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 portion is used for factory automation by means of pneumatic (linear) drives. Therefore, the aim of the present 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 motion characteristics and a higher efficiency than the state of the art.

 

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Up to now, exhaust air throttling has primarily been used to adjust the movement behavior of pneumatic drives. The speed is adjusted by building up a counterpressure in the counteracting chamber. As a rule, 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 load.

Supply air throttling can be viewed in contrast to exhaust air throttling. Here, the cylinder chamber, which acts in the direction of movement, is only pressurized to the extent that the load moves. The opposing chamber is vented to the surroundings. Supply air throttling thus represents the energy-optimal pneumatic realization of a movement. However, due to the load-dependent density in the drive chamber, there is a load-dependent movement. For this reason, this supply air throttling, which is clearly preferable in terms of energy, is rarely used industrially.

An integrated solution for simple and efficient control of the movement behavior of pneumatic drives by combining an exhaust air throttle for speed adjustment with a supply air throttle that adapts to the load in order to reduce the compressed air requirement could combine the advantages of both technologies in the best possible way. For this reason, adaptive supply air throttling is to be investigated as a new type of circuit within the scope of the project.

 

Development of alternative 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
 
 

Whereas the exhaust air throttles in the new system, analogous to today's systems, directly preset the cylinder speed independently of the load, the setting of the supply air throttle tracks the back pressure. 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 variant has an additional requirement from a control engineering point of view, namely that the directional control valve assumes the center position when the cylinder reaches the end position. This prevents the pressure balance of the supply air throttle from being opened at the end of the movement, 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. The new design incorporates a direct fluid-mechanical implementation of the aforementioned function by means of a sophisticated circuit.

The function optimization is based on pneumatic detection of the piston reaching the cylinder end position. This causes a sudden pressure drop in the exhaust-restricted chamber, which acts on the control surface of the valve regulating the supply air. By means of an adapted nonlinear characteristic of the control valve, the necessary cut-off of the supply pressure of the controlled side in this position can be achieved directly by fluid mechanics. This means that the previously required electronic mapping 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 neutral position, can be omitted.

 

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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