Sensors

5.3.4.1 Accelerometer

The purpose of the accelerometer for our system is to detect movement after the receiver is deployed. For example, let’s say the receiver is attached to a vertical surface and the operator has relocated to a safe location without line of sight to the receiver. If the receiver dislodges and falls down the operator has no way of knowing whether the receiver is still functional. With the accelerometer implemented, the operator will get a warning that the receiver has moved since it was deployed.

To cover this functionality we have implemented a 3-axis accelerometer that has a mea-surement range of ±3.6g [32]. This is a general purpose accelerometer, not something you would use in high performing applications, but it will be more than good enough to de-termine if the receiver has moved or not. It is also shockproof for up to 10000g [32] which should make it robust enough to handle the demanding circumstances our system has to endure. This is a low priority subsystem for our customer so there is not much time spent in its development or design.

5.3.4.2 GPS

The GPS module in our system will provide the location of the deployed receiver. This will allow the operator to monitor the receiver’s whereabouts and see if someone has moved it from its original location. It also makes it easier to retrieve the receivers after the operation is concluded, assuming the receiver is still intact.

There are two fully functional GPS satellite systems, the russian GLONASS system and the NAVSTAR ownd by the U.S. but there is more coming, as both china and the EU is working on finalizing their own equivalent systems. China is working on the BEIDOU system and the EU is developing the Galileo GPS system.

All the mentioned systems will have, or has, a global coverage; how much they differ in performance is still unclear. We plan to implement a GNNS module in our system, this is a module that is capable of receiving and handling data from all the previous mentioned systems and will provide our customer with more flexibility when it comes to further de-velopment.

For the GPS system to calculate the location of the GPS receiver it needs to receive a signal from 4 different satellites, 3 of them are used to calculate the location, and the fourth is used as a clock signal as every GPS satellite has a high precision atomic clock.The GPS receiver can then use the time delay to calculate its position.

How long it takes for a GPS to get a connection and a location depends on several factors, most of them from the surrounding environment. The ones we want to focus on is the data needed for the GPS receiver to determine its location. When starting up the GPS has to check if it has valid Ephemeris and Almanac packages [72] for the area it is and if not, it has to download the packages from the satellite network.

If the GPS has a cold start it must download the Almanac package from the satellites.

Each satellite sends a signal every 30 seconds and part of that signal holds a fragment of the Almanac package. The download normally takes about 15 minutes with normal reception [50], but can take longer depending on the number of satellites in range.

The last is the hot start and has a typical time of a few seconds before the first fix, after which the system holds valid Ephemeris and Almanac packages, and only needs to calcu-late its position.

When the GPS system is turned on for the first time it will be in a cold start situation where it must update all of its data. This will also be necessary when the GPS system has been off for a long time or when the system has been moved far from the last active location, like traveling to another part of the world.

The warm start is common when the GPS system has been off for a few days or you trav-eled to another part of the country. A hot start is when the GPS system has been off for a few hours and you are close to the same location where the GPS was last active.

The system we are implementing is not bound to be used in just one country or one loca-tion, so some sort of routine or protocol around when to start up and let the GPS system update will be necessary. Especially for operations where the location of the receiver is mandatory. Maybe towards SR-035.

The active antenna for the GPS has been chosen as a result of the bandwidth and the built-in filtering capabilities, as this saves us some time in development because we don’t need to design any filter circuitry, which in turn means we get less components that take up space on the PCB. This is a low priority subsystem for our customer and therefore little time is used on the design and component selection.

The GNNS module will be able to cover all the functionalities needed for our system. It will also be a building platform for further development and testing as it is capable of utilizing the two existing systems and the once that are on their way. The size of the component and the few external components needed to implement this IC was two major contributors for why this IC was chosen.

5.3.4.3 Battery capacity estimation

To fulfill SR-049 we must be able to estimate battery condition during runtime. We use the built in Analog to digital converter (ADC) of the ATmega2560. By using the ADC together with an transistor controlled voltage divider we can measure the voltage of the battery. By knowing the voltage of the battery it is possible to estimate the battery con-dition, and estimate when it will need to be replaced. The battery discharge curve can be found in Figure 60. We observe that the flat regions is where the battery will have a fairly high capacity. The battery voltage is strongly correlated with the temperature of the battery as can be seen in Figure 60. Therefore the design integrates a temperature sensor as described in EL-006. The battery capacity can therefore be calculated in software using voltage and temperature data. A simple switch case function could be used to correlate dif-ferent predefined temperature ranges with the output voltage, thus estimating the capacity.

The STLM20 temperature sensor is an ultra-low current 2.4 V precision analog temperature sensor for low current where maximizing battery life is important. It can be ordered as an IC and only uses 4 pins. The output voltage is ideally proportional with the ambient temperature of the device, and the parabolic transfer function can be expressed as:

V_O = (−3.88·10⁻6·T²) + (−1.15·10⁻2·T) + 1.8639 (14) Solving for T in Equation 14 yields

T =−1481.96 + r

2.1962·10⁶+1.8639−V_O

3.88·10⁻6 (15)

By using Equation 15 we can estimate the temperature of the device with an accuracy of

±2.5^◦C[132]. The STLM20 can handle captive loads up to 300pF. Since the ATMega2560 ADC has an input capacitance of 10pF the two devices are compatible.

In Figure 108 you can see the schematic of the battery measurement circuit with off-page connectors to the ATmega2560 and power supply. Here, we are using an enhancement MOSFET, which is normally open. This is to ensure power consumption is minimized as much as possible. The ATmega2560 needs to activate the E-MOSFET to take a mea-surement, and only then will there be a small current. The leakage current through the transistor in series with 499k Ω resistor is negligible. A future improvement of this design is to add additional components to protect against reverse polarity, since this design does not take that into account.

Figure 108: Schematic for battery measurement for SPARK PCB

5.3.4.4 Watchdog

The demand of the implementation of a watchdog in the system was given to us 16.05.2020 , where the watchdog shall serve as an extra safety barrier that ensures that nothing goes wrong in the event that the main controller fails. The main concern was aimed at situations where the system was connected to explosives, in which case a failure would be catastrophic.

The watchdog will be implemented between the main controller and the circuitry that controls and executes the ARM, DISARM and DETONATE functionality of the electrical system shown in Figure 109.

Figure 109: Watchdog Implementation

The watchdog is powered by the ATmega 2560 through the WD enable pin and the timer is reset with a rising edge on the WD timer pin. The duration of the timer is 102ms [133]

and when the timer runs out the output of the watchdog goes low which in turn disables the 350V and -12V step up circuits. With the 350V step up disabled, the system cannot charge up the capacitors and hit the -12V, and with the step up system disabled the system cannot send the energy to the plasma igniters. The circuitry that uses the -12V step up as supply is by default in discharge mode 5.3.3.2.

With this configuration we minimize the chance of an accidental detonation even further.

If the MCU fails there is no telling which state the I/O pins take, but with the watchdog there to disable the most critical subsystems, the MCU would have a hard time energizing