Keywords

1 Introduction

Formvariable handling systems can adapt to different geometries of the handling objects and fulfil various tasks and operations. Shintake et al. reviewed the so called soft grippers that consist of polymer material [1]. The polymer material is advantageous because of its elastic behaviour. In combination with several physical effects like granular jamming [2] or Fin Ray Effect [3], a universally usable handling system is realised. In contrast to the benefits of the elastic polymer material, the max. operating temperature of 300 \({}^{\circ }\text {C}\) limits the usability of a soft gripper [4]. For most use cases, this temperature is not exceeded. In this work, the application in the forging sector is considered. Here, the objects undergo massive geometric changing processes and reach temperatures up to 1200 \({}^{\circ }\text {C}\). The workpiece of a bevel gear, for example, has a simple cylindrical geometry. After forging, the bevel gear is conically shaped with teeth on the cylinder surface without parallel surfaces for grasping. A handling system that can adapt to the varying shape of the handling object would be beneficial for the automation of these processes. Additionally, using one gripper instead of several reduces the number of necessary grippers, resulting in a cost reduction. Further, tooling times are saved.

The Tailored Forming Process can, for instance, be considered such a procedure. It is a novel forging process investigated in the Central Research Centre (CRC) 1153 at the Leibniz University Hannover [5]. The main focus is to develop a process chain to manufacture particularly tailored hybrid components consisting of multiple materials. The joining process is the main difference between the Tailored Forming Process and other hybrid manufacturing processes. Conventional processes place the joining process at the end of the manufacturing process, at which state the components have almost their final shape. This fact limits the possible geometries for the hybrid components. In contrast, the joining is located at the beginning of the process chain in the Tailored Forming Process. Here, the materials are merged into semi-finished workpieces with simple shapes followed by the forging.

To match the forging properties of the combined materials, the workpiece has to be heated up. Depending on the different materials, a temperature gradient is necessary and is set by induction heating. Currently, the combination of steel-steel and steel-aluminium is being investigated. The steel-steel paring requires temperatures up to 1200 \({}^{\circ }\text {C}\), which defines the max. process temperature.

Several demonstrator components are investigated in the CRC 1153. Their shapes vary from cylindrical shafts over a conical bevel gear to a wishbone and are depicted in [6]. The bevel gear, for example, has a cylindrical workpiece and undergoes a massive change in shape, which is challenging to handle with the same handling system.

Furthermore, the accuracy of the handling system is essential for the Tailored Forming Process quality. Induction heating is used to prepare the workpieces for forging. They are placed on the induction coil, which requires high precision. Contact between the induction coil and the workpiece could damage the coil. In addition, the workpieces have to be placed in an exact position in the forging die. Otherwise, the forging could fail.

The brief overview of the Tailored Forming Process indicates the need for a handling system which withstands high temperatures of up to 1200 \({}^{\circ }\text {C}\), adapts to changing geometries and fulfils the accuracy requirements.

In this work, a concept for a previously developed form variable pin gripper for use in forging environments is investigated further [6]. Therefore, the Tailored Forming Process has been introduced to define the boundary conditions. In the following, the functionality of the automation of different pin grippers is briefly presented to outline the state of the art. Afterwards, the prototype of the previously developed handling system is presented and the experimental validation results are discussed. A summary and an outlook complete this paper.

2 Related Work

This section gives an overview of pin grippers and their grasping process. At the end of this section, the gap between the currently available pin grippers and the boundary condition of the Tailored Forming Process is pointed out.

The first gripper of this kind is the Omnigripper by Scott [7]. Scott has an arrangement of 8\(\,\times \,\)16 pins on two slightly separated plates with the same orientation. The pins can move independently in the vertical direction when it comes to contact with an object. The Omnigripper lowers over an object, the pins in contact retract, whereby the negative of the shape is adapted. The plates then move together and the pins clamp the object. The pins are telescopically designed. When the pin retracts by contact, electrical contact is established between the inner and outer tubes activating a switch. Thus, the Omnigripper can acquire 3D data from the pin positions with this sensing. A host computer starts the gripping process. The Omnigripper is attached to a robot, and the host computer order to grasp, release or reorient the object. Afterwards, the control is given back to the robot for doing the movement. Scott proved the ability to handle a wide range of objects with his work but did not mention high temperatures or the reached accuracy.

Similar to Scott, Mo also developed a pin gripper [8]. Mo’s gripper has pins that move independently vertical and the shape is adapted by lowering the gripper over the object to handle. In contrast to Scott, Mo’s pins are arranged on one plate, and the grasp is realised by active rotational actuation of the pins. Some pins have an elliptical shape, whereby the object is clamped. Both of these grippers need an active actuation.

Meintrup developed a system based on pins that consists of two opposing pin jaws [9]. The system is primarily utilised as a manual clamping device without automation. The pins on each jaw are close-packed and in contact with each other, and they can move independently in the horizontal direction. The jaws can be brought together to grasp an object. Thereby, a mechanical system is activated that presses the pins together. The resulting friction between the pins blocks their horizontal movement and the pin position is fixed. The activation of the mechanical system depends on the position of the jaws. The jaws move over a ramp, whereby the clamping is realised. This passive actuation is advantageous for high temperatures but not adaptable to changing diameters of the handling objects.

Kim [10] developed a similar gripper to [6] with two opposing pin jaws. The pins are actuated with pressurised air, whereby the jaws are electrically driven. The stroke of the pins regulates the process. A resistive foil is attached inside the gripper. If the pin touches the resistive foil, its resistance changes and the process is considered completed.

The systems shown are adaptable to different shapes and diameters. However, none of them is designed for the operation in conditions like the Tailored Forming Process or other forging processes. Pin grippers do not need to be made of polymer material to achieve their shape variability. In addition, the gripper’s jaws can be configured and arranged differently allowing further adaptation to the handling task. Electrical grippers or sensors cannot be utilised to detect the pin position at high temperatures and have to be adapted. Due to those facts, the pin system is investigated further to realise a handling system for hot forging workpieces.

3 High Temperature Flexible Handling System

As described before, the boundary conditions of the Tailored Forming Process are exceptional. Thus, standard parts like parallel gripper cannot be utilised. Therefore, a flexible handling system consisting of two pin jaws and a grasping device was developed. The jaws, Fig. 1b, are the shape variable part of the system and were part of the previous work [6]. The grasping device, Fig. 2, aligns and moves the jaws and is designed in this work for the use case of the forging sector.

3.1 The Pin Jaws

In the case of the Tailored Forming Process, the high temperatures are problematic for the handling task. For that reason, the system developed in [6] is actuated by pressurised air instead of electric motors or hydraulics. Electrical units are not heat resistant, which is why they are not considered. Hydraulic systems have to be sealed and the sealings on the pin can be damaged by the heat the pin reaches. If the sealing breaks, the system will fail, and the fluid will contaminate the environment. Pressurised air also needs sealing but can also work without sealing. In the circumstance of a leak, only air is released, which is uncritical for the environment. The high temperature also affects the pressurised air, which causes expansion that the control circuit can compensate.

In order to use the pressurised air as actuation for the pins, each pin is integrated into a cylinder like a piston-cylinder-system, depicted in Fig. 1a. The pin consists of the piston and a screwable head. The subdivision is required for assembly purposes and allows to change the pinheads for different tasks. For example, if the gripper handles objects with polished surfaces, metallic heads could damage them. A seal is attached at the other side of the piston to prevent pressure loss. The seal is critical because it consists of polymer material such as the elastic gripper mentioned above. Therefore, a thermal simulation was carried out to investigate the influence of different settings for material data and contact time between object and gripper. The results indicate temperatures in an allowed range for a high temperature stainless steel with a low conduction coefficient. The cylinders with the inserted pin pistons are assembled on a base plate, whereby the matrix arrangement occurs.

Fig. 1
figure 1

a Construction of the cylinder-piston-system. b Pin gripper holding a bearing bushing

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3.2 Grasping Device

The jaws have to be aligned and moved, which requires a grasping device that can withstand the heat and water from an additional cooling unit to maintain the handling object’s heat distribution, described in more detail in [6]. Due to the temperatures, sealings could be harmed and the vaporised water can damage the electrics of the grasping device. To overcome these difficulties, a particular grasping device is designed within this work as follows: The jaws are mounted on a linear guiding system and can move independently. The linear guiding system also has a clamping system that can be activated at every position and fix the jaw. Additional double-acting cylinders actuate the jaws by pressurised air. To protect these jaw cylinders against heat and the cooling fluid, they are mounted on the backside of a plate while the linear guidance systems and the jaws are on the front side. The grasping device is depicted in Fig. 2.

Fig. 2
figure 2

Components of the pin gripper divided into grasping device and pin jaws

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4 Controlling Concept

After the presentation of the mechanical part of the handling system, the introduction of the developed control unit follows. The gripper’s control has a significant influence on the accuracy and reliability of the grip. Therefore, this section addresses the basics of the underlying control concept and the gripping routine carried out.

4.1 Concept

For the realisation of the control, the following setup in Fig. 3 is chosen: A programmable logic controller (PLC) is utilised as a central control unit. On the pin gripper, sensors are attached and connected with the PLC. A hall sensor on each jaw actuation cylinder is used to measure the jaw’s position. Furthermore, there are laser distance sensors under the jaws. Their task is to detect the position of the handling object and transmit the data to the PLC. They operate as placeholders in the concept as their final application is still being investigated. Due to the thermal radiation of the hot object, their use is not possible without further considerations. The PLC processes and corrects position deviations of the object by adjusting the jaw positions and the pressure in the pin cylinder, which improves the gripping accuracy. Preliminary grasping experiments showed that the pins of one jaw press in the pins of the other depending on the handling object. An assumption is that the production tolerances cause varying friction forces in the cylinder-piston system. That is why the force imbalance between the pins occurs. The imbalance is compensated by adapting the pressure value in the jaws and their position. As seen in Fig. 3 the pressures are set by a valve system that is connected with the PLC. The valve system measures and adapts the pressures to equalise air expansion caused by heat impact.

Fig. 3
figure 3

Setup of the control system

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4.2 Gripping Routine

A three-phase gripping routine is performed to grip a workpiece securely. In the first phase, both jaws are moved to the extended position using the hall sensors on the cylinders to check the position. Then the pins are brought back to their initial position by shortly applying pressure to revise previously set contours.

In the second phase of the gripping cycle, the gripper is closed. The cylinders first close the jaws. Thereby, the closing speed of the jaws is adjusted by the exhaust air throttling that controls the airflow out of the jaw cylinder. It is then integrated in the pressure regulating valve and can be adjusted continuously. When the pins come in contact with the workpiece during the closing process, the pins retract and the pin matrix maps the workpiece surface’s profile. The pneumatic clamping system is activated, and the pins are re-pressurised for a secure grip.

In the third phase of the routine, the grip is released. The cylinders are pressurised again in the opening direction and the pins are vented. The clamping elements are deactivated, so the grip is released abruptly as the cylinders have already been pressurised. All components are vented when the defined extended position is reached again. Next, the handling and control systems are presented. Then, the gripping accuracy will be validated to examine if the required handling precision is reached.

5 Validation

The designed system is analysed experimentally to verify the accuracy and provide fundamental knowledge for subsequent system optimisation. This section describes the experimental setup first and is followed by the results.

5.1 Experiments

The gripping routine presented in Sect. 4.2 is performed to investigate the gripping process. The aim is to measure the deviation of the object caused by the handling system and to minimise the deviation. The output variable by which the results are evaluated is the relative position change of the workpiece during gripping. In all experiments, a hybrid hollow cylinder workpiece with an outer diameter of 62 mm and a height of 84 mm is gripped. Two laser distance sensors measure the x- and y-position of the cylinder. The x- and y-direction of the handling system are shown in Fig. 3 and the set up of the testbed is depicted in Fig. 2. During preliminary tests, the closing velocity of the jaws was identified as the main parameter. The velocity is determined by the inflow and outflow of air from the jaw cylinder. Therefore, the air inlet and exhaust air throttling are considered. For the exhaust, a max. value of 2% is chosen. Higher values result in a too fast movement of the jaw. For the air inlet, the range from 0.01% to 100% is tested. Initially, a mechanical synchronisation of the jaw cylinders is not utilised. Afterwards, a synchronisation is integrated because asynchrony was observed in the closing movement. The jaw cylinders are mechanically coupled through a lever. When the piston of one cylinder moves, the other is automatically set in motion, whereby synchronisation of the jaws is achieved. In total, an experimental design with 20 parameter settings is carried out. Each experiment is repeated three times in randomised order.

5.2 Results

The results are depicted in a box plot in Fig. 4. Here, only the x-direction is evaluated because it has a more significant deviation than the y-direction due to asynchrony. The results without synchronisation are presented first followed by the synchronised, including the explanation of the mechanical jaw coupling. The deviation for an air inlet throttle of 0.01%, 50% and 100% are high, and a precise handling could not be realised. An exhaust air throttle of 0.4% has, in every uncoupled case, the worst results. Furthermore, 1.6% and 2% for the exhaust have the best repeatability results for all uncoupled settings. Based on the results, one can conclude that the exhaust air throttle has a more significant effect on the accuracy than the air inlet throttle. During the experiments, observations were made that explained the inaccuracies. Depending on the exhaust air throttle, there was a delay in the closing movement between the jaws, whereby one jaw reached the object earlier and forced it out of the centre. The accuracy achieved with non synchronised jaw cylinders is insufficient for the Tailored Forming Process.

Fig. 4
figure 4

Box plot of the experimental investigation. The influence of the parameters on the accuracy is tested. Three sets without synchronised jaw cylinders and one set with synchronised jaw cylinders

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The coupling minimised the deviation drastically as seen for the coupled air inlet throttle of 50% (synchronised) in Fig. 4. The best results are achieved for the exhaust air throttle of 1.6% with a deviation between 0.2 mm–0.7 mm. The deviation has to be optimised further to operate in the Tailored Forming Process successfully.

The validation provides the potential for further mechanical improvements. Some pins do not retract when they come into contact as assumed, whereby the object is moved. Investigations show that the manufacturing tolerances do not match every pin. The coaxiality of the cylinder drilling and the base plate drilling must be chosen more accurately to prevent pin clamping.

6 Summary

This work points out the advantages of shape variable grippers and their ability to adapt to varying geometries as well as their benefit for the forging sector. Current shape variable grippers can not be utilised in the forging sector because they are made of a polymer material that has a limited operating temperature of 300 \({}^{\circ }\text {C}\). The temperature of the forging object exceeds this limit by reaching temperatures up to 1200 \({}^{\circ }\text {C}\).

Therefore, a pin gripper that is variable in shape as well as resistant to high forging temperatures was developed. The pin gripper’s construction and the control to achieve precise handling were presented. The controlling is necessary for the automation of the handling process and to carry out the gripping routine.

After implementing the routine, experiments were carried out to verify the accuracy of the handling system and provide fundamental knowledge for optimisations. For the experiments, two parameters were investigated. First, the exhaust air throttle and second, the air inlet throttle. The exhaust air throttle impacts the system more than the air inlet throttle. The overall accuracy was insufficient, which is why a mechanical coupling of the jaws was installed. This had a positive impact on the accuracy. The experiments showed other critical points that must be investigated further, like the manufacturing tolerances.

7 Outlook

The observation during the experiments showed optimisation potential for the jaw’s construction. The current jaw consists of a base plate with mounted pin cylinders where the experiments identified an error source. The base plate and the pin cylinders can be manufactured as one part to eliminate the error. Consequently, the drillings achieve a better coaxiality and the risk of clamping during retraction decreases.

Furthermore, the routine could be enhanced by involving the sensors for the jaw’s positioning in the closing process. Currently, the jaws close until they are in contact with the object and cannot move further. In future, the jaws could be stopped before a contact appears, followed by pushing out the pins. This would be an additional electronic coupling to the mechanical one.

For experiments under forging conditions, secure and precise handling must be ensured beforehand. The experiments have shown where improvements are required, which means that the adjustments can be completed and the influence of temperature examined in future.