The use of Smart Sensors in Industrial Control

A smart sensor is the integration of a processor directly into the sensor assembly, which gives direct control of the actuator and digital communication to a central controller i.e. it, allows for the direct conversion of an analog signal to a digital signal, conditioning of the signal, generation of a signal for actuator control, and diagnostics.

Let’s consider the following diagram:

The above diagram represents a process where they are mixing two liquids in a fixed ratio, the flow rates of both liquids are monitored using differential pressure sensors (DP). The temperatures of the liquids are monitored to correct the flow rates for density changes and any variations in in the sensitivity of the DP cells using Temperature sensors (T),

The electronics in the smart sensor contains all the circuits necessary to interface to the sensor, amplify and condition the signal, and apply proportional, integral and derivative action (PID).

When the usage is varying, the signals from the sensors are selected in sequence by the multiplexer (Mux), and then converted by the ADC (Analog to Digital Converter) into a digital format for the internal processor.

After signal evaluation by the processor, the control signals are generated, and the DACs (Digital to Analog Converters) are used to convert the signal back into analog format for actuator control.

Communication between the central control computer and the distributed devices is via a common serial bus.

The serial bus or Fieldbus is a single twisted pair of leads used to send the set points to the peripheral units and to monitor the status of the peripheral units. This enables the processor in the smart sensor to receive updated information on factors such as set points, gain, operating mode etc. and to send status and diagnostics information back to the central computer

Smart sensors are available for all the functions required in process control, such as flow, temperature, humidity, pressure and level control.

You can also read: 

• The Basics of Industrial Control Systems
• How to integrate PLC into a Control System

The implementation of smart sensors has the following advantages over central control systems:

• Smart sensors use a common serial bus eliminating the need for discrete wires to all sensors, greatly reducing the wiring cost, large cable ducts, and confusion over lead destination during maintenance or upgrades.
• The smart sensor takes over the conditioning and control of the sensor signal reducing the load on the central control system, allowing for faster system operation.
• Uniformity in programming means that the program only has to be learned once and new devices can be added to the bus on a plug and play basis.
• Individual controllers can monitor and control more than one process variable.
• Smart sensors have a powerful inbuilt diagnostics, which reduces commissioning, and start-up costs and maintenance.
•        The set points and calibration of a smart sensor are easily changed from the central control computer. 
•    Direct digital control provides high accuracy, not achievable with analog control systems and central processing.
•    The cost of smart sensor systems is higher than the conventional systems, but when the cost of maintenance, ease of programming, ease of adding new sensors is taken into account, the long term cost of sensor system is less. 

Limitations of Smart sensors

Since these sensors are connected to a common serial bus, if the bus fails, the total system is down, which is not the case with discrete wiring; this problem can be prevented by use of a redundant backup bus.

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Stepper and Servo motors

Stepper and Servo motors find key applications in motion control.

Generally a motion control system consists of:

• The mechanical part being moved
• The motor (stepper or servo) with a feedback to provide an indication of actual position
• Intelligent controller
• The motion driver unit
• Programming and operator interface software

Driving a controlled current through a number of coils within the motor generates the magnetic forces in a motor. Depending on their design, motors have many coils oriented in specific magnetic positions within their housing. By pulsing or steady control of current through different motor coils in a known pattern, electromagnetic fields develop in the motor, causing incremental or continuous motion.

The current and voltage that drives a motor typically comes from a power electronics device, known as an amplifier or power drive. Stepper and servo motors are located between the motion controller elements.

So what are stepper motors?
Stepper motors are discrete motion devices that move to positions that relate directly to the number of input control pulses, at a velocity that relates directly to the pulse rate.

Stepper motors rely on the principle of commutation or alternating magnetic forces to provide predictable controlled motion.

Commutation in motion applications is the controlled sequencing of drive currents and voltages in motor coil winding to provide torque and therefore, movement.

In a stepper motor system, individual signals from a motion controller are converted into an energizing pattern for the motor.

As the commutation pattern varies, the motor moves from one discrete position to another.

When the pattern is held in a single state, the stepper motor holds its position with a known torque also called holding torque. These single-state locations are known as the full-step locations of a stepper motor. One key stepper motor specification is the number of full steps per revolution (rotary motion) or full steps per unit length (linear motion).

The steps/revolution parameter of a stepper motor indicates the basic resolution of the motor.

Let’s consider a stepper motor with a resolution of 200 steps/revolution, which can also be referred to as a 1.8 degree/step motor. If the motion controller outputs 200 steps to a full-step motor driver connected to a 1.8 degree stepper motor, the resulting movement would be a full 360° of the movement or one revolution of the motor. If those 200 steps were generated evenly over a period of one minute, the speed of rotation of the motor would be one revolution per minute (RPM).

Having looked at the stepper motors, let us now learn more about Servo motors.

Servomotors are continuous motion devices that use feedback signals to provide position and velocity control in a closed loop system.

The primary types of Servomotors are DC brush servo and brushless servo.

An open loop servomotor rotates or moves uncontrolled as long as the power is applied to it.

By implementing a control loop around a servomotor using a PID controller and feedback from an encoder device mounted on the motor, it is possible to accurately and reliably move to the desired position at well controlled velocities following your specified motion trajectory paths.

All servomotor systems use a motor driver pattern power unit to control the voltage and current that flows through the motor armature and motor winding.

The basic principle of motion in servo motors is based on the flow of current through wire coil, generating a magnetic field that reacts with permanent magnets in the motor to cause attraction and repelling forces that cause movement.

You can also read: Closed Loop Control System

DC brush servomotor

This is the simplest servomotor design. Actually it is cost effective for its performance and power in general servo applications.

DC brush servo motors are self-commutating motion devices that rotate continuously while current is applied to the motor brush contacts. The current flows through the brushes to the armature and then through the motor coils, creating the magnetic forces that cause motion. Changing the direction of the current flow through the motor reverses the direction of rotation.

Encoder feedback to the motion board is required to provide accurate control of position and velocity with a DC brush servomotor. Encoders are mounted on the shaft of a motor or on the coupled mechanical unit as a linear or rotary device, directly translating movement into feedback data.

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