The key component in a motion system is the motor because it determines the design of the associated motion controller as well as the motor drive. From a power-management viewpoint, the important motion-system design considerations are providing the appropriate control signals as well as the required drive power for the specific motor. Each motion controller is unique for a specific motor. Fig. 19-1 shows the typical motion system that includes a motion controller and a motor drive.
19-1.Typical motion system.
An electric motor is an electrical machine that converts electrical energy into mechanical energy. In normal motoring mode, most electric motors operate through the interaction between an electric motor’s magnetic field and winding currents to generate force within the motor.
Electric motors are used to produce linear or rotary force (torque). That is, the motion controller controls the motor’s rotary speed or its linear position. You can convert rotating motion to linear motion using mechanical components or use a linear motor to provide linear motion by itself.
19-2. Maxim’s MAX14871 dc motor driver provides a low-power and simple solution for driving and controlling brushed motors with voltages between 4.5V and 36V.
The motor’s moving part is the rotor that turns the shaft to deliver mechanical power. The stator is the stationary part of the motor’s electromagnetic circuit and usually consists of either windings or permanent magnets. Windings are wires that are laid in coils, usually wrapped around a laminated soft iron magnetic core to form magnetic poles when energized by current.
In a brushed motor, a commutator switches the input of most dc machines and certain ac machines consisting of slip-ring segments insulated from each other and from the electric motor’s shaft. The motor’s armature current is supplied through the stationary brushes in contact with the revolving commutator, which causes a current reversal and applies power to the machine in an optimal manner as the rotor rotates from pole to pole. In absence of such current reversal, the motor would brake to a stop.
The MAX14871 dc motor driver provides a low-power and simple solution for driving and controlling brushed motors with voltages between 4.5V and 36V. Very low driver on resistance reduces power during dissipation (Fig. 19-2).
The MAX14871 features a charge- pump-less design for reduced external components and low supply current. Integrated current regulation allows user-defined peak startup motor currents and requires minimal external components.
The MAX14871 includes three modes of current regulation: fast decay, slow decay, and 25% current ripple modes. Current regulation based on 25% ripple simplifies the design and enables regulation independent of motor characteristics. A separate voltage sense input (SNS) reduces current-sensing errors due to parasitic trace resistance.
The term “electronic commutator” is usually associated with self-commutated brushless dc motor and switched reluctance motor applications. Some problems with the brushed dc motor are eliminated in the brushless dc (BLDC) design. In this motor, the mechanical “rotating switch” or commutator is replaced by an external electronic switch synchronized to the rotor’s position. BLDC motors are typically 85% to 90% efficient or more.
BLDC motors are commonly used where precise speed control is necessary. They have several advantages over conventional motors:
• Without a commutator to wear out, the life of a BLDC motor can be significantly longer compared to a dc motor using brushes and a commutator. Commutation also tends to generate electrical and RF noise. Without a commutator or brushes, a BLDC motor may be used in electrically sensitive devices like audio equipment or computers.
• Hall effect sensors provide the commutation and can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications.
• They are also acoustically quiet motors, which is an advantage if being used in equipment affected by vibrations.
• The BLDC motion controller must provide the proper electronic commutation interface.
19-3. ON Semiconductor MC33035 brushless dc motor controller.
ON Semiconductor’s MC33035 is one of a series of high performance monolithic DC brushless motor controllers(Fig. 19-3). It contains all of the functions required to implement a full−featured, open loop, three or four phase motor control system. In addition, the controller can be made to operate DC brush motors. Constructed with Bipolar Analog technology, it offers a high degree of performance and ruggedness in hostile industrial environments. The MC33035 contains a rotor position decoder for proper commutation sequencing, a temperature compensated reference capable of supplying a sensor power, a frequency programmable sawtooth oscillator, a fully accessible error amplifier, a pulse width modulator comparator, three open collector top drive outputs, and three high current totem pole bottom driver outputs ideally suited for driving power MOSFETs.
Included in the MC33035 are protective features consisting of undervoltage lockout, cycle−by−cycle current limiting with a selectable time delayed latched shutdown mode, internal thermal shutdown, and a unique fault output that can easily be interfaced to a microprocessor controller.
Typical motor control functions include open loop speed control, forward or reverse rotation, run enable, and dynamic braking. In addition, the MC33035 has a 60°/120° select pin that configures the rotor position decoder for either 60° or 120° sensor electrical phasing inputs.
Switched Reluctance Motor (SRM)
19-4. Control system for an SRM senses angular position, which is sent to the controller. Deriving the position in the time domain allows computing the angular speed of the rotor. The controller compares the actual speed with the reference value and calculates the error signal for the hysteresis comparator. Each phase is supplied within a certain rotor position range in order to maximize the developed torque. The hysteresis controllers send the gate signals to the power switches of the converter.
The SRM has no brushes or permanent magnets, and the rotor has no electric currents (Fig. 19-4). Instead, torque comes from a slight misalignment of poles on the rotor with poles on the stator. The rotor aligns itself with the magnetic field of the stator, while the stator field stator windings are sequentially energized to rotate the stator field.
The magnetic flux created by the field windings follows the path of least magnetic reluctance, meaning the flux will flow through poles of the rotor that are closest to the energized poles of the stator, thereby magnetizing those poles of the rotor and creating torque. As the rotor turns, different windings will be energized, keeping the rotor turning. The SRM motion controller must provide the appropriate signals.
An induction motor is an asynchronous ac motor where power is transferred to the rotor by electromagnetic induction, much like transformer action. An induction motor resembles a rotating transformer, because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Polyphase induction motors are widely used in industry. Fig. 19-4 shows a microcontroller-based induction motor drive.
Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars determines the speed-torque characteristics. At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to be in the outer parts of the rotor cage. As the motor accelerates, the slip frequency becomes lower, and more current is in the interior of the winding. By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit. However, the majority of such motors have uniform bars.
A servomotor is a motor, very often sold as a complete module, used within a position-control or speed-control feedback control system mainly to control valves, such as motor-operated control valves (Fig. 19-5). Servomotors are used in applications such as machine tools, pen plotters, and other process systems. Motors intended for use in a servomechanism must have well-documented characteristics for speed, torque, and power. The speed vs. torque curve is quite important and is high ratio for a servo motor. Dynamic response characteristics such as winding inductance and rotor inertia are also important; these factors limit the overall performance of the servomechanism loop. Large, powerful, but slow-responding servo loops may use conventional ac or dc motors and drive systems with position or speed feedback on the motor. As dynamic response requirements increase, more specialized motor designs such as coreless motors are used. AC motors’ superior power density and acceleration characteristics compared to that of dc motors tends to favor PM synchronous, BLDC, induction, and SRM drive applications.
19-5. IRMCF143S from International Rectifier is a high-performance flash-based motion-control IC designed primarily for position servo applications based on an incremental encoder.
A servo system differs from some stepper motor applications in that the position feedback is continuous while the motor is running; a stepper system relies on the motor not to “miss steps” for short-term accuracy, although a stepper system may include a “home” switch or other element to provide long-term stability of control. For instance, when a typical dot matrix computer printer starts up, its controller makes the print-head stepper motor drive to its left-hand limit, where a position sensor defines home position and stops stepping. As long as power is on, a bidirectional counter in the printer’s microprocessor keeps track of print-head position.
Stepper motors are a type of motor frequently used when precise rotations are required. In a stepper motor, an internal rotor containing permanent magnets or a magnetically soft rotor with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a dc electric motor and a rotary solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the stepper motor may not rotate continuously; instead, it “steps”—starts and then quickly stops again—from one position to the next as field windings are energized and de-energized in sequence. Depending on the sequence, the rotor may turn forward or backward, and it may change direction, stop, speed up, or slow down arbitrarily at any time.
Simple stepper motor drivers entirely energize or entirely de-energize the field windings, leading the rotor to “cog” to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position between the cog points and thereby rotate extremely smoothly. This mode of operation is often called “microstepping” (Fig. 19-6). Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.
19-6. Texas Instruments’ DRV8811 is a motor microstepping motor driver with two H-bridge drivers, as well as microstepping indexer logic to control a stepper motor.
A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a piezoelectric material when an electric field is applied. Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or ultrasonic vibrations in order to produce a linear or rotary motion. In one mechanism, the elongation in a single plane is used to make a series stretches and position holds, similar to the way a caterpillar moves.
19-7. Texas Instruments’ DRV8811 is a motor microstepping motor driver with two H-bridge drivers, as well as microstepping indexer logic to control a stepper motor.