Motor Controllers

     

A motor controller is a device that provides or facilitates accurate control of a motor, which is an essential ingredient of any motion control application. A motor controller IC is simply a motor controller in integrated circuit form.

               

There are many types of motors used in various industries today, all of which can now be controlled by one type of motor controller IC or another.  Commonly used types of motors include the DC motor, the  stepper motor, and the servo motor, each of which has many variants of its own.

   

Needless to say, the type of input required and output provided by an IC motor controller depends on the type of motor it controls.  It must be noted, however, that some advanced motor controller IC's can control several different types of motors.  Below are brief descriptions of what motor controller IC's generally offer.

   

DC Motor Controllers

   

There are two major types of DC motor - the common DC motor with brushes, and the brushless DC motor. Both types has a non-moving source of magnetic fields, known as a stator, and a rotating source of magnetic fields, known as the rotor.  The interaction of the magnetic fields from the stator and the rotor is what makes the motor shaft turn.

    

A brushed DC motor, which is operated simply by applying a DC voltage across its terminals, has a 'permanent magnet' stator and an 'electromagnet' rotor. It uses its brushes to deliver commutating current from the motor's external terminals to the moving rotor coils inside. A brushless DC motor, on the other hand, has a 'permanent magnet' rotor and an 'electromagnet' stator. It requires a more complex form of energization to operate - proper sequencing of the delivery of commutating currents to its stator coils.  Brushless DC motors are not subject to the arcing caused by brushes, and therefore have a longer life. 

   

Brushed DC motors are not widely used in precision motion control application because they are difficult to control. Brushless DC motors are more widely used, which is why many motor controller IC's for DC motors cater to the brushless type.

   

A typical brushless DC motor controller IC is equipped with a control circuit and a driver circuit.  The control circuit, which is the 'brain' of the controller IC, generates a control output based on some form of input. For instance, it may receive feedback about the state of the motor, usually in the form of input data based on Hall Effect, which it decodes. The logic circuit then applies a built-in commutation logic that interprets the decoded feedback information and outputs the appropriate commutation commands to the driver circuit. 

  

The driver circuit, which translates the logic circuit commands into motor-useable currents, typically consists of a set of integrated power drivers that supply the correct sequence of currents to the brushless DC motor.  It may also have a built-in pulse width modulation (PWM) circuit that varies the amounts of DC currents delivered to the motor to control its torque and speed.

  

DC motor controller IC's can control DC motors over a wide range of motor supply voltages (up to 50 V) and motor winding currents (several amperes).  They may also provide specialized outputs such as tachometer readings for use in speed control loops.

   

Stepper Motor Controllers

                    

A stepper motor is a motor that converts digital pulses into mechanical shaft rotation. These pulses control the rotation of the shaft in small angular 'steps', hence the name 'stepper motor'.  Stepper motors come in a wide range of angular resolution, i.e., from 90 degrees per step to as low as 0.72 degrees per step. The speed and torque of a stepper motor are determined by the amount of current through its windings. Stepper motors are simple, low-cost, yet highly reliable motors that can operate in almost any environment.

   

A stepper motor rotates in discrete 'steps' because its movement is achieved by aligning certain 'teeth' of the rotor with certain poles of the stator (depending on which windings are energized and which are not) at any given time. As such, there are only specific equilibrium points at which the rotor can 'rest.' Every time a new set of pulses is delivered, the rotor rotates to the next 'equilibrium point', and its angular position with respect to the stator is locked in place until a new set of pulses arrives. 

   

There are three stepping modes in which a stepper motor can be operated, namely, full stepping, half stepping, and micro-stepping.  Every step taken under full stepping mode results in a rotation equal to 100% of the angular displacement specified for a single step.  Half-stepping results in only 50% of this, so it would take twice as many steps under the half-stepping mode as what it would take under the full-stepping mode to cover the same angular displacement.  Micro-stepping further reduces the angular displacement equivalent to a single step of the motor. 

   

A stepper motor controller must be able to handle the generation and conditioning of pulses needed to operate the stepper motor easily even in complex applications. To accomplish this, a typical stepper motor controller IC consists of three basic elements:  1) an indexer, which generates low level signals that correspond to step pulses and direction signals (collectively referred to as 'indexer commands') needed to control the stepper motor; 2) a motor driver circuit, which translate the indexer commands into power that energizes the appropriate windings of the stepper motor; and 3) an interface that would allow it to be controlled more conveniently by a computer, PLC, or microcontroller.

   

Characteristics or features that designers take into consideration when choosing a stepper motor controller IC include the following: 1) the ability to put a stepper motor in continuous 'run' mode at various speed profiles, or in 'step' mode with precision control; 2) directional control; 3) available stepping modes for resolution control; 4) programmability; 5) specialized I/O controls; 6) feedback mechanisms about the state of the stepper motor; 7)  effective and efficient energization of the stepper motor; 8) industry-standard user interfaces; and 9) ease of use. 

                

Servo Motor Controllers

                    

A servo motor is a motor whose angular displacement at any one time is determined by a coded signal, which is usually the width of the pulse applied to its control terminal.  It is operated in a closed loop, i.e., it requires some form of analog feedback (usually provided by a potentiometer) to let it know the current rotor position.  Thus, the repeatability of a servo motor's positioning depends greatly on the stability of the potentiometer and other components used in the feedback circuit. Since a stepper motor operates without feedback, a servo motor is a better choice than a stepper motor if monitoring of the rotor position at any given time is important.

   

Since a servo motor operates on the widths of the control pulses it receives, a servo motor controller must be capable of pulse width modulation (PWM).  The servo motor expects to see a pulse regularly, say, every 20 milliseconds. The pulse width influences the amount of power delivered to the motor and, therefore, its angular displacement as well, i.e., the longer the pulse, the larger the rotation will be.

           

Examples of features offered by servo motor controller IC's in the market include : 1) ability to support multiple motors; 2) velocity and trapezoidal profiling; 3) directional control; 4) programmability; 5) specialized I/O controls; 6) stable feedback mechanisms; 7)  overcurrent and power failure protection; 8) industry-standard user interfaces; and 9) ease of use.

                

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