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Digital DC Motor Speed Controller

Various techniques can be used to control the speed of a dc motor, such as using the phase-locked-loop principles, digital inputs, or analog inputs. If desired, the speed of the motor may also be monitored with LED or LCD displays. The project digital DC motor speed controller illustrates the use of digital inputs to control the speed of a dc motor. To process the digital inputs, a D/A converter will be used, while a combination of a speed sensor and F/V converter will be used to sense and convert the speed into an appropriate voltage.

Working of the system

Figure 1-1 shows the block diagram of a digitally controlled dc motor. The output of the D/ A converter is proportional to the binary equivalent of its digital inputs. The differential amplifier compares the D/A converter output with the output voltage of the F/V converter. The resulting difference voltage is an input to the power amplifier/driver stage. The output of the power amplifier/driver then drives the dc motor. The speed sensor converts the motor’s speed into a pulse waveform, which is in turn converted into a proportional voltage by the F/V converter since the output of the F/V converter is processed using a negative feedback formed with the differential amplifier, the motor is kept at a constant speed corresponding to the setting of the digital inputs. In fact, the key to the operation of the circuit is that the differential amplifier maintains a specific difference between two input voltages so that motor speed is constant at the selected digital input setting.

Since the output of the D/A converter is directly proportional to the binary equivalent of its digital inputs, the output voltage of the D/A converter will be maximum positive when all the inputs are logic 1. This means that when all inputs are logic 1 the motor will run at a maximum speed.

Now suppose that the motor is initially running at a certain speed and digital inputs have just been set to lower the speed. This action will reduce the output voltage of the D/A converter, which in turn reduces the difference between the two input voltages of the differential amplifier, resulting in a reduced drive for the motor. Therefore, the speed of the motor will be lowered until the output of the F/V converter is such that a specific input difference voltage for the differential

amplifier, which is required to keep the motor running at a constant speed, is reached. The difference voltage necessary to maintain the constant motor speed is a function of the physical dimensions and electrical characteristics of the motor. These include torque, speed, inertia, and current and voltage ratings of the motor. Thus the constant difference voltage and, in turn, a constant motor speed is maintained through the use of negative feedback.

The digital inputs may be calibrated in terms of revolutions per minute (rpm). In addition, the output of the speed sensor may be applied to the frequency meter/indicator to monitor the motor’s speed.

Circuit Description

Figure l-2 shows the schematic diagram of a digitally controlled dc motor. As shown in the figure, the following ICs are used: SN74LS241 octal tri-state buffer, MCl408 8-bit D/A converter, MCl403 2.5-V voltage reference, LF353 dual op-amp, 9400 converter, and the Hall-effect transducer. The desired digital inputs are selected by the use of a switch assembly. The eight LEDs indicate the state of the digital inputs applied to the D/A converter. When a switch is open, the corresponding LED lights up. The 74LS24I octal tristate buffer is used here because it isolates the LEDs from switches and also provides a current drive for the LEDs.

 A number of materials exhibit the Hall effect in that when a current-carrying semiconductor strip (usually silicon) is placed in the transverse (perpendicular) magnetic field, the combination produces an electromotive force (emf) between the opposite edges of the strip. This emf is proportional to the product of the current and field strength. On the other hand, when the magnetic field is zero, or of specific polarity, an emf between the opposite edges of the strip is also zero. Thus the Hall-effect transducer is a magnetically activated electronic switch that can be used for sensing a magnetic field.

Figure l-3 shows the equivalent circuit, operating arrangement, and an electrical switching characteristic with hysteresis of Texas Instruments TL170 bipolar Hall-effect switch. The TL170 is a three-terminal plastic package that consists of a silicon sensor, signal conditioning and hysteresis function, and an open-collector output stage integrated onto a monolithic chip [see figure 1-3(a)]. The output of the device is compatible with bipolar or MOS logic circuits. Figure 1-3(b) shows the practical setup for the on state. The sensor is on (output voltage ≤ 0.04 V) when the magnetic field (BON) associated with the permanent-magnet North Pole is perpendicular to the surface of the sensor and below a certain level, called the operate point or the threshold. On the other hand, the sensor is off when the magnetic field (BOFF) emitted from the south pole of a permanent magnet is perpendicular to the surface of the sensor and above a certain level, called the release point. The TLl70 has a typical operate point of ≤-350 gauss and a release point of ≥350 gauss with a magnetic switching hysteresis (BON – BOFF) of 200 gauss typically. The negative and positive magnetic fields are defined as those fields that are emitted from the north and south poles, respectively, of a permanent magnet. The magnetic switching hysteresis curve of the TLl70 is shown in Figure 1-3(c). The sensor is designed so that its output stage can withstand up to 20 V in the off state and can sink up to 16 mA in the on state.

 

To operate, the TL170 sensor is positioned so that the plain surface of the sensor faces the permanent magnet. In addition, to obtain two samples per revolution and hence help to control the motor speed more accurately, four permanent magnets are used in Figure 1-2. These magnets are glued to the 4-in. diameter disk with alternately south and north poles up, as shown in Figure 1-3(d). The disk is then mounted on the motor’s shaft. When the motor is running, the TLl70 is turned on due to the magnetic field strength of the North Pole and turned off due to the magnetic field strength of the South Pole. Therefore, because of four permanent magnets the sensor will generate two cycles per revolution. The distance between the magnets and the sensor, however, depends on the strength of the magnets. For the TL170 used in Figure 1-2, a magnetic field strength magnitude of ≥350 gauss is necessary. When the motor is running, the distance between the disk and sensor can be adjusted so that the output of the sensor is a pulse waveform. Remember that the output amplitude of the sensor depends on the supply voltage and is independent of the rpm of the motor.

Now let us reconsider the circuit of Figure 1-2. The D/A and F/V converters in this figure should be adjusted initially as follows:

1. With all the inputs high (logic I), adjust R17 so that the output voltage of the converter is 3.0 V.

2. Disconnect pin 11 of the F/V converter from the junction of R27 and R26. First, use the zero-adjust circuit connected to pin 2 to reduce the output voltage to zero. Second, apply 160-Hz, PP symmetrical square wave to pin 11 and adjust R29 until the output voltage is equal to 3.0 V.

Once the adjustments are performed in this order, they should not have to be repeated. Note that the F/V is calibrated for the maximum expected speed of the motor. In Figure 1-2, the motor speed is

4800 rpm = (4800)(2)/60 = 160 Hz.

The motor in Figure 1-2 initially starts running when the input binary code is (00000110)2. Thereafter, the motor speed increases with the digital input until the motor attains a maximum speed at (00111111)2. After (00111111)2, however, the motor speed does not increase further even though the digital input is increased. In other words, we get 6-bit resolution instead of 8-bit. To obtain 8-bit resolution, an appropriate DAC with better resolution, a motor having favorable electromechanical specifications and a differential amplifier with proper gain must be selected. The principles illustrated in the digital dc motor speed control of Figure 1-2 are used in the cruise control of automobiles.

PARTS LISTS

Resistors (all ¼-watt, ± 5% Carbon)

R1 – R8 = 4.7 kΩ

R9 – R16 = 220 Ω

R17 = 1kΩ Potentiometer

R18, R19, R25 = 1kΩ

R20 = 1.5 kΩ

R21, R23, R26, R27, R28, R31 = 100 kΩ

R22 = 2.5 MΩ at 2.2 MΩ potentiometer

R24 = 2.5 MΩ at 2.2 MΩ potentiometer

R29 = 1 MΩ potentiometer

R30 = 2.2 kΩ

R32 = 20 kΩ

Capacitors

C1 = 15 pF

C2 = 0.1 µF

C3 = 75 pF

C4 = 0.001 µF

C5 = 1 µF

Semiconductors

IC1 = SN74LS241 tristate buffer

IC2 = MC1408 8-bit D/A convertor

IC3 = MC1403 2.5-V voltage reference

IC4 = LF353 dual op-amp

IC5 = Teledyne 9400 F/V converter

IC6 = TL170 bipolar Hall-effect switch

Q1 = 2N6594 [complementary power transistors (Ic = 12A, VCEO = 40V)]

Q2 = 2N6569 [complementary power transistors (Ic = 12A, VCEO = 40V)]

L1, L8 = light-emitting diodes

Miscellaneous

DC- Motor = TRW 405A100-1

SW1 – SW2 = Miniature 8-slide or rocket DIP switch

Heat sink for power transistor

Four small magnets with field strength magnitude ≥350 gauss

Nonmagnetic disk 4 in. in diameter

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