Why we don't use micro controller in industries instead of PLC'S ?

The reason why PLC is prefered over Microcontrollers can be divided into passive reasons(varying) and active reasons(technical).

Passive reasons:

1. Microcontrollers are multipurpose device. Where as PLC is a device that is made for industrial purpose only. This reason maybe seem in favour of Microcontroller, But infact it is relevant in terms of external conditions. Since PLC is developed for industrial scenario it is constructed from high density thermosetting plastics that can withstand extreme physical conditions like temperature, pressure, density, humidity, acidity etc. Also with stands vibrations and shocks.

2. Ladder programming, the most popular programming language in PLC is comparatively simpler when compared to microcontroller programming languages (this also depends from user to user). Ladder is analogous to relay wiring and it gives a pretty good idea to the programmer in relation to practical application.

3. PLCs tend to have a longer life span when compared to Microcontrollers due to its built.

4. PLCs have lesser 'down-time' when compared to microcontrollers. Troubleshooting is easier in PLCs.

5. PLCs are of plug and play type once its programmed. thereby making it easier for technicians to swap processors or modules in case of problems.

Now lets see the active reasons:

1. PLC has an optical isolation(most cases optocouplers) between its Input module and processor ; between processor and output module. Thereby reducing the risk of direct wired connecton problems .

2. Industrial machines are working mostly on AC voltage and the sensors are mostly of DC voltage. This control scenario is difficult for microcontrollers as they require additional devices to handle it. Whereas PLC is available in both DC and AC; it also has different output types like transistor output, relay output etc that helps us to handle varying voltage scenario.

3. PLC is available in almost all types of communication protocols and thereby is easily interfacable with other automation devices like SCADA, HMI, VFD, DCS, Smart sensors etc and also with PCs and other interactive devices.

4. PLCs are available with various scan times depending on our process/application. PLCs are also of various types that can support any industry/process.

5. The digital input signal voltage of most PLCs being 24 Volt DC and that of Microcontrollers being 5- 12 volt DC; this has a profound affect on the tolerance level of the signal loss. 24V DC is more apt for industrial scenarios. 

PLC actually embeds a microcontroller + several circuits like communication, IO, filters...etc

The advantages:

1. PLC is plug and play technology

2. PLC programming was made easy for any technician even to program it unlike C or other micro languages

3. PLC is made into a rigid form so it can be put in a factory without any additions

4. It is macro and not micro. So it doesn't envolve doing things from scratch or re-inventing the wheel.

Microcontroller doesn’t work as a stand alone controller but it came as a part of electronic circuit or device while the PLC is a stand alone controlling device that I can program it for any process that I have

also number of i/o in plc is more than microcontroller

PLCs:

Well-protected, electrically isolated inputs and outputs 24 VAC or 100-240VAC

Simple, Robust Power needs - 24 VDC or 100-240 VAC

More robust physical construction with screw terminals

Constructed for high temperature and humidity environments - protected circuit boards

Simple to program with simple, standard languages

Micro-controllers:

Smaller, usually not waterproof circuit boards, not mounted in plastic housing

Smaller, more fragile physical I/O typically with pin headers

Input and outputs usually not electrically isolated nor very powerful - power spikes can destroy board

Non-standard Input/Output signal voltages (5V, 3.3V, 1.8VDC)

Requires non-standard Supply Voltages (5V, 3.3V, 1.8V DC), more sensitive to Voltage fluctuations 

More complex to program with different programming languages

Micro-controller has limited resources in terms of IO,RAM, ROM and are suitable for embedded products.

While PLC requires to handle large IO and data, suitable for industrial use,

inside both micro-controller and PLC there is a microprocessor eventually they are all same it depend on the use, if your control is small use Embedded system otherwise use PLC.

PLCs are robust, and made for more rugged usage than microcontrolelrs

The PLC is a dedicated controller designed for industrial usage. It contains the various types of I/Os that are very suitably matching with the industrial instrumentation and plant control interfaces. It provides the flexibility in the programming options specially the "ladder language" that is similar to industrial standard "ladder Network" used for designing automatic control schemes in the plants. The overall system is modular and the user need to add modules to increase the I/Os. It  has the standard programming interface along with the online monitoring features of the various plant parameters.

However the microntroller provides the TTL outputs used and can be programmed in assembly, and C langauges. It has less programming memory, available as a single chip solution for smaller applications. Hardware periphery requires a low level design to get the extended functionality and and there is no support for the industrial conventional control scheme,the "ladder networks". The I/O extension is also not available in standard modular form.

A variety of reasons

1. Much stronger

Easier when maintenance

Borne vibration

Programming easier

1. Micro-controller is not rigid to use in industrial environment.

2. It does not  have I/P and O/P  card.

3. It has voltage and current limitation.

4. For the isolation of I/O pin you have to use extra relay circuit.

5. Not user friendly and not easy to program.

1- one of the most reason the performance PLC have high level performance in Industrial application .

2- Modification in PLC easy rather than the Micro .

3- Some application need embedded system which is more expensive ..

4- Programing Library in PLC cover all needs , and suppliers like drivers create the GSD file matching with PLC .

5- I/O modules in PLC cover all needs.

overall PLC Performance and price better than micro .