“The global power stolen and non-technical losses are huge, which greatly affects the interests of power supply departments and users. Dismantling the meter is a common non-technical loss, and the electricity bill is reduced by stopping the meter and slowing the meter. In order to prevent this phenomenon as much as possible, the tamper detection function is very important.
The global power stolen and non-technical losses are huge, which greatly affects the interests of power supply departments and users. Dismantling the meter is a common non-technical loss, and the electricity bill is reduced by stopping the meter and slowing the meter. In order to prevent this phenomenon as much as possible, the tamper detection function is very important.
Figure 1 Appearance of the watch case
Traditional meter disassembly detection can be done by mechanical design. Make a downward protruding part on the case, and then place a button on the corresponding position of the PCB so that the protruding part of the case presses the button, and the output of the button is connected to the input port of the MCU. When someone opens the case, the case and the buttons are separated, and the input of the MCU is changed, and the MCU alarms. This traditional approach has low cost and low energy consumption, but there are many restrictions on its use. The first is the issue of reliability, because the different heights of the button depression may falsely trigger the tamper detection during transportation and installation. In addition, if the button is jammed, the tamper detection will directly fail. In order to prevent this from happening, it is necessary to use high-reliability switches and reset chips, which will increase the cost of the system.
Here is an introduction to the anti-dismantling detection scheme based on the high reliability of the Hall sensor, and the good power consumption and low cost.
1 Anti-dismantling detection scheme using Hall switch
The output of the Hall switch can Display the appearance and disappearance of the magnetic flux density relative to the threshold value. The output of the unipolar Hall switch and the bipolar switch are shown in the figure:
Because the bipolar Hall switch can respond to both the south magnetic pole and the north magnetic pole, it has a wider application range and is more suitable for anti-dismantling occasions.
Put a magnet on the opening of the watch case. When the watch case is opened and the magnet is far away from the Hall device, the detectable magnetic flux density disappears and the output of the Hall switch is changed to realize monitoring. The power consumption of this scheme can be very low, for example, the average current of DRV5032 is 0.54uA.
The reference design TIDA-00839 provides a tamper-proof solution for electric meters. Use the bipolar Hall switch DRV5033 to achieve. The design uses two sets of DRV5033, three in each set, to realize X/Y/Z detection in three directions, and the two sets of Halls are respectively placed near the current transformer and power transformer that are vulnerable to magnetic attacks.
Figure 4 TIDA-00839 circuit diagram
2 Anti-dismantling detection scheme using linear Hall sensor
Unlike Hall switches and latches, the output of a linear Hall sensor is not a two-level switch, but a value proportional to the magnetic flux density, which can achieve more accurate measurement.
Imagine that if a large external magnet is placed near the Hall switch when the case is disassembled, the magnetic flux density will be saturated, the output of the Hall switch will not change, and the fact that it was disassembled cannot be correctly reflected. And if we use a linear Hall sensor, the change of the magnetic field is likely to change the output range, and thus be detected, which provides a more robust and more accurate solution.
The output of linear Hall sensor has different types, such as:
1) DRV5055 can respond to north and south magnetic poles, and the output voltage is proportional to the magnetic flux density
2) DRV5056 only responds to the south magnetic pole. For applications that sense one magnetic pole, this response can maximize the output dynamic range. For the change of the unit magnetic flux density, the output range doubles, and the magnetic sensitivity also doubles.
3) DRV5057 outputs a clock with a frequency of 2kHZ and a duty cycle that varies with the magnetic flux density. When there is voltage noise or ground potential mismatch, signal integrity can be maintained. This signal is suitable for long-distance transmission in a noisy environment, and the always-present clock enables the system controller to confirm a good interconnection.
Figure 5 DRV5055 output
Figure 6 DRV5056 output
Figure 7 DRV5057 output
3 Anti-dismantling detection scheme using 3D linear Hall sensor
As mentioned in the first part of “Tamper Detection Solution Using Hall Switches”, in order to detect the magnetic flux density on a three-dimensional level, TIDA-00839 combines three single-axis linear Hall sensors into a group for detection.
Using a single 3D linear Hall sensor, such as TMAG5170, can achieve a wider detection range, making the placement of the magnet more flexible.
Figure 8 Schematic diagram of 3D linear Hall sensor
In summary, this article introduces three anti-dismantling detection schemes based on Hall sensors with high reliability and low power consumption. The scheme of the Hall switch is simple and easy to implement, with low power consumption. The linear Hall sensor solution solves the scene of external magnetic field interference during disassembly, and provides a solution with stronger robustness and higher accuracy. Finally, a brief introduction of the tamper detection scheme using 3D linear Hall sensors can achieve a wider detection range and make the placement of the magnet more flexible.