TechArchive

Original article date: April 1996

Using technological advances in electronics it’s possible to use pneumatic drives in complex control functions says Heinz Godert of Robert Bosch.

Pneumatic drives in the past have been consigned to relatively simple functions because of their inability to hold position. But analogue valves combined with suitable control electronics and sensors allow the physical capabilities of a pneumatic drive to be exploited to the full. This technology opens up possibilities of realising new solutions which could not be achieved with conventional pneumatic equipment. Critical in the development of servo control and positioning systems has been valve design.

In industrial applications pneumatic drives with position servo control compete directly with established drive systems using electric motors. The pneumatic drive is only accepted as an alternative where concrete benefits regarding cost or function can be demonstrated yet the additional costs involved can be compensated for by enhanced benefits to the user.

The different systems can be subdivided into two groups namely analogue and digital position servo control systems according to the signal processing involved.

Digital systems will usually work with incremental measuring systems which allow accurate positioning independent of stroke. The use of a digital controller allows the control loop to be adapted closely to the complex controlled system. Special handling modules usually serve as drive elements.

These pneumatic NC axes have excellent dynamic qualities and offer functional advantages over electric motor drives in special applications. Price levels are roughly comparable.

Pneumatic drives with position servo control which are based on analogue signal processing represent an economical alternative particularly for applications involving short strokes. Since the associated measuring system operates with absolute values the achievable accuracy depends on the stroke.

The drive system consists of a standard cylinder with guide unit which has a position measuring system (linear potentiometer) mounted to it. A wide range of standard cylinders is available. The valve is controlled by a valve amplifier which accommodates the position controller as a piggyback board.

Due to the greatly differing properties of the controlled systems (cylinder diameter stroke mass) the controller needs to be specially adapted to each particular application.

Force Control

For the input of defined forces pneumatics has distinct advantages over other systems. A minimal amount of equipment is required. A mechanical pressure regulator stipulates a certain pressure a pneumatic cylinder converts this pressure into a corresponding force.

But for many industrial applications this minimum configuration is no longer sufficient. In particular there is now a demand for greater flexibility such as the possibility of modifying the force by means of an electrical signal.

If the mechanical pressure regulator is replaced by an electrical proportional pressure regulator this demand can be met.

But both these versions only provide the control of force without taking into account the friction characteristics of the cylinder.

There are many applications where it is no longer viable to ignore the resulting errors. Particularly in cases where the point of application of force is not constant there are noticeable deviations from the proportional ratio between pressure and force.

Typical applications are for instance the exertion of force to elastic materials or maintaining a constant force acting on a moving piston rod. In cases like these a high level of static accuracy can often only be achieved through a force servo loop involving the direct measuring of force.

The benefits offered by force control using an analogue valve with 5/3-way characteristic as actuator are:

• Tensile and pressure forces can be applied
• Simple compensation for forces due to weight where units are mounted in a vertical position
• Highly dynamic response to changing force values since the two chamber pressures are changed inversely.

Unlike the change to position servo control a transition from an analogue controller to a digital controller would not afford any distinctive benefits for force control. For one thing the transducers used in industry work on analogue principles and therefore usually also supply an analogue output signal. This can be processed directly by an analogue control system.

The accuracy of a force control system is not determined primarily by the method of signal processing in the controller. It is rather the quality of the transducer and the design aspects governing the application of the force which are decisive.

When making a comparative analysis of different drive systems the relative levels of energy consumption are important. The losses suffered in the generation of compressed air and its distribution through the network result in a relatively low level of overall efficiency. One of the aims in designing a pneumatic drive solution must therefore be to keep the air consumption as low as possible.

• Robert Bosch
• Tel: 0116 289 2828
• Fax: 0116 289 2723

April 1996