Sensor and Actuators
What is a sensor?
A sensor is a device that senses something. Today we have sensors that can see, feel, hear, smell, and even taste.
Without sensors, our home and work lives would be quite difficult. For example, as you drive to work, the traffic lights at an intersection are controlled by sensors embedded in the road. These sensors detect your arrival at the intersection.
As you approach the grocery store, the door automatically opens because of a sensor.
In your plant, the batch process temperature and pressure are displayed and controlled as a result of output from Sensors.
Industrial sensors
In the world of instrumentation and process control, we define a Sensor as a device that detects changes in physical, electrical, or chemical properties and produces an electrical output in response to that change.
Types of sensors
What are the typical physical properties that sensors are detecting? Let’s name a few… Level, Temperature, Flow, Pressure, Speed, and Position.
Classification of sensors
From a process control perspective, we can classify sensors as either Passive or Active.
Passive sensors
A Passive Sensor requires an external source of power to operate while an Active Sensor does not.
Active sensors
A Thermocouple is an Active Sensor as it does not require any external power supply to operate.
Active sensors examples
As a thermocouple is exposed to an increase in temperature, it will develop an increasing voltage across it.
Another example of an Active sensor is a piezoelectric sensor.
Passive sensors examples
A Resistance Temperature Detector (RTD) is a Passive Sensor. It is a device that’s resistance will change with a change in temperature. To take advantage of this change in resistance, an external supply, or an excitation circuit is required to produce a change in voltage.
Another example of a Passive sensor is a Strain Gauge.
Sensors in the industry
Alright now that we’ve talked about different sensor types and the physical properties that they can sense, let’s discuss how they are used in the industry.
Almost every sensor used in process control will be connected to a Transmitter because a sensor’s output needs to be conditioned or amplified.
Here’s an example…We’ve already talked about a thermocouple and the voltage output created when it is heated. Unfortunately, the voltage output of a thermocouple is minuscule!
In our example, the thermocouple will produce a voltage output from 8 mV to 18 mV over a 450 degree Fahrenheit change in temperature!
In-process control, we condition that 8mV to 18mV thermocouple voltage and convert it to a 4 mA to 20 mA industry-standard signal that represents our controlled temperature range.
What is actuator?
An actuator is a device that makes something move or operate. Every one of us takes advantage of at least one actuator every day. Let’s look at some examples of actuators.
Grocery Store Door
When we go to the grocery store, the door opens automatically for us. An actuator makes the door open.
Car Seat
We can move the car seat forward or backward before we drive away in our car. An actuator makes the seat move.
Types of Actuators
Linear Actuator
An actuator can move something in a straight line, also referred to as linear.
Rotary Actuator
An actuator can make something move in a circular motion, also referred to as rotary.
What Makes an Actuator Move?
We’ve talked about the movement of an actuator, but we haven’t discussed how, or what makes it move.
Actuators receive a source of energy and use it to move something. To put it another way, the actuator converts a source of energy into a physical-mechanical motion.
A butterfly valve is operated by turning the Handwheel connected to the rotary actuator. In industrial applications, the Handwheel could be replaced by one of three typical sources of energy, which can operate the rotary actuator:
– Electric actuators use some form of electric energy to operate.
– Hydraulic actuators use a variety of liquids as a source of energy.
– Pneumatic actuators are operated by compressed air.
Types of Actuators
Typical actuator types in the industry include:
– Electric Motors
– Hydraulic motors
– Pneumatic Control Valves
4-20 mA Pneumatic Actuator
Let’s look at a typical Pneumatic Actuator in action! The PLC analog output card produces a 4-20 mA current to move the valve from fully open to fully closed.
The 4-20 mA current will be converted to pneumatic pressure which becomes the source of energy to operate the actuator.
Electrical Drive
The system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive. In other words, the drive which uses the electric motor is called electrical drive. The electrical drive uses any of the prime movers like diesel or a petrol engine, gas or steam turbines, steam engines, hydraulic motors and electrical motors as a primary source of energy. This prime mover supplies the mechanical energy to the drive for motion control.
The block diagram of the electrical drive is shown in the figure below. The electrical load like fans, pumps, trains, etc., consists the electrical motor. The requirement of an electrical load is determined regarding speed and torque. The motor which suited the capabilities of the load is chosen for the load drive.
Parts of Electrical Drive:
The main parts of the electrical drives are power modulator, motor, controlling unit and sensing units.Their parts are explained below in details.
Power Modulator: The power modulator regulates the output power of the source. It controls the power from the source to the motor in such a manner that motor transmits the speed-torque characteristic required by the load. During the transient operations like starting, braking and speed reversing the excessive current drawn from the source. This excessive current drawn from the source may overload it or may cause a voltage drop. Hence the power modulator restricts the source and motor current.
The power modulator converts the energy according to the requirement of the motor e.g. if the source is DC and an induction motor is used then power modulator convert DC into AC. It also selects the mode of operation of the motor, i.e., motoring or braking.
Control Unit : The control unit controls the power modulator which operates at small voltage and power levels. The control unit also operates the power modulator as desired. It also generates the commands for the protection of power modulator and motor. An input command signal which adjusts the operating point of the drive, from an input to the control unit.
Sensing Unit : It senses the certain drive parameter like motor current and speed. It mainly required either for protection or for closed loop operation.
Advantages of Electrical Drive
The following are the advantages of electrical drive.
- The electric drive has very large range of torque, speed and power.
- Their working is independent of the environmental condition.
- The electric drives are free from pollution.
- The electric drives operate on all the quadrants of speed torque plane.
- The drive can easily be started and it does not require any refuelling.
- The efficiency of the drives is high because fewer losses occur on it.
The electric drives have many advantages shown above. The only disadvantage of the drive is that sometimes the mechanical energy produced by the prime mover is first converted into electrical energy and then into a mechanical work by the help of the motor. This can be done by the help of the electrical link which is associated with the prime mover and the load.
Because of the following advantages, the mechanical energy already available from a non-electrical prime mover is sometimes first converted into electrical energy by a generator and back to a mechanical energy of an electrical motor. Electrical link thus provides between the non-electrical prime mover and the load impact to the drive flexible control characteristic.
For example – The diesel locomotive produces the diesel energy by the help of the diesel engine. The mechanical energy is converted into an electrical energy by the help of the generator. This electrical energy is used for driving the other locomotive.
Disadvantages of Electrical Drive:
The power failure completely disabled the whole of the system.
- The application of the drive is limited because it cannot use in a place where the power supply is not available.
- It can cause noise pollution.
- The initial cost of the system is high.
- It has a poor dynamic response.
- The output power obtained from the drive is low.
- During the breakdown of conductors or short circuit, the system may get damaged due to which several problems occur.
Application of Electric Drive
It is used in a large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots and washing, etc.
