Design of a Differential Safety Mechanism (DSM) Dedicated to the NSW Micromegas Wedges

The New Small Wheel Micromegas detector system for the Upgrade of ATLAS Muon Spectrometer is in the phase of integration and commissioning at the Laboratories BB5 and Building 191 at CERN respectively. In this framework, the produced modules are evaluated and tested at a Cosmic Ray Stand or at their final position on New Small Wheel. Providing gas mixture to the Micromegas Wedges, the static gauge pressure inside the detector’s layers must be kept below a nominal value around 3 mbar. Pressures above 10 mbar, due several reasons or gas line blocking, could cause serious damages in the detectors. In this work we describe the principle of operation and the design of a low cost intelligent unit, the “Differential Safety Mechanism”, dedicated to protect the Micromegas Wedges against unexpected slow or sudden increase of the static gauge pressure. The internal detailed structure, the simulation and the prototype tests of the DSM are presented analytically in this work.


Introduction
The New Small Wheel (NSW) Micromegas detector Wedges for ATLAS experiment upgrade at CERN [1,2,3], during their integration pass from different sequential installations and QA/QC procedures, while a final performance operation test is performed in a Cosmic Ray Stand (CRS). In this test the two single Micromegas Wedges, that constitutes a double Wedge, is a complete detector to be installed to the NSW detector system at CERN. The protection of the Wedges against an unexpected gauge pressure increase at their operation test at CRS and also at their final position at NSW is essential. Because the gauge pressure inside the volume of the Micromegas volumes is not feasible, the only way to sense an unexpected pressure growing is to use an intelligent mechanism acting with an indirect way by sensing the growing of the static gauge pressure in the input gas line. We must note that a bubbler or a passive safety valve is improper for this purpose, as we explain in the next section. The Micromegas Wedges are designed to operate at low static gauge pressure, around 3 mbar, with an upper typical limit of 10 mbar. Higher pressures must be avoided due to deformations caused by volume expansion and the subsequent risks for damage. In the present work we describe the design and implementation a "Differential Safety Mechanism" (DSM) for protecting the Micromegas Wedges against unexpected and sudden increasing of the static gauge pressure inside its volume. This is achieved in a indirect way by sensing the sudden growing of the inlet gauge pressure in the gas line. The DSM is essentially an asynchronous digital sequential system based on  a adjustable voltage comparator and other digital logic circuits for driving a 3-way solenoid  valve at the correct time interrupting the pressure growing and releasing the Wedges to the atmosphere.

The basic idea and motivation
During the final performance operation of the Micromegas Wedges at CRS there is finite probability of static gauge pressure sudden increase due to several unexpected reasons. The most likely one is the blocking the intermediate part of the output gas line by accident by human action. In Fig. 1 the location of the DSM at the input gas line, at the point is shown. The risky gas line part for blocking is the A 2 A 3 . If this happens, the static pressure in this line and that inside the Micromegas Wedges IP and HO tend to be equalized converging rapidly to the level of the gas mixer output, that is, around 1 bar. Because the location of the blocking point is unknown, we are forced to put the safety mechanism before the Wedges, at the point A 1 . On the other hand, the maximum allowed static gauge pressure in the Wedge, around 10 mbar, is much lower than that in the input gas line (in the region of point A 1 due to the installed gas impedances in both sides of the two Wedges). For this reasons an appropriate mechanism must sense the growing of the pressure by a predefined amount, that is, to operate differentially. Based on this idea, the "heart" of the DSM is a voltage comparator by using OPAMP in which the reference voltage level is adjustable. The comparator gives a digital voltage signal (command) for activating a 3-way solenoid valve, normally open. This valve can perform two functions at the same time: a) interruption of the input gas line and b) gas release from the wedges to the atmosphere. The timing diagram of the DSM operation cycle is given in Fig. 2. Other needed functions are also included and are described next.  Diagram showing the operation of the DSM. Assumed a blocking at time t 0 , the pressure starts to increase until the predefined limit for DSM activation at time t 1 . Without the DSM protection the pressure at time t 2 should be of the order of 40 mbar.

Electronic configuration and logic
The main circuit of the DSM mechanism is a comparator driving a Flip-Flop. The sensing of the pressure increase is done by a high precision differential pressure transducer (DPT), type 144LP50D-PCB, from First Sensor [4], while the voltage difference can activate the 3way normally open solenoid valve, type VLV 11-10-4-BV-5P88 [5]. In the basic electronic configuration of DSM in building blocks, given in Fig. 3, we can see the location of the voltage comparator, the driven J-K Flip-Flop which, in turn, provides the activation command to the During the normal operation of the Micromegas wedges we provide gas at a nominal flow rate which some times is adjusted manually in lower or higher level. At any flow rate the static gauge pressure in the input is developed due to the "external impedances" used for uniform gas distribution among the Micromegas wedges (see ref. [?] for more details) of each wedge by a non-linear way. The relationship between the flow rate and the static pressure is known and therefore, in any selected flow rate we expect a corresponding value of the nominal static pressure. The differential pressure transducer (DPT) we use in the DSM has a full scale 0-50 mbar with an output voltage in the range 0-5 V leading to a pressure to voltage conversion factor equal to 0.1 V/mbar. For a typical nominal static pressure corresponds a voltage output of the DPT which we call "Threshold Voltage", V TR .
In order to set a tolerance above the nominal static pressure (and thus in voltage) as a critical acceptance limit we define the voltage that we call "Reference Voltage", V REF , which corresponds to a static pressure 7 mbar above the nominal one. This value comes from the maximum allowed static pressure in a Wedge, 10 mbar, including the typical pressure in the output due to the bubbler, which is around 3 mbar. Therefore, the pressure difference for sensing any sudden blocking of the gas line is 10-3=7 mbar. The reference voltage is calculated as folows: . Both solenoid valves remain in the same state until reset (recovery) procedure is performed. A buzzer is also used for providing a "beep" signal during the activation. (ii) For performing the reset function, we must firstly zeroing the gas flow rate and secondly to press the RST (reset) button for at least 2 seconds. These steps are required because, even though the Micromegas Wedge is diverged to the atmosphere reversely from its input gas line, the pressure is suddenly increased by pressing the RST button. (iii) If an electric power cut happens, DSM can not be inactive for protection but the gas should be still flowing uninterrupted. However, there is an option to use a Li-ions battery to extend its operation for about 3 hours. (iv) By pressing the test button we can test and verify the functionality of the activation procedure of DSM.

Electronic schematic and simulation
The detailed electronic design is given in Fig. 4 while a 3D model of the pcb card is presented in Fig. 5. In the Table 1 the gas flow variable and the corresponding voltage output of the DPT are presented.
During the design phase, simulations were performed using PSpice [6], for optimizing the performance of operation. A test voltage input of triangle-shape pulse was used. The schematic and the resulting plots are shown in Fig. 6 and Fig. 7 respectively. Also a test with a prototype PCB has been performed by using again the same voltage input (triangle-shape). In Fig. 8 the voltage output observed in a screen of a oscilloscope is shown.

Performance tests and implementation
The overall functional test of the DSM prototype has been performed in two stages: a) under real conditions with a setup emulating the Micromegas wedges connected in parallel to a gas line (channel) including two external impedances. We used also an electronic flow meter, a shut-off valve and a portable differential manometer with display. We were regulated the flow rate at the values given in Table 1. In this test the main goal was to verify the functionality of the logic of the mechanism and as well as its stability against electrical voltage spikes inside the Laboratory  BB5 where high electrical current instruments, like chillers, cranes and voltage power supplies are in use. b) in connection to a real Micromegas Quad to study the pressure rising curves and the time constant of the system. Moreover, this test helped us to construct a precise table of settings and a corresponding model. In Fig. 9 the testing setup in connection to a Micromegas Quad LM2-M38 is presented. Blocking the output line by using a shut-off valve, the Quad due to its inherent elasticity presents a time constant during its overpressure (about 10 min). The time constant depends only on the volume of the detector under protection. Therefore, of a wedge the time constant is expected to be twice, that is, about 20 min. As an indicative result, after blocking the output line the pressure in the Quad output was increasing fron 1.3 mbar while the pressure in the input was increased from 17 mbar to higher values with slower rate than that of the Quad. After 3 min from the blocking time, the pressure in the input reached the cut-off value, 24 mbar.  Figure 9. The setup used for the test of the baseline DSM-V1 providing gas mixture to the Quad LM2-M38 by using two external solenoid valves N.O. and N.C. respectively. These two solenoid valves have been used because the 3 way, 2 position solenoid valve was not yet available.

Design of advanced version DSM-V2
The advanced version of DSM, essentially, is based on a micro-controller which manages two inputs (the flow rate and the static gauge pressure) and one output (the command to the solenoid valve) shown in Fig. 10. The functionality of DSM-V2 is based on a the Finite State Machine configuration, given in Fig. 11. In an appropriate software code the state diagram should be realized. As in the baseline version DSM-V1, the 3-way, 2 position solenoid valve operates as follows: the gas line becomes closed at port 3 while the gas of the Micromegas Wedge volume is diverted to the atmosphere from port 2 to port 1. The main events that must be identified and the associated actions are the following: (i) Detection of a slow rate rising of the gas flow rate, with respect to pre-set one, r q , and a corresponding pressure rising rate, r p . This event corresponds to a conscious desired