Rivals or running mates?

Picking a winner between pneumatic and electrical control is no easy task. Design Engineering asked SMC Pneumatics and Mitsubishi Electric for their input.

For many industrial applications, choosing between traditional electrical and pneumatics solutions has been a matter of habit and familiarity rather than carefully weighed up choices. Generally, pneumatics provide excellent motion control solutions. Typically, simpler electrical motion control with inverters and steppers have been a `good enough’ solution.

By contrast, servo control has, in the past, been a costly option, rarely justifiable. That is now changing: the price of servos is coming down, so that high precision and – with encoder feedback devices – excellent electrical motion control is coming within easier reach of more companies.

Both pneumatics and servo control technologies have raced ahead in recent years. Pneumatic control has become smarter. Programming is so much easier with electro-pneumatics so that many tasks traditionally difficult with pneumatics have been made much easier. For example, stopping in shorter distances, profiling speeds very accurately and varying torque are now easy hurdles to jump.

There is no doubt too that, if servos used to be regarded as difficult to handle, they no longer demand the same degree of specialist expertise. Servo control has become much more user friendly. Many new products are emerging, bringing it well within the capabilities of any competent engineer.

There are three criteria where servo drives can justify their cost if your application’s demands are high: if you need very high precision, interpolation between axes and repeatability, servo drives are probably the way ahead.

But what level of precision do you need for multiple positions? With pneumatics, you are typically talking millimetres – though finer tolerances can be achieved. Accurate enough for most applications, pneumatics cannot achieve the micron accuracy of servo drives.

BUT IT COSTS

But such accuracy does not come cheap. If you need to perform mid-position stopping in microns rather than millimetres, it will cost thousands rather than hundreds of pounds – typically companies are spending twice or three times as much on an electrical motion control system compared with pneumatic movements.

Interpolation also remains a problem for pneumatics. If you want to generate a constant path, in a curve or straight diagonal line between two axes, that is difficult task. XY interpolation, and controlled movement between multiple axes is where servos come into their own. Changing one precise path for another is simply a matter of programming.

However, if an application needs to move between two points very accurately, but the precise path between them is not critical, pneumatics is still likely to be the best runner for the course unless there are any other reasons to justify incurring the higher cost of components and programming a servo.

Another important criterion is repeatability – greater speeds and accuracy are more easily achieved with servo drives.

Because pneumatic control is best suited for end to end movements, stopping midway or varying speeds presents quite a challenge – it can be achieved but is typically less accurate.

Proportional pneumatics can provide acceleration and deceleration and force control – but electrical servo drives permit much greater control of force, acceleration and deceleration at all positions through a movement. While pneumatic PLC control can be simpler and cheaper, there is significantly less flexibility than with servos. By contrast, multiple positions are no problem with servo drives, and programming precise rates of acceleration can be relatively straightforward.

With servos, achieving very high velocity is almost effortless, with motors able to move from stationary to 4000rpm and back to rest, and provide very accurate positioning in 10ms. High cycling is possible for electrical drives, for much longer movements than pneumatics but adds complications – for example, sometimes requiring higher rated motors and thus adding to costs.

If you are looking for very slow movements, both technologies provide solutions. Ask a pneumatics expert, and he would be able to find ways of bringing down speeds and keeping firm control – for example with air hydro systems. Ask a servo control expert, and it is probably a simple question of programming to deliver any low speed.

STRENGTHS IN PNEUMATICS

For a sequence of simple end-to-end movements, pneumatic control is likely to be a simpler and more cost-effective solution than servos.

In some applications, pneumatic control literally has particular strengths – for example, for lifting applications, for applying a constant force for minutes and hours, for pumping, and for operating process valves.

CLOSER TOGETHER

Today, many engineers are choosing to harness both technologies – exploiting the strengths of both pneumatics and servo drives. Some, for example, are developing more sophisticated gantry style robots with grippers pneumatically driven, and motion controlled by servos. Filling lines are exploiting electrical motion control moving bottles, with pneumatics pumping the fluids, holding the bottles, and servos capping the bottles to precisely the right torque.

The dual approach is also valuable with heavy loads. Since pneumatics can apply higher forces than traditional servo systems – and especially heavy loads in conjunction with hydraulics, so some applications, for example in the automotive industry, are utilising pneumatics for rendering heavy products – such as engines – weightless and entrusting complex robotic movements to servo control.

In the UK, service and support is not an issue, but for a machine built for export, it may be.

Although more engineers in the UK are learning to speak the language of servo control fluently, in some countries the knowledge may be rudimentary. Faced with a problem with stepper motors, let alone servos, some end users around the world are – at the moment – less likely to be able to solve it, whereas there tends to be more expertise for diagnosing and solving problems for pneumatics, and correction is much easier.

Embedded intelligence in modern servo drives should make this a concern of the past. However, if you have to design your applications keeping in mind the capabilities of other end users, the availability of expertise could be an issue in the short term. Some companies have found that it pays to invest in special pneumatics multi-stop rodless cylinders as that can save on costs of support and maintenance at a distance.

SMC Tel: 01908 563888

Mitsubishi Tel: 01707 276100