Clamping systems for autonomous production cells (SFB 368)
Hydraulic actuated clamping systems are established in modern machine tools. Today hydraulic clamping actuators are supported by external pressure supply and controlled by machine control. The hydraulic actuators are usually connected to the external pressure supply by horses, pipes and coupling systems. This components have drawbacks concerning flexibility, leakage and system bleeding at change of attachment.
In the framework of the special research array 368 "Autonomous production cells" different clamping systems are build up for milling and laser process.
Figure 1: SFB 368
In consideration of the very different demands of providing clamping systems caused by milling and laser process, at first clamping systems for the milling process are developed at the IFAS and subsequent clamping systems for the laser process.
For the milling process different autonomous clamping systems with integrated pressure supply and all needed hydraulic components are developed. The electric drive is integrated in the clamping system as well as the valves, hydraulic actuators and pressure transducers. Further on the closed loop control as well as a eligibly power supply should be integrated into the clamping system as a matter of fact the clamping system does not required a connection to an external power supply and the data exchange between the clamping system and the production cell can be minimised.
For the laser process clamping systems are developed, which could be adapted very flexible to different three dimensional moulded plates. On the basis of a CAD-File the clamping system should be adjusted full automatic.
For the milling process different clamping systems and a few therefore needed components were developed.
The concept "power-by-wire" is a big step to increase automation and flexibility of clamping systems. Pressure supply including electric drive as well as the circuit technique for the pressure control loop are integrated into the realised autonomous clamping systems. Further on NiMH-accumulators are integrated into the system to allow operation in the production cell without a connection to an external power supply. At present the electronics including a micro-controller is integrated into the module. Therefore hydraulic and electric interfaces are redundant.
Special importance is attached to the choice of the electric drive, because the force density of electric drives in proportion to hydraulic drives is lower. Therefore the clamping cycle is divided in two phases. During the first phase the clamping cylinders pull out fast and the system pressure is negligibly small because it serves only the purpose of overcoming the hydraulic flow resistances in the system and the friction between seal and housing. The second phase is distinguished by contrary demands. This second stage raises the force and thus the system pressure (max. 500bar) in the active element. The volume flow demand in this period of time coincides with the compression volume flow and is relatively insignificant. The maximum of the two parameters determining the power, delivery flow and system pressure, do not occur simultaneous and therefore the needed electric power can be minimised. Two circuit variants were developed at the IFAS which realise a two stage concept. In the first system the concept is realised with a combination of a gear pump and a pressure intensifier. The second system works with a special two stage pump. In the first system a piezo-proportional-throttle valve is used for pressure control and in the second systems piezo switching valves with pulse width modulated control.
The dynamic of the piezo-proportional-throttel valve is very high. The -3dB frequency is higher than 600 Hz. Significant is the flat and, over a wide range of the effective shift valve stroke, almost linear correlation between slide valve stroke and pressure difference across the valve. Besides high dynamic valves the choice of closed loop parameters is very important for a satisfying pressure control in miniature hydraulic systems. Therefore a model of the clamping system was designed with the simulation software DSHplus. The purposeful optimisation of the parameters was realised with the procedure of Hooke and Jeeves.
In future clamping systems for the laser process will be developed.