One common nonintrusive way to steal electricity
is to apply a strong permanent magnet or an AC magnet near the electricity meter,
thus tampering with the meter. A permanent magnet or an AC magnetic field can affect
meter components like current transformer current sensors, shunt current sensors
(shunts are only affected by AC magnets), or any power supply transformer. As a
result of the weaknesses of these components to magnetic tampering, utility
customers can be undercharged for their energy consumption, thereby allowing
consumers to essentially steal electricity.
Due to the susceptibility of meters to magnetic
tampering, magnetic sensors are often used in electricity meters to detect external
magnetic fields to take appropriate action, such as disconnecting services to the
meter or applying a penalty fee for magnetic tampering. In this design, magnetic
tamper detection is done with the TMAG5273 linear 3D Hall-effect sensor, which has
the following advantages compared to other magnetic sensing devices and designs:
- Ease of assembly: Hall sensors in general
are not as fragile as reed switches, the latter of which can break during
assembly.
- Only one surface mount IC needed: Sensing
in three directions with the TMAG5273 requires only one surface mount IC for 3D
linear Hall-effect sensors instead of the three ICs in the case of 1D
Hall-effect sensors. 3D linear Hall-effect sensors therefore enable a more
compact printed circuit board (PCB) layout. In addition, having a surface
mount-only implementation can reduce PCB manufacturing costs compared to a 1D
Hall-effect sensor implementation that can require through-hole sensors for
detecting some of the directions.
- Flexibility for defining magnetic tampering
threshold: Since 3D linear Hall-effect sensors provide information about
the actual sensed magnetic flux density value, it is possible to select the
magnetic tampering threshold of each axis to anything within the magnetic
sensing range of the 3D linear Hall-effect sensor. This enables configuring how
to define what is tampering, which can vary between designs since the magnetic
flux density sensed depends on the distance from the magnet to the sensor as
well as the characteristics of the external magnets to be detected. This type of
flexibility is not possible for Hall-effect switches with fixed magnetic
operating point (BOP) thresholds. Finding the appropriate tamper
threshold definition can be done by using a magnetic calculation tool to determine what is the
resulting magnetic flux density seen for the different magnet-to-sensor
distances and magnet types that must be detected. The magnetic threshold can be
then set to something lower than the magnetic flux density seen by the sensor
when exposed to the desired tamper conditions. Typically, it is desired to set
the threshold to be small enough to detect tamper magnets but also large enough
so that the system does not see any false positives from any nearby equipment
that causes a magnetic field that does not affect the functionality of the
meter. The magnet-to-sensor distance depends on where the sensor is placed on
the PCB as well as the dimensions of the e-Meter case. For small-sized systems,
the magnetic sensor can be placed near the center of the board for symmetrical
sensing coverage across the meter case, or the sensor can be placed near any
components that are affected by magnetic tampering. For large-sized systems like
certain polyphase meters, sometimes it is not possible for one magnetic sensor
to sense tampering across the entire meter surface, so multiple 3D Hall sensors
can be used and placed spread out with respect to each other on the PCB to cover
a large sensing area. The TMAG5273 has four sets of device orderables that are
factory programmed with different I2C addresses, which enable multiple devices
to share the same I2C bus.
- Ability to change between multiple device
power modes: The TMAG5273 supports switching between multiple power
modes, depending on if it is desired to reduce system current consumption. The
TMAG5273 has an active mode for taking measurements, a sleep mode for minimizing
current consumption, and a duty-cycle mode that automatically switches between
active and sleep modes. Typical use-cases of the different power modes for
electricity meters are described below:
- Active mode is used for
taking measurements and requires the most power out of the different
power modes. An example scenario where active mode is typically used is
when the Mains are available and the meter is running off the AC/DC
power supply. When running off the AC/DC power supply, the relatively
high active mode current consumption (2.3 mA) of the TMAG5273 is
negligible.
- In duty cycle mode, the
device takes measurements and then automatically goes to sleep for a
user-specified amount of time. Duty-cycle mode is good for minimizing
current consumption while still detecting magnetic tampering, such as
when low-speed magnetic tamper detection is necessary when running off a
backup battery. To reduce average current consumption in duty cycle
mode, select a long sleep time. When selecting the sleep time, set the
sleep time to be less than the desired response time for magnetic
measurements. As an example, to sense magnetic tampering every 2 ms
using wakeup and sleep mode, set the sleep time to 1 ms instead of 1
second.
- In sleep mode, the device
does not take any magnetic measurements. An alternative to wakeup and
sleep mode is to have the MCU manually set the sensor to sleep mode and
then manually set the sensor to wake up after the desired sleep time has
passed. This requires more overhead from the MCU; however, this option
can reduce the system current consumption if the MCU is going to have
its own wakeup and sleep mode that allows the MCU to reconfigure the
TMAG5273 during each wakeup and sleep mode cycle. For systems that do
not require detecting magnetic tampering when running off a backup
battery, the TMA5273 can just be put in sleep mode to reduce system
current consumption when running off a battery and then put back in
active mode when the system is able to run off the AC/DC power supply
again.
- GPIO pin interrupts when magnetic tampering
detected (depends on device): The TMAG5273 has the ability to set an
interrupt pin when the sensed magnetic flux density of any axis goes beyond a
user-defined magnetic switching threshold. To detect tampering, the user can set
the magnetic switching point for interrupts to the desired magnetic tampering
threshold. Since the interrupt pin of the Hall-effect sensor can wake up the
microcontroller when the MCU is in low-power mode, and since the microcontroller
does not have to read the Hall-effect sensor to determine magnetic tampering,
the MCU can go to low-power mode when running off a backup power supply until
woken up by the interrupt pin of the Hall-effect sensor. Used simultaneously,
the general-purpose input/output (GPIO) pin interrupt feature and duty-cycle
power mode can reduce system current consumption and extend the lifetime of the
backup power supply. Once the GPIO pin of the Hall-effect sensor wakes up the
microcontroller, the MCU can then retrieve the value of the sensed magnetic
field reading that caused the interrupt and then enable wakeup and sleep mode
with GPIO interrupts again.
- Detection of AC magnets: AC magnets do not
only affect current transformers. AC magnets can also affect shunt and Rogowski
coil current sensors. To detect AC magnets, a linear 3D Hall sensor can also be
used. Detecting AC magnets requires a fast-enough effective sampling period and
a small enough sleep time to properly capture enough samples along a cycle of
the AC magnet waveform, as Figure 2-2 shows.
The effective sampling period corresponds to the time needed to get one set of
samples, which is dependent on the internal sampling rate of the device. Since
linear Hall sensors provide information on the actual sensed magnetic flux
densities, the sensors are better able to detect AC magnets than a low-sample
rate Hall switch.