Josep Sabater

IEEE 802.15.4 based wireless monitoring of pH and temperature in a fish farm

In the last years the number of papers related to wireless sensor networks has increased substantially. Most of them focus in raising issues as routing algorithms, network lifetime, and more recently, Multiple Input Multiple Output wireless networks. In contrast with all those studies, we present a practical application of wireless networks: The sensing of the pH and temperature for a fish farm. The application requires two different kind of modules: the sensor itself and the wireless module. The sensor collect and transmit the information to a wireless module using a wired connection. Once the information reaches the wireless node, it is forwarded to the central unit through a wireless protocol. The central unit starts and manages the network, as well as stores all the received data. The sensor module includes an pH sensor based on a specially designed ISFET and a commercial temperature sensor. The wireless node collects the sensed data by means of an asynchronous wired serial polling communication. The use of this kind of protocol allows to connect a single master with multiple slaves. In our particular case, we have connected one master with four slaves using a transmission rate of 9600 b/s. The wireless transmission follows the standard IEEE 802.15.4, and implements the routing protocol based on the ZigBee standard. The number of nodes distributed in the fish farm has been limited to 30 while the maximum number of hops to 6. Moreover, between the MAC and the routing layer an energy management layer have been included. This layer reduces the power consumption of the wireless network using an RF activity duty cycle for the reception stage at the final end device of around 0.02%.

System model of an image stabilization system

The Polarimetric and Helioseismic Imager (PHI) instrument is part of the remote instruments for the ESA Solar Orbiter (SO), which is scheduled to launch in 2017. PHI captures polarimetric images from the Sun to better understand our nearest star, the Sun. A set of images is acquired with different polarizations, and afterwards is processed to extract the Stokes parameters. As Stokes parameters require the subtraction of the image values, in order to get the desired quality it is necessary to have good contrast in the image and very small displacements between them. As a result an Image Stabilization System (ISS) is required. This paper is focused in the behavior and the main characteristics of this system. This ISS is composed of a camera, a tip-tilt mirror and a control system. The camera is based on a STAR1000 sensor that includes a 10 bits resolution high-speed Analog-to-Digital Converter (ADC). The control system includes a Correlation Tracking (CT) algorithm that determines the necessary corrections. The tip-tilt mirror is moved based on this corrections to minimize the effects of the spacecraft (S/C) drift and jitter with respect to the Sun. Due to its stringent requirements, a system model has been developed in order to verify that the required parameters can be satisfied. The results show that the ISS is feasible, although the margins are very small.

Software management of power consumption in WSN based on duty cycle algorithms

In battery-operated nodes of a wireless sensor network, energy is a scarce resource that should not be wasted. During the recent years, energy efficient algorithms and protocols to maximize network lifetime have attracted the attention of many researchers. As the radio transceiver is one of the elements of the node that has higher power consumption, a typical approach to save energy is to minimize the period of time it is active. That can be achieved by introducing a duty cycle in which the node periodically enables its radio for a short time to later return to an idle state. In this paper, we discuss the importance of the duty cycle window size and present an expression to determine it according to the particularities of the IEEE 802.15.4 specification. But, to obtain a more accurate expression, we show that it is necessary to consider the characteristics of a noisy channel, especially in industrial applications, where interferences cannot be underestimated. The proposed technique tries to minimize electromagnetic pollution in order not to disturb the normal operation of other radio devices in the environment. The results of a real implementation are also presented and its efficiency discussed in terms of boundary conditions.

Towards an IEEE 802.15.4 SDR transceiver

One of the goals of a software defined radio system is to be adaptable to send and receive data according to several specifications. That can be achieved if that system implements most of the communication operations, including physical layer tasks, in reconfigurable platforms. This paper introduces an all-digital transceiver based on the 868 MHz band of the IEEE 802.15.4 standard. The design consist of a transmitter with a bandpass delta-sigma modulator and an undersampling receiver. A Xilinx Virtex-5 FPGA is used to implement the proposed model.

Using clustering for disperse objects fields segmentation in MIRADAS instrument

Mid-resolution InfRAreD Astronomical Spectrograph (MIRADAS), a near-infrared multi-object spectrograph for Gran Telescopio Canarias (GTC), has 12 deployable optomechanical Integral Field Units (IFU). Based on a robotic probe arm with a pick-off mirror, each of these units can observe a different user-defined sky object. MIRADAS can work with target sets where their components are spread over such a wide area so that all of them do not fit in the field-of-view of the instrument. Therefore, data sets of that kind require, prior to capturing them, some arrangement that groups its elements in different subsets where the distance between the two most remote elements is inferior to the field-of-view diameter. This field segmentation is achieved using a hierarchical clustering technique. Our method relies on determining mutual nearest-neighbors, which will be merged if they show a given degree of similarity known beforehand. Moreover, we also compute a geometric center for these clusters, information to be delivered to the telescope’s pointing process. This step is formulated as the minimum bounding disk problem, which founds the center of the smallest radius circle enclosing all points of a cluster. Finally, we consider several real science cases and analyze the performance of the proposed solution.

APS fixed pattern noise modelling and compensation

In this paper we describe different procedures to compensate the fixed pattern noise (FPN). These procedures have been developed and tested for a camera designed by us, which is based on a CMOS active pixel sensor (APS). In this kind of sensors, the FPN mainly depends on the noise coming from the column selection electronics. In this sense, we suggest and assess three different methods for FPN compensation: using a simple model taking into account this dependency, using an external test signal routed through the image chain and finally using a region of the sensor which is not illuminated. The first two procedures are of special interest for camera systems lacking a shutter or a not illuminated region in the sensor. The last one is more similar to the typical approach, but requires using a larger sensor only to have some covered light sensitive area.

Design and test of a tip-tilt driver for an image stabilization system

The tip/tilt driver is part of the Polarimetric and Helioseismic Imager (PHI) instrument for the ESA Solar Orbiter (SO), which is scheduled to launch in 2017. PPHI captures polarimetric images from the Sun to better understand our nearest star, the Sun. The paper covers an analog amplifier design to drive capacitive solid state actuator such ass piezoelectric actuator. Due to their static and continuous operation, the actuator needs to be supplied by high-quality, low-frequency, high-voltage sinusoidal signals. The described circuit is an efficiency-improved Class-AB amplifier capable of recovering up to 60% of the charge stored in the actuator. The results obtained after the qualification model test demonstrate the feasibility of the circuit with the accomplishment of the requirements fixed by the scientific team.

Target allocation and prioritized motion planning for MIRADAS probe arms

The Mid-resolution InfRAreD Astronomical Spectrograph (MIRADAS) is a near-infrared multi-object echelle spectrograph for the 10.4-meter Gran Telescopio Canarias. The instrument has 12 pickoff mirror optics on cryogenic probe arms, enabling it to concurrently observe up to 12 user-defined objects located in its field-of-view. In this paper, a method to compute collision-free trajectories for the arms of MIRADAS is presented. We propose a sequential approach that has two stages: target to arm assignment and motion planning. For the former, we present a model based on linear programming that allocates targets according to their associated priorities. The model is constrained by two matrices specifying the targets’ reachability and the incompatibilities among each pair of feasible target-arm pairs. This model has been implemented and experiments show that it is able to determine assignments in less than a second. Regarding the second step, we present a prioritized approach which uses sampled-based roadmaps containing a variety of paths. The motions along a given path are coordinated with the help of a depth-first search algorithm. Paths are sequentially explored according to how promising they are and those not leading to a solution are skipped. This motion planning approach has been implemented considering real probe arm geometries and joint velocities. Experimental results show that the method achieves good performance in scenarios presenting two different types of conflicts.

A space grade camera for image correlation

In this paper we describe the space grade camera we developed for the Polarimetric and Helioseismic Imager (PHI) instrument of the Solar Orbiter (SO) mission. The camera, called Correlation Tracking Camera (CTC) will be part of the Image Stabilization System (ISS) used to compensate the spacecraft jitter. Since the ISS works on a correlator basis, the CTC requires a high frame rate while keeping the power consumption as low as possible. The CTC works at a nominal frame rate of 414 fps for 128 pixels square images with a latency below the microsecond. The images have a 10 bit resolution and the tests shows an effective number of bits (ENOB) above 9.3. Also, the full ISS closed-loop has been successfully tested with this camera.

Demonstration of high-performance cryogenic probe arms for deployable IFUs

We describe the design, development, and laboratory test results of cryogenic probe arms feeding deployable integral field units (IFUs) for the Mid-resolution InfRAreD Astronomical Spectrograph (MIRADAS) - a near-infrared multi-object echelle spectrograph for the 10.4-meter Gran Telescopio Canarias. MIRADAS selects targets using 20 positionable pickoff mirror optics on cryogenic probe arms, each feeding a 3.7x1.2-arcsec field of view to the spectrograph integral field units, while maintaining excellent diffraction-limited image quality. The probe arms are based on a concept developed for the ACES instrument for Gemini and IRMOS for TMT. We report on the detailed design and opto-mechanical testing of MIRADAS prototype probe arms, including positioning accuracy, repeatability, and reliability under fully cryogenic operation, and their performance for MIRADAS. We also discuss potential applications of this technology to future instruments.