts are connected in parallel in each of the K1.1 and K1.2 groups to increase reliability). Switching of contacts occurs when the electric motor is de-energized (K3 = 0), which significantly reduces the requirements for their switching capacity.

Depending on the rated current of the motor, a more powerful switching device may be required to switch the field winding. When controlling a non-reversible electric drive, there is no need to use these elements at all.

The program provides for the formation of a signal at the output of the RB6 MK, which includes a brake element to quickly stop the drive when it is turned off or to limit the speed in the stabilization mode with negative loads on the motor shaft. If there is no such element, the specified signal is simply not used.

The output RB7 receives pulses from the photoelectric speed sensor. It consists of an IR emitting diode VD5, a VD6 photodiode, an amplifier based on a VT3 transistor [3] and a disk mounted on the motor shaft with two diametrically spaced holes about 10 mm in diameter. When the shaft rotates, the IR rays illuminate the photodiode twice in one revolution for a short time, and voltage pulses are formed in the collector circuit of the transistor VT3. Entering the input of RB7, they cause interruptions of the MC from port B. Based on these interruptions, the MC measures the time of each revolution of the motor shaft and converts the measured interval into a rotational speed normalized relative to the nominal percentage. In this case, the rotational speed of 100 min-3000 is taken as 1%.

If the duty factor has reached zero (power off), and the motor continues to rotate with an angular frequency exceeding the specified one, the MC issues a braking command to the actuator through the bit RB6 of port B.

The five-bit port A configured for output is used to dynamically control the seven bits of the HG1 digital indicator. Through the bit RA3, the input C1 of the binary counter DD3 receives information (in the form of the corresponding number of pulses) about the displayed decimal digit, and through the bit RA4 the counter is reset. The DD4 decoder converts the binary code at the output of the counter into a seven-element indicator code.

From the outputs of the RAO-RA2 MK to the address inputs of the decoder DD2, the bit number of the HG1 indicator is supplied in binary code, in which the contents of the counter DD4 should be displayed. The voltages at outputs 0 - 6 of the decoder sequentially activate the corresponding bits of the indicator, providing the display of seven digits, and in the intervals of voltage formation at the unused output of the decoder, the indication is turned off and the displayed digit is loaded into the counter.

When the device is turned on, the MK is automatically reset and the execution of the program recorded in its memory begins. The initial initialization of the MC and the control program is performed: the timer / counter prescaler and the lines of ports A and B for input / output are configured, the necessary initial constants are entered into the variables used, interrupts from the timer / counter and from changes in the input voltage level in the RB7 bit of port B are allowed. After these actions, the program cyclically displays information on the digital indicator HG1 and polls the status of the buttons SB1-SB3.

The electric drive can be controlled according to two algorithms selected by the user.

Stabilization mode is on. The user sets the required speed of the motor shaft, and the MC measures the actual speed several times per second and, depending on the result, corrects the fill factor K3 in such a way as to maintain the set frequency regardless of the supply voltage fluctuations and changes in the moment of resistance on the motor shaft.

To turn on the stabilization mode, it is necessary to simultaneously press the buttons SB2 ("Stop") and SB1 ("Forward") when the drive is stopped, to turn off - SB2 ("Stop") and SB3 ("Back"). The indicator in this mode displays information in the format 5_XXX_YYV, where 5 is a sign that the MC is operating in stabilization mode, XXX is the current duty cycle in percent from 0 to 100% in 2% increments, formed by the MC to maintain the set speed, a YYY - set drive speed as a percentage of the nominal in the range from 0 to 100% in 5% increments.

Stabilization mode disabled. The user sets the required fill factor K3. The speed feedback signal is not used. The indicator displays information in the format XXX_YYY, where XXX is the measured current speed of the motor shaft (measured several times per second), and YYY is the specified duty factor K3 from 0 to 100% in 2% increments.

Using the timer/counter built into the MK, the program counts the time worked by the engine in minutes, periodically saving its value in the non-volatile data memory. The corresponding information is displayed on the indicator after pressing the SB2 button when the drive is stopped. When the minute counter reaches the value 8192 (about 136,5 hours), it is reset to zero.

The control pulses of two power switches are formed by the MK at the outputs RB4, RB5 by interrupts from the timer / counter in the sequence shown in Fig. 1. As a result, at K3 ≤ 0,5, only one of the two motors is connected to the power supply at any given moment, and at K3 > 0,5, the electric motor consumption currents are partially superimposed, which improves the power source operation mode.

The constants necessary for the formation of time intervals according to relations (1), (2) and fig. 1 are loaded into the timer from a table located in the MK program memory. The address in the table is determined by the required fill factor K3.

Codes "firmware" ROM MK are shown in the table.

(click to enlarge)

In case of unexpected behavior of the control program, caused by any reasons, the watchdog timer command resets the MC and stops the drive in an emergency.

Program source code

When programming the MK, the following information must be indicated in the configuration byte: generator type - HS, Watchdog timer and Power-up timer - are enabled. The program is designed for the maximum allowable speed of 3000 min -1 To change this value, you need to set other constants in the procedure for its measurement (see comments in the text of the original program).

In addition, the value of the maximum speed can be changed in steps by varying the number of holes in the tachometer disk. For example, to obtain a maximum frequency of 1500 min -1, four holes must be drilled.

To power the low-voltage part of the regulator, you can use any low-power source that provides a voltage of 5 V at a current of up to 150 mA. MK PIC16F84 without changes in the control program can be replaced by a cheaper PIC16C84, also designed to operate at a clock frequency of 10 MHz. As a digital indicator HG1, you can use any other with a similar control. Rectifier bridge diodes VD3, transistor VT2 and relay contacts K1 determine the maximum drive power that can be controlled by the regulator.

The regulator has been tested in operation with a 400 W universal collector motor. In this case, the VT2 transistor was installed on a heat sink with a total cooling surface area of about 100 cm2.

A regulator correctly assembled from serviceable components with an accurately programmed MK does not need to be adjusted.

The described device can be used not only to control the speed of electric drives, but also to maintain the set values of other physical parameters, such as the temperature in a room, incubator, pool, aquarium or other objects. In such a case, instead of a speed sensor, a temperature-frequency converter is connected to the RB7 MK input. The unused bits of port B can be programmed to control other external devices, for example, turning on the ventilation in the room when the air is overheated, lighting and compressor in the aquarium at certain intervals, etc. All this requires making minimal changes to the control program.

Literature

Birznieks LV Impulse direct current converters. - M.: Energy, 1974.
Energy electronics. Reference guide: Per. from German / Ed. V. A. Labuntsova. - M.: Energoatomizdat, 1987.
Bayanov K. Magnetic tape consumption counter. - Radio, 1994, No. 5, p. 5-7.

Author: S.Koryakov, Yu.Stashinov, Shakhty, Rostov region

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