Will there be room for CNC and PLCs in the factory of the future?

15 October 2017

As automation in manufacturing increases control systems are having to become increasingly complex and efficient.  We investigate what role different industrial controllers might have to play in the future.

While it is not new, the concept of automation is changing as Industry 4.0 and the IIoT become more prevalent. Alongside the picking, packing, palletising and conveying of former years, automation now also encompasses concepts such as zero-downtime, increased precision, high speeds, efficiency and proactive maintenance.  All of this is powered by digital, programmable systems that have been developed and perfected over many decades.

Computer Numerical Control (CNC), for example, is a technology that facilitates the automation of machine tools by means of computers that execute pre-programmed sequences.

“CNC was developed during the 50s and 60s as a logical progression from Computer Aided Manufacturing (CAM) and tracer-based automation,” explains Ian Baird, CNC applications manager for FANUC’s Factory Automation Division.  “Established alongside computer and servo system developments, it helped manufacturers to meet their increasing requirements for repeatable, high-precision production.  Today, it’s formed of five main parts – a sequencer, interpolator, servo controllers, logic controller and operator control interface – and is synonymous with precision and control.”

Twenty years after CNC was introduced, a cheaper and simpler form of computer-aided control was developed –  Programmable Logic Control (PLC).

Programmable Logic Control
“PLC was developed in the 1980s to supersede relay logic control systems, which were often less cost-effective, flexible and easy to use because they relied on hardware to perform their key functions.  PLC has input and output functionality and can be programmed to perform sequential operations, data processing, or simple axes control.”

 PLC was never intended to replace CNC.  “Both serve very different purposes and marketplaces, with their own advantages and disadvantages.  Therefore, it would be limiting for anyone in industry to say: ‘I’ve invested in CNC – there’s no room for PLC here’, because they are two entirely different controls,” continued Baird.

CNC is more frequently associated with the concept of automation because the scope of its use extends beyond a simple input-output algorithm. Modern CNC is a flexible, digitally-controlled system that can be tailored to suit a manufacturer’s needs without needing to re-programme the entire system.

“Most modern CNCs also include user interfaces with built-in operation, maintenance and diagnostic screens.  For this reason, CNC is popular with people who want full control over their machines, because its functionality does allow you to fly solo after a bit of training,” said Baird. The flexibility of CNC lends it to complex, multi-axis machining in almost any industry. “A CNC’s applications are limited only by imagination. Any application that requires precision motion control needs CNC, whether that be the manufacture of watch parts and medical devices, or reactive atomic plasma etching.”

Simple control tasks
PLC, on the other hand, is a good solution for simple control tasks. “If you’ve got an application that doesn’t need a high level of accuracy or flexible motion control, such as an AC motor conveyor, then PLC is often the best choice.  It is cheaper than CNC, which would arguably be better invested in more complex applications,” continued Baird. 

However, there are some disadvantages to the simplicity of PLC, as Baird explains. “PLC does not have the flexibility of CNC. If you need to change the programme even slightly, you have to re-programme it entirely.  It also doesn’t offer the precision of CNC and is therefore best used as a low-cost solution for basic tasks.” According to Baird, despite PLC’s low cost, many manufacturers are choosing CNC, due in part to its lower total cost of ownership. “It is interesting to see many designers turning to CNC after investing in PLC, largely for reasons of flexibility, reliability and cost. “The initial cost of CNC is higher than that of PLC, but the return on investment can be higher in the long term because of the CNC’s higher reliability and control.  It also gives system designers the flexibility to dictate how much control they want users to have over their machines.”

The long-term cost-effectiveness of CNC can be attributed to its advanced user-programmable features, which can minimise downtime and control the energy usage or output of the machine.

Many CNCs now also come equipped with AI contour control.  This means you can control the machine to be within a certain workload, or adaptively control the machine for working overnight.  For example, you can programme it so that it only works at 80% load, allowing you to be more economical with your energy usage. CNCs also come equipped with energy efficiency functions, such as energy charge modules.  

The added safety functions of CNCs also lend themselves well to collaborative working with humans.  Baird said: “CNCs come equipped with a digital algorithm that looks after the motion control.  This digital system is formed of two parts – a real digital data system and an observer digital system.  The observer acts like the ‘ideal’, providing the machine with the parameters in which it should be working. The real and the observer are both driven by the same command, so they should be working in exactly the same way.

“If the real system encounters a disruption, such as an unexpected load, then this causes the real data to deviate from the observer data. The machine will translate this as a collision and respond in one of two ways.  If it’s moving slowly, it will stop, and if it’s moving quickly, it will perform a ‘vectored back-off’, where it will retract any moving machinery to avoid damage.

“For high-end machines, you can also incorporate 3D technology, which stops five-axis machinery from moving outside of its pre-determined work envelope.”

Alongside safety, zero-downtime is an important consideration for manufacturers looking to automate their processes. Unplanned downtime is expensive: it can halt production for days, weeks, or even months. An undetected fault could cause irreversible damage to machinery, and even be hazardous to human operatives.

Minimising downtime
“Although it’s unrealistic to expect factories to work seamlessly 24/7, we can aim to minimise downtime caused by minor faults or errors,” said Baird. This is where the concept of predictive maintenance comes in, which, as Ian explains, is facilitated by CNC control technology.  “Predictive maintenance allows us to spot potential problems before they occur, and act accordingly before they become serious. We do this by employing the automation technology that controls the machine as a kind of watchman.

So, is CNC destined to become the sole tool of the factory of the future? “It’s tempting to dismiss PLC as cheap and cheerful, but it still has a vital part to play in automation,” said Baird. “The best example of this is a production line. CNC may be controlling the robot arms, the tooling, milling, and grinding, but PLC is powering the belt that takes a product or material from one part of the line to the next.  The complexity of CNC does not lend it well to such tasks and would be wasted. As part of a factory floor, where simple and complex tasks are done simultaneously, CNC and PLC work perfectly together.”

With CNC and PLC both maintaining a place in manufacturing’s tool-kit, it is now important to look at how they can be developed. “Industrial control systems will continue to evolve, and this will largely come in the form of specialisations suited to specific industries”, concludes Baird. “Third-parties will also exploit the concept of an open interface in order to integrate the factory with the Internet of Things.  With this will come intelligent machines and data collection and analysis on a vast scale, which will help us to identify further process improvements.”

Only time will be able to tell how CNC and PLC adapt to the factory of the future, but it is clear that they will both form a part of it, if not always working in collaboration.

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