Adaptive Supply Air Throttle for Increased Efficiency of Meter-out Contolled Pneumatic Drives

  Image of a pneumatic linear drive with adaptive inlet throttle Copyright: © ifas

Compressed air generation is responsible for about 10% of industrial electricity consumption in the European Union. A significant part is used for factory automation by means of pneumatic (linear) drives. The aim of this project is to investigate a novel system architecture for the control of pneumatic linear actuators, which has the potential to achieve a good controllability of the dynamic behavior with a higher efficiency than the state of the art.

 

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Up to now, downstream throttling has primarily been used to adjust the cylinder speed of pneumatic actuators. The speed is adjusted by building up a pressure in the counteracting cylinder chamber. In general, the high differential pressure at the exhaust air throttle and the associated supercritical flow result in a load-independent movement of the cylinder. However, due to the adjustment of the velocity by the pressure build-up opposing it, the movement takes place with low efficiency. As a result, the cylinder always consumes the maximum amount of compressed air, regardless of the applied load.

Upstream throttling can be considered in contrast to downstream throttling. Here, the cylinder moves with a reduced load dependent driving pressure. The counteracting chamber is vented to the ambient pressure. Upstream throttling thus represents the energy-optimal pneumatic realization of a linear movement. However, due to the load-dependent density in the drive chamber, there is a load-dependent cylinder movement. For this reason, upstream throttling, which is clearly preferable in terms of energy, is rarely used industrially.

A solution for simple and efficient control of pneumatic drives by combining a downstream throttle for speed adjustment with a load adaptive upstream throttle in order to reduce the compressed air consumption is proposed. The new system could combine the advantages of both technologies in an optimal way. For this reason, adaptive upstream throttling is to be investigated as a new type of circuit within the scope of the project.

 

Description of the developed concepts

Figure 1 shows an exemplary comparison of supply and exhaust air throttling with a possible embodiment of the novel system.

  Novel circuitry for efficient speed adaptation Copyright: © ifas Figure 1: Novel circuitry for efficient speed adaptation
 
 

While the downstream throttles in the new system directly set the cylinder speed independently of the load, as in today's systems, the counteracting pressure is used to set the upstream throttle. This is important to ensure that the pressure gradient across the exhaust throttle is just sufficient for supercritical flow. The result is a load-adaptive self-regulating component for speed adjustment of pneumatic drives with significantly increased efficiency compared to the classic exhaust air throttle.

  Pneumatic circuit of adaptive throttle Copyright: © ifas Figure 2: Optimized circuit of the adaptive supply air throttle

However, this design alternative has an additional requirement from a control point of view, namely that the directional control valve should quickly return to neutral position when the cylinder reaches the end of its stroke. This counteracts the effect of the upstream throttle opening fully when the cylinder suddenly stops, so that the corresponding cylinder chamber remains in the partially filled state. This additional complexity due to sensors and a more complex valve can be circumvented by an optimized version. Here, a direct fluid-mechanical implementation of the aforementioned shut-off function is achieved by means of a sophisticated circuit.

The function optimization is based on pneumatic detection of the piston reaching the end of its stroke. This causes a sudden pressure drop in the exhaust throttled chamber, which acts as a pilot for the upstream throttling valve. By means of an optimized nonlinear flow characteristic of the throttling valve, the necessary shut-off of the supply pressure at this position can be achieved directly by the throttling valve itself. This means that the previously required electronic implementation of the function in the machine control system, which requires electronic detection of the end position and rapid switchover of the 5/3-way valve to the center position, can be avoided.

 

Further project development

First, the control stability and dynamic behavior of the previously explained concepts are optimized. For this purpose, a simulative investigation is carried out on a wide range of parameters. The goal is to show the application limits of the system by means of a robustness analysis and to work out a design guideline.

The testing of the system and the validation of the simulation results will be done by means of a prototype, which will be realized with the help of the design guideline. The prototype will be tested experimentally in a test rig which allows both static and dynamic loading of the drive. In addition to the functional and stability verification of the system under different conditions, the achievable savings potential will also be validated. In order to support a successful transfer of possible findings from the project, a function-integrated design draft will be elaborated. Finally, a cost estimate of the optimized design and a comparison of the costs with the potential compressed air savings will be made.

 

Acknowlegdement

The IGF research project 21381 N / 1 of the research association Forschungskuratorium Maschinenbau e. V. – FKM, Lyoner Straße 18, 60528 Frankfurt am Main was supported from the budget of the Federal Ministry of Economic Affairs through the AiF within the scope of a program to support industrial community research and development (IGF) based on a decision of the German Bundestag.

 
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Relevant Publications

O. Reinertz, K. Schmitz: „OPTIMIZED PNEUMATIC DRIVES THROUGH COMBINED DOWNSTREAM AND ADAPTIVE UPSTREAM THROTTLING“, Proceedings of the 2020 Bath/ASME Symposium on Fluid Power and Motion Control, Bath, UK, 2020