Saturday, November 16, 2013

Overview of industrial motor control system

Summary : DC motors , brushless DC motors , and an overview of the industrial motor control system this is , and watch for the sub- system and differences of the AC induction motor . In a sweeping analysis of critical subsystems , control detection monitoring and measurement of current , temperature detection , motor speed , position, and behavior , and monitoring of current and voltage of the multi-channel , motor and high precision with encoder data interface I will focus on control .
The electric motor has consumed about 50 percent of the electricity in the world . Because energy costs are rising steadily , be replaced by variable speed drive of microprocessor -based drive motors and constant speed inefficient , are addressed to focus on the industrial field . You can by this new motor control technology , compared to the older drives , to reduce by more than 30% energy consumption . Cost of the motor will increase by variable speed controller of these , by the improvement of motor function and energy saving expected , it can be easily offset the initial cost within a few years .
Motor design general

DC motor , brushless DC, and AC induction motor is a motor design that are commonly used in industrial applications today . Each motor type of these is equipped with its own characteristics in each , but it works based on the same basic principle of electromagnet . In other words, is that you place a perpendicular external magnetic field in the conductor , the conductor , such as a wire coil with current flowing , conductors , and subjected to a normal force to both the external magnetic field and the conductor itself .
Precise driving performance at low cost : DC motorDC motor , is one of the motor type is the first practical use , but it is often used now to be required to drive superior performance and low initial cost . The intended simplest form , the stator ( i.e. , fixed portion of the motor ) is a permanent magnet , the armature winding is given rotor ( i.e. , the rotating portion of the motor ) to the armature winding , the winding it is connected to a mechanical commutator for switching on / off of the current flowing through the . Electromagnetic torque is generated by the field magnetic flux is generated by the magnet , which interacts with the armature current . It is possible to operate the motor by this . You can control the speed of the motor , you can adjust the DC voltage applied to the armature winding .
Depending on the application , to drive the armature winding by using one of just a step down converter full bridge , half bridge or , . I get the desired voltage pulse width modulation to (PWM) switch in the converter of these . You can if you use the IC bridge driver or the high side of the Maxim such as MAX15024/MAX15025, to drive the FET in the half-bridge circuit or full-bridge circuit .
In addition , DC motors are widely used in servo applications speed and accuracy is important . To meet the requirements of speed and accuracy , and closed-loop control using a microprocessor , information about the rotor position is essential . By using the MAX9641 Hall effect sensor of Maxim , it is possible to obtain information about the rotor position .

Block diagram of the industrial motor control standard .

Durability and simplicity : AC induction motor

Because of the durability simple , AC induction motor , is often used in the industrial field . This motor is a transformer intended simplest form , has a primary side voltage which is connected to the AC voltage source , the secondary side is shorted to flow induced secondary current . The name of the motor called " induction " is , is based on the induced secondary current . Windings of the three phases is given the stator , for design of the rotor is simple , it is referred to as a " cage " in general. Here , we have short-circuited at both ends of the bar aluminum and copper by the end ring of cast aluminum . With that said there is no brush and winding of the rotor , the reliability of the motor design has improved significantly .

Rotor and the stator of the induction motor
Induction motor operates at a constant speed when you drive at a voltage of 60Hz. However, when using microprocessor-based system and power electronics , it is possible to change the speed of the motor . Variable-speed drives are configured in the control of microprocessor-based inverter , and signal conditioner , . Inverter is using a half-bridge three switch up and down is controlled complementary . The Maxim , half-bridge drivers, such as MAX15024/MAX15025 to independently control the top and bottom FET offers several .
To efficiently closed-loop control of the induction motor , three -phase current of the motor , rotor position , and must be precisely measured rotor speed . The Maxim , in order to measure accurately in harsh environments these parameters , analog of simultaneous sampling current amplifier of high-side and low-side of a large number , and the Hall effect sensor , - to - digital converter (ADC) is provided .
Microprocessor generates a logic signal of the three-phase bridge for using the data on the current and position . It has faster transient response detach field current vector of the stator magnetic flux loop control common technique called vector control , as is independently controllable .
Brushless DC motor : high reliability and high output power

For commutator and brushes not to (BLDC) Motor Brushless DC, I corner maintenance is less than the DC motor . In addition, the output power of 1 frame per size will be more than the DC motor and induction motor .
The stator of the BLDC motor is very similar to the stator of the induction motor . However, the rotor of the BLDC motor , even able to take a variety of forms , everything is permanent magnet . Air-gap flux is fixed by the magnet , will not be affected by the stator current . In the case of the BLDC motor , you need to detect the position of the rotor in some way . It detects the position of the rotor using a Hall effect device that is embedded in the stator normally . Magnetic poles of the rotor and through the neighborhood of the Hall effect sensors , a signal indicating N or S pole is whether they pass is originated. By using built-in , such as the digital logic and Hall effect sensors of the two , to provide both the output of the position and orientation of the magnet , we are prepared to reduce the system cost by simplifying the design , the Hall effect sensor several .
Sensor , signal conversion , and the importance of the data interfaceSome of the sensor , are also available to provide information feedback in motor control loop . These sensors , has improved the reliability by detecting fault conditions that may damage the motor . Is described in detail and took up the role of the sensor in the motor control in the next section . I also lists notes specific variable reluctance current-sense amplifier , and a Hall effect sensor , for (VR) sensor . Examples of other important matters , high-speed analog - control and monitoring of voltage and current of multi-channel using (ADC) digital signal conversion , there is a data encoder and interfaces required for the motor control of high precision .
Monitoring and measurement of current for optimal motor control

Current monitoring

Current is the signal common to perform detection , monitoring, and feedback to the motor control loop . It allows you to easily monitor and high precision current output or input to the system by the current sense amplifier . If you use the current-sense amplifier , the electrical signal itself is measured , there is no need for transducers . You can by the current sense amplifier , to the monitoring of reverse connection status and battery power and detection of transient and short-circuiting .
Current measurement

There are a variety of techniques to measure the current, approach the best known is a method of using a current sense resistor . In this method , amplified by the operational amplifier the voltage drop across the current sense resistor is provided in the differential gain stage in the first, and is then measured . Conventionally, this technique was implemented using discrete components . However , discrete solutions involves resistor values ​​match , low drift , and each requirement of the mounting area and wider , also some disadvantages . Fortunately , various disadvantages of many of these , can be overcome by incorporating the design the current-sense amplifier . Amplifier can be used not only to measure the current , to detect the direction of the current , in response to a wide common-mode range , to perform accurate measurements even more .
To make a current measurement , you can use one of the high-side method to connect the sense resistor in series with the hot wire or low-side system , connect the sense resistor in series with the ground path . In the low-side measurement , circuit corresponds to the low input common-mode voltage , output voltage is referenced to ground . By low-side resistor , unwanted electrical resistance undesirable is added to the ground path . In the high-side measurement and the load is grounded , but there have to deal with large common- mode signal is relatively high-side resistor . In addition , the high-side detection , it is also possible to detect a fault condition , particularly in the windings and motor case that is shorted to ground .
High-side current-sense amplifier such as the MAX4080/MAX4081 , use the current-sense resistor placed between the power input of the monitored circuit and the positive terminal of the power supply . Avoid unnecessary electrical resistance of the ground plane by this arrangement , you can simplify the layout , it has improved a uniform performance of the entire circuit . Current sensing IC of one-way / two-way , such as Maxim MAX9918/MAX9919/MAX9920 , is provided by the type you do not want to built-in types and with a built-in sense resistor . For the type of product is rich , design flexibility is greatly improved , selection of parts for applications and a wide variety of ADC is simplified .
Speed ​​of the motor , position detection and operation

The Hall effect sensors , motor speed , position, and used to detect the direction. With a built-in logic device , the sensor sends this data to the system in order to provide feedback in real time . Moreover , as take corrective action , sensor , and to detect and report the failure of any motor . In general, to detect the direction of motion , you can use the Hall effect sensor two .
The system includes a Hall effect device having the same phase of the motor , when there is correlation in the form of electrical phases of the motor and forms a mechanical Hall effect devices , it is possible to synchronize the edges of the hole to communicate you . A combination of sensor signal conditioning and Hall effect sensors of two 2, MAX9641 Maxim has the output of both position and orientation .
In addition , Hall effect sensors can be used with a special Hall -effect sensor interface products such as MAX9621. The interface device , some features , such as protection diagnosis and filtering, fault protection from transients of power , and the detection of the current Hall effect sensor draws are available.
Hall effect sensor can be used to improve the reproducibility and robustness compared to the system of photo-interrupter -based mechanical exposed to humid environments and dust . In order to detect the magnetic field generated by the current or the magnet , Hall effect sensors , can be operated continuously in harsh environmental conditions such .
Depending on the application , vibration , dust , and you may sensor running by high temperature to operate improperly . In this situation , it is possible to detect the operation of the motor by using the passive element , and sends to the system interface IC that data . Alternatively, you can use in harsh operating conditions of these variable magnetic resistance (VR) sensor .
The VR sensor interface , such as a MAX9924 ~ MAX9927, coils for detecting the rotational speed and the motor is used. Gear shaft attached to the motor passes through the surface of the magnets, magnetic flux passing through the coil as a result and magnet will change . Magnetic flux is maximum tooth approaches the sensor , the magnetic flux will decrease with distance . By the gear rotates , the voltage varying magnetic flux time is generated , proportional thereto is induced in the coil . Digital waveform is obtained by processing the signal with electronic circuits that follow , it is possible to measure or the time or count more easily . VR sensor interface solutions built , has many advantages over and the like accuracy of phase information and enhanced noise immunity , the other solutions .

Control and monitoring of voltage and current of multi-channel

To monitor and control of the motor , you need to measure the voltage and current of more than one , to ensure the integrity of the phase between channels . Designers will be able to choose from two options for ADC architecture . The first , intended to be used in in parallel with a single-channel ADC with multiple , synchronized conversion timing is very difficult to design . The second is to use the simultaneous sampling ADC. The simultaneous sampling architecture , use (also known as track-and-hold amplifier ) sample-and-hold amplifier for multiple analog input or , to use the conversion trigger every single using the ADC of multiple packages in one you . In the case of the sample-and-hold amplifier multiple multiplexer are still used between the ADC and a single multiple analog inputs . Complex digital signal processing algorithm eliminates the need for simultaneous sampling .
Sampling the general speed of the motor control application is 100ksps or more . It continuously monitors the motor at a speed of these , ADC is to see if there is a risk of failure and error . When the sign of an abnormality is found in the first , the system will be able to shut it down if necessary , or self-correction . If the ADC does not perform the sampling at a sufficient rate , because it is not possible to identify early error condition may not be able to deal with this .
Value of the dynamic range of the measurement depends on each motor control applications . Is sufficient resolution of 12 bits in some cases , but in a more precise motor control applications , 16-bit resolution is the criteria more general . High-performance 16-bit ADC , such as MAX11049 or MAX11044 , which allows the system to achieve a dynamic range of 90dB or more .
Maxim offers a wide range of products of simultaneous sampling ADC that are designed for motor control . These devices , has both interface of serial and parallel , each operation of the 16- bit and 12, 14 or , .

Motor control with high accuracy by using an encoder Data Interface

Accuracy required for motor control depends on the system requirements . Such as bottling and industrial robots , depending on the application requirements of accuracy is very high . For example , welding robot will be required to operate high-speed and high-accuracy . Similarly, since it must be stopped at the correct position for filling bottles , attached lid , and the labeling , must be precisely controlled motor bottling plant . In order to accurately control the motor , there is the rotor speed , the direction , the need and determining the position . These can be monitored using the analog sensor , such as a rotary potentiometer resolver , synchro , or RVDT,. In order to achieve high accuracy , you can use the encoder such as a Hall effect sensor and an optical encoder . Encoder provides the controller the absolute position information and incremental information of the shaft angle .
By Standard , motor controllers implemented as algorithms digital signal processor by (DSP) is to adjust the operating power stage by calculating the angle and the current speed of the rotor can be obtained efficiently optimum desired response you can . Reliable information from a robust sensors are required for this feedback control loop . Typically , the information is sent to the controller from the encoder via a long cable .
Usually , the incremental information is transmitted to the controller by two signals in phase quadrature That signal is shifted 90 °. In some cases analog format (sine + cosine ) , can also be a binary format these signals . On the other hand , the absolute position information is sent through the RS-422 or RS-482 is only binary serialized data stream .
For work environment is harsh , data path is required robustness and reliability . That EMI level is high is that it uses differential signaling . In order to access to the motor , it will usually result in , exposed to high temperatures .
Interface device and PROFIBUS RS-485/RS-422 a wide range of Maxim , is intended for motor control applications of these . Interface devices such as high-speed MAX14840E RS-485 transceiver features consistency and robustness of high signal required for the maintenance of up-time consisting of a significant capital expenditures and strict safety management .
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