Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats. Brushed DC (BDC) motors are inexpensive, easy to drive, and readily manufactured in various sizes and shapes. This application note will discuss the working principle of BDC motors, methods for driving BDC motors, and methods for interfacing drive circuits with PIC microcontrollers.
Detailed Explanation of the Working Principle of Brushed DC Motors
Figure 1 shows the structure of a simple BDC motor . All BDC motors share the same basic components: stator, brushes, and commutator. Each component will be described in more detail later.
Figure 1 shows a simple dual-pole brushed DC motor.
stator
The stator generates a fixed magnetic field around the rotor. This magnetic field can be generated by permanent magnets or electromagnetic windings. The types of BDC motors are distinguished by the structure of the stator or the way the electromagnetic windings are connected to the power supply (for information on the different types of BDC motors, please refer to the types of stepper motors).
Rotor
The rotor (also called the armature) consists of one or more windings. When these windings are energized, they generate a magnetic field. The poles of the rotor's magnetic field attract the opposite poles of the stator's magnetic field, causing the stator to rotate. During the motor's rotation, the windings are continuously energized in different sequences, so the magnetic poles generated by the rotor never overlap with those generated by the stator. This transformation of the magnetic field in the rotor windings is called commutation.
Brushes and commutator
Unlike other motor types (i.e., brushless DC motors and AC induction motors), BDC motors do not require a controller to switch the direction of current in the electrode windings; instead, commutation of the BDC motor windings is achieved mechanically. A segmented copper bushing, called a commutator, is mounted on the shaft of the BDC motor. As the motor rotates, carbon brushes slide along the commutator, contacting different segments of the commutator. These segments are connected to different rotor windings, thus generating a dynamic magnetic field inside the motor when energized through the brushes. It is important to note that the brushes and commutator are the most easily worn parts of a BDC motor due to the relative sliding between them.
Types of stepper motors
As mentioned earlier, the various types of BDC motors are distinguished by the method of generating the fixed magnetic field in the stator. This section will discuss the different types of BDC motors, as well as the advantages and disadvantages of each type.
permanent magnet
Permanent magnet brushed DC (PMDC) motors are the most common type of BDC motor in the world. These motors use permanent magnets to generate the stator magnetic field. PMDC motors are commonly used in applications including fractional horsepower motors because permanent magnets are more cost-effective than wound stators. A disadvantage of PMDC motors is that the magnetism of the permanent magnets gradually decays over time. Some PMDC motors also have windings on the permanent magnets to prevent magnetism loss. The performance curve (voltage versus speed curve) of PMDC motors is very linear. Current and torque are linearly related. Because the stator magnetic field is constant, these motors respond very quickly to voltage changes.
and stimulate
In a shunt-wound brushed DC (SHWDC) motor, the field coil is connected in parallel with the armature. The current in the field coil is independent of the current in the armature. Therefore, this type of motor has excellent speed control capabilities. SHWDC motors are typically used in applications requiring five or more horsepower. SHWDC motors do not suffer from magnetism loss, making them generally more reliable than PMDC motors.
Series
In a series-wound brushed DC (SWDC) motor, the excitation coil is connected in series with the armature. Because the current in both the stator and armature increases with the load, this type of motor is ideal for high-torque applications. A disadvantage of SWDC motors is that they cannot offer the same precise speed control as PMDC and SHWDC motors.
Re-excitation
A compound-wound (CWDC) motor is a combination of a shunt-wound and a series-wound motor. As shown in Figure 5, a CWDC motor can generate both series and shunt magnetic fields. The CWDC motor combines the performance of SWDC and SHWDC motors, offering greater torque than an SHWDC motor and better speed control than a SWDC motor.
Basic drive circuit
Drive circuits are used in applications employing a certain type of controller and requiring speed control. The purpose of the drive circuit is to provide the controller with a method to change the winding current in the BDC motor. The drive circuit discussed in this section allows the controller to pulse-width modulate the supply voltage of the BDC motor. In terms of power consumption, this speed control method is much more efficient than traditional analog control methods in changing the speed of the BDC motor. Traditional analog control requires an additional rheostat connected in series with the motor windings, which reduces efficiency. There are various methods for driving BDC motors. Some applications only require the motor to rotate in one direction. Figures 6 and 7 show circuits for driving a BDC motor in one direction. The former uses a low-side drive, and the latter uses a high-side drive. The advantage of using a low-side drive is that a FET driver is not required. The uses of a FET driver are:
1. Convert the TTL signal driving the MOSFET to the level of the supply voltage.
2. Provide sufficient current to drive the MOSFET (1)
3. Provides level conversion in half-bridge applications.
Note 1: For most PIC microcontroller applications, point 2 usually does not apply because the I/O pins of a PIC microcontroller can provide 20mA of pull-up current.
Note that in each circuit, a diode is connected across the motor terminals to prevent back electromagnetic flux (BEMF) voltage from damaging the MOSFET. BEMF is generated during motor rotation. When the MOSFET is off, the motor windings remain energized, generating a reverse current. D1 must have an appropriate rating to dissipate this current.
Resistors R1 and R2 in Figures 6 and 7 are crucial for the operation of each circuit. R1 protects the microcontroller from damage caused by sudden current surges, while R2 ensures that Q1 is turned off when the input pin is in a tri-state.
Bidirectional control of a BDC motor requires a circuit called an H-bridge. The H-bridge gets its name from its schematic appearance; it allows current to flow in both directions through the motor windings. To understand this, the H-bridge must be divided into two parts, or two half-bridges. As shown in Figure 8, Q1 and Q2 form one half-bridge, while Q3 and Q4 form the other. Each half-bridge controls the switching on and off of one end of the BDC motor, making its potential either the supply voltage or ground potential. For example, when Q1 is on and Q2 is off, the left end of the motor will be at the supply voltage potential. Turning on Q4 and keeping Q3 off will ground the opposite end of the motor. The arrow-marked IFWD indicates the current flow direction in this configuration.
Note that a diode (D1-D4) is connected across each MOSFET. These diodes protect the MOSFET from damage caused by the BEMF current spike when the MOSFET is turned off. These diodes are only needed if the internal diodes of the MOSFET are insufficient to dissipate the BEMF current. Capacitors (C1-C4) are optional. These capacitors are typically no more than 10pF and are used to reduce RF radiation caused by commutator bulging.
Table 1 shows the different driving modes of the H-bridge circuit. In forward and backward modes, one end of the bridge is at ground potential, and the other end is at VSUPPLY. In Figure 8, the arrows IFWD and IRVS depict the circuit paths for the forward and backward operating modes, respectively. In coast mode, the motor winding terminals remain suspended, and the motor coasts by inertia until it stops. Brake mode is used to quickly stop the BDC motor. In brake mode, the motor terminals are grounded. When the motor is rotating, it acts as a generator. Short-circuiting the motor leads is equivalent to the motor having an infinite load, which can quickly stop the motor. The arrow IBRK depicts this.
When designing an H-bridge circuit, a crucial consideration must be taken into account. When the circuit input becomes unpredictable (e.g., during microcontroller startup), all MOSFETs must be biased to the off state. This ensures that the MOSFETs on each half-bridge of the H-bridge will never conduct simultaneously. Simultaneous conduction of MOSFETs on the same half-bridge will cause a short circuit, ultimately damaging the MOSFETs and rendering the circuit inoperable. Pull-down resistors on the input of each MOSFET driver achieve this function (see Figure 8 for the configuration diagram).
Speed control
The speed of a BDC motor is directly proportional to the voltage applied to it. When using numerical control technology, a pulse width modulation (PWM) signal is used to generate the average voltage. The motor windings act as a low-pass filter, so a PWM signal of sufficient frequency will generate a stable current in the motor windings. The relationship between the average voltage, the supply voltage, and the duty cycle is given by the following formula:
Formula 1: VAVERAGE=D×VSUPPLY
Speed and duty cycle are directly proportional. For example, if a rated BDC motor rotates at 15000 RPM at 12V, then when a signal with a 50% duty cycle is applied to the motor, the motor will (ideally) rotate at 7500 RPM. The frequency of the PWM signal is a key consideration. Too low a frequency will result in excessively low motor speed, high noise, and a slow response to changes in duty cycle.
Too high a frequency will reduce system efficiency due to switching losses in the switching devices. A rule of thumb is to modulate the input signal frequency in the range of 4kHz to 20kHz. This range is high enough that motor noise can be attenuated, and switching losses in the MOSFET (or BJT) can be ignored. Generally, it is a good approach to find a satisfactory PWM frequency experimentally for a given motor. How to use a PIC microcontroller to generate a PWM signal to control the speed of a BDC motor? One method is to write specialized assembly or C code to alternately toggle the level of the output pin (1). Another method is to choose a PIC microcontroller with a hardware PWM module. Microchip provides CCP and ECCP modules with this functionality. Many PIC microcontrollers have CCP and ECCP modules. Please refer to the product selection guide for devices with these functional modules.
Note 1: Microchip's Application Note AN847 provides assembly code examples for pulse width modulation of I/O pins using firmware.
The CCP module (an abbreviation for Capture Compare and PWM) can output a 10-bit resolution PWM signal on a single I/O pin. 10-bit resolution means the module can achieve 2^10 (1024) possible duty cycle values within a range of 0% to 100%. The advantage of using this module is its ability to autonomously generate PWM signals on the I/O pin, freeing up the processor to perform other tasks. The CCP module only requires the developer to configure its parameters. Module configuration includes setting the frequency and duty cycle registers. The ECCP module (an abbreviation for Enhanced Capture Compare and PWM) not only provides all the functions of the CCP module but can also drive full-bridge or half-bridge circuits. The ECCP module also features automatic shutdown and programmable dead-time delay.
Note: Microchip's application note AN893 provides detailed instructions on configuring the ECCP module to drive a BDC motor. This application note also includes firmware and drive circuit examples.
Feedback mechanism
While the speed of a BDC motor is generally proportional to its duty cycle, no motor is perfectly ideal. Heat, commutator wear, and load all affect the motor's speed. Introducing some form of feedback mechanism is a good idea in systems requiring precise speed control. Speed control can be achieved in two ways. The first is by using some type of speed sensor. The second is by using the BEMF voltage generated by the motor.
Sensor feedback
Various sensors can be used for speed feedback. The most common are optical encoders and Hall effect sensors. An optical encoder consists of several components. A grooved wheel is mounted on the shaft at the non-drive end of the motor. An infrared LED provides a light source on one side of the wheel, and a phototransistor detects the light on the other side (see Figure 9). Light passing through the grooves in the wheel turns the phototransistor on. As the shaft rotates, the phototransistor turns on and off depending on whether light passes through the grooves. The frequency of the transistor's on/off state characterizes the motor speed. In applications where the motor is displaced, optical encoders are also used to provide feedback on the motor position.
Hall effect sensors are also used to provide speed feedback. Similar to optical encoders, Hall effect sensors require a rotating element connected to the motor and a stationary element. The rotating element is a wheel with one or more magnets mounted on its outer edge. The stationary sensor detects the passing magnets and generates TTL pulses. Figure 10 shows the basic components of a Hall effect sensor.
Anti-electromagnetic flux (BEMF)
Another form of providing fast feedback for a BDC motor is through BEMF voltage measurement. BEMF voltage is proportional to speed. Figure 11 shows the location for measuring the BEMF voltage in a bidirectional drive circuit. A voltage divider is used to reduce the BEMF voltage to the 0-5V range so that it can be read by the analog-to-digital converter. The BEMF voltage is measured between PWM pulses with one end of the motor floating and the other grounded. In this case, the motor acts as a generator and produces a BEMF voltage proportional to speed.
Due to differences in efficiency and materials, the behavior of all BDC motors will vary slightly. Experimentation is the best method to determine the BEMF voltage at a given motor speed. The reflective strip on the motor shaft helps a digital tachometer measure the motor's speed (in RPM). Measuring the BEMF voltage while reading the digital tachometer will reveal the relationship between motor speed and BEMF voltage.
Note: Microchip's application note AN893 provides firmware and circuit examples for reading the BEMF voltage using the PIC16F684.
in conclusion
Brushed DC motors are very easy to use and control, resulting in a shorter design cycle. PIC microcontrollers, especially those with CCP or ECCP modules, are ideal for driving BDC motors.
