Press machine manufacturers are confronted with increasing technological and cost pressure: Many customers demand faster and ever more precise presses. The more precisely force is applied in a press machine, the higher the quality of the manufactured part is. This is why, increasing the press machines accuracy is one of the most important challenges for manufacturers of these assets. In addition, the market requires increasingly faster press machines that, at the same time, offer higher bandwidth to increase production output in existing systems.
Nowadays, the torque of the press gear shaft is measured indirectly from the force that is applied in the connecting rod. This measure is quite precise but it needs to be continuously recalibrated to obtain the accurate quality. To solve this problem, the technological answer is to measure the torque directly by using wireless sensors placed in the press gear shaft.
As a solution, IKERLAN has designed and manufactured a prototype of a shaft-adapted wireless sensor node which comprises a transducer based on torque oriented gauges, a signal conditioning circuit and a signal processing software, the latter allowing a local preprocessing and treatment of the collected data, by means of intelligent functions.
The design process has been made following two main phases:
Phase 1: Testbed validation
Before starting the development of the wireless torque sensor, a previous validation was made in testbeds both in IKERLAN. This was an initial requirement to ensure the proper functioning of gauges, generic electronics and wireless for working in press-based conditions.
With regards to wireless communications, two main challenges were tested: (i) signal attenuation due to the rotation of the emitter around the shaft and (ii) multipath fading due to RF signal reflections in the metallic (steal) elements of the head of the press in which the torque sensor is to be installed. Tests were successful being outlined that depending on the angular position of the shaft, and therefore, on the relative position of the transmission and reception antennas, more or less amount of power is received periodically.
A similar test has been performed in the Try-Out press machine from FAGOR ARRASATE. In this case, both the emitter and the reception antenna have been placed in a realistic place within the head of the press machine as in can be seen in the next figure.
Once the top cover is closed, creating a complete metallic case, it was observed how the received signal was not as clean as the one in the previous measurements due to multipath reflections. The statistical features obtained from this signals were used in the selection of the most suitable wireless communication technology to be used in the torque sensor.
Phase 2: Design and development
Once the concept and the elements of the device (gauges, conditioning and processing, radio) were validated in a rotational environment, the system design and development was started. A prototype of the wireless sensor node was designed and developed. It consists of a single PCB with the necessary interfaces to attach torque gauges, besides the conditioning, processing and wireless communication electronics. The whole system is powered by a rechargeable lithium ion polymer battery and it is encapsulated and protected by a plastic cover in the shape of the press’ secondary driving shaft, which is prepared to avoid oil leakage.
Once the design and fabrication of the wireless torque sensor was finished, the sensor was installed in the Try-Out press machine from FAGOR ARRASATE.
First tests regarding the overall performance of the sensor were successful providing signals with the torque measurements were sent to an external laptop were they could be visualised. Later, the complete validation process was carried out. This process aimed to test the accuracy of the sensor’s measurement against several torque and speeds and the robustness of the wireless communication protocol employed.
Several tests were carried out combining different values of the nominal torque and speed of the press as well as several configurations of the sensing electronics. These results were compared with an estimation of the torque at the drive shaft obtained from an overload pressure evolution analysis. Besides, some measurements regarding the performance of the wireless communication were also taken. As an example of it, Figure 6, shows the results of the test in which the maximum torque (87%) and the maximum speed (100%) were configured at the press machine.
The measured torque values at almost each stroke are close to 60kN•m which corresponds to the estimated torque values. Moreover, the clutch brake engage and disengage events are still captured.
In general terms, it is considered that the obtained results are valid, taking into account that they are compared with estimated values and not with another measurement obtained by a commercial system. However, regarding the amount of data shown at the measured torque values, some data can be missed either on the positive or the negative peaks, as the same amplitude should be acquired for each stroke. With regards to wireless communications, in general the expected performance in terms of data throughput and network availability has been achieved. However, the loss of some data packets has been detected which should be corrected in future versions.
As an important upgrade of the system, it is expected that the inclusion of antenna diversity inside the shell of the press machine will improve the communication between emitter and receiver. This new configuration should decrease the number of packets lost.
Another point of improvement is the detection of low depths of penetration. To achieve this, new tests will be performed modifying the gain parameter of the wireless sensor node and the obtained results will be analyzed.
Last but not least, the energy management of the system is a key feature if it is pretended to leave it permanently attached to the press machine’s drive shaft. With this in mind, a more energy efficient redesign will be carried out together with the development of an energy harvesting system to power up the wireless sensor node