An Electrical Power Monitoring System for Automotive/Marine Applications
This project (in the following text referred as PowerMonitor) was initially targeted
to serve as a measurement tool
for 12V/24V electrical power systems in harsh environments. In particular for an
off shore sailing yacht where it was and is intended to act as some kind of
multimeter for some electrical power buses. There it is being used to watch
over various voltages and currents that are involved in a system consisting
of multiple batteries, generators, and photovoltaic solar panels.
Meanwhile, this system is serving its duty within the target application
Here are a few of the basic requirements for the PowerMonitor:
- Direct power supply from 12V or 24V raw power rail
That is, the PowerMonitor can be directly attached to a more or less unregulated
and unstable power bus. This also means that it must survive intermediate
voltage spikes of up to 60V. Such high voltages are typically assumed for automotive systems
that include generators and motors.
In the end, the PowerMonitor can be supplied from a wide input range of 7V-42V DC.
- Distributed measurement capability
The PowerMonitor needs to be able to measure voltages and currents directly at
various critical locations.
Due to the accuracy of the measurements it is not possible to just feed some wires
to a central unit that is finally carrying out the measurements.
So the overall design has been laid out so that it is consisting of so-called
Measurement Units carrying out the actual measurements very close to the
area of interest by means of performing the needed analog-to-digital conversion.
The digitized voltage and current samples are sent digitally to a Central Unit
where they are processed further.
The final system supports up to four Measurement Units, where each one is
capable to measure one voltage and two currents. Each Measurement Unit can
be attached to a different electrical circuit and the two current and one
voltage channel share a single reference, which is typically the plus pole
of a battery.
- Very accurate and wide range of high-side current measurement capability
Well, what does "very accurate and wide range" mean? Wide range means that currents
into the hundreds of Amperes need to be measured. But at the same time it shall
be possible to measure currents in the range of Milliamperes. In fact, the nominally achieved
current measurement resolution is 3-4mA assuming an 100A/60mV shunt.
"high-side" means that the currents are to be measured at the plus pole of the according
system rather than in the ground path. This is the preferred way for measurements involving
batteries where you are interested in the currents flowing into and from various other
systems. However, precise high-side current measurements cause additional complexities,
especially when higher voltages are involved.
- Also very accurate voltage measurement capability in the interested range
The voltage measurement is not that difficult as the current measurement. It has
been laid out to formally work at a nominal resolution of 5mv (10mV was the requirement) and is
working across a range of +/-160V (DC, of course). Although this is unnecessarily high,
this has been chosen in order to be able to have a rather high protection level of the
voltage measurement channel to survive even higher discharges or whatever.
It needs to be mentioned that in the configuration that has been finally chosen the
voltage measurement is slowly becoming inaccurate when the input voltage is between -5V and
+5V or so. However, this is not an issue, as the nominal system voltage is usually around
at least 12V.
- Low power design
Low power consumption is always an issue, especially in self-sufficient systems.
The System is consisting of
Central Unit Main Board
- A Central Unit Main Board
This board is containing the primary processing hardware and firmware.
Up to four measurement units can be attached to the main board.
- A Central Unit IO Board
This is a supplemental board that is being hooked up to the main board
and just contains a LC display and a few buttons
- The Measurement Units
Those are more or less identical units that are located directly at the
position where the measurements are to be carried out.
The following picture is showing a photograph of the primary side of the main board.
Here are a few facts about the main board:
- The board is a 4-layer board and is 12cm by 9cm in size.
- As processor the 8051 that is integrated within a Cypress FX2LP USB Controller
has been selected (above the USB connector). This decision has been made because
the USB connection seemed useful for reading back logged measurement data
(never implemented, actually....) and it was also quite convenient and simple in
view of the firmware development.
Last but not least the FX2LP was a well-known device, which is also an important
The firmware has been implemented in C (partially assembly language) using
SDCC. However, as it turned out, this processor is hardly at its limit for this
application. Not because of its calculation power, but because of its limited
This processor is also running a tiny multi-threading like task scheduler that
has been specifically developed for this project. More detailed information
about the task scheduler is available on a dedicated project page
(An Ultra Small Real Time Multi Tasking Kernel for Embedded Applications).
- The firmware code as well as other vital configuration information and
some logging information is stored within I2C EEPROMs located left to the
- At the upper/left area a real time clock and an associated backup battery can be seen.
The RTC is a Maxim (formerly Dallas) DS3231 very high precision RTC. The battery
is a Lithium Mangan Dioxide rechargeable one which will become automatically charged when the
system is being powered up. It should keep the RTC running for about 2 years
without turning the PowerMonitor on.
The primary purpose of this RTC was for accurate time-stamping of logged
measurement data. However, that has never been implemented. So the RTC is
just acting as a precision timekeeper now....
- At the upper area there are some switching power regulators. Specifically
important is the main input regulator which one is deriving 6V from the unregulated
input power supply. A Linear Technology LT3435 is being used here. The remaining
two regulators generate 1.8V resp. 3.3V for various devices.
- The Lattice Chip (LFXP6C) is an FPGA that is containing vital logic resources
especially for the packet communication with the measurement units. It does also
provide some additional RAM to the 8051. Furthermore it allows the 8051 to communicate
with some SPI NOR Flash EEPROMs that are located above the FPGA.
- The NOR Flash EEPROMs are M25Pxxx types from ST Microelectronics (now Numonyx).
Only two of them are soldered on the board shown on the photograph. Initially they
were intended to log measurement data for a later analysis. However, that has never
been implemented, and doing so at the level of interest seems not possible with the
- To the right the connector for the measurement units is visible, and left
to this connector the RS485 transceivers for the electrical communication level.
For simplicity, an RJ45 connector system has been used. The electrical connection
to the measurement units is being equipped with some automatic detection mechanism
as well as with an electronic fuse mechanism in order to automatically turn off
a measurement unit in case an unusual over-current has been detected. This mechanism
has been tested by just shortcutting the power supply of a measurement unit
(outch! :-) and is working well.
- The connectors at the bottom provide a standard RS232 serial interface (that
has never been used so far) and the USB connection. As previously mentioned, one
of the primary purposes of the USB connection was to be able to read back logged
measurement data into a regular computer (notebook etc.) for analysis purposes. However, this has
never been implemented. So far, there has been just defined some protocol to
allow an in-field firmware update (I2C EEPROM access) via USB.
- The Connector at the top/left (mounted on the reverse side) is the connector
for the IO board for interfacing with the display and the buttons.
- On the left edge of the board there is visible a provision for another
connector where some additional hardware might be attached (who knows.... ;-)
Central Unit IO Board
The following picture is showing the most interesting side of the IO board.
Here are a few facts about the IO board:
- The board is also 12cm x 9cm wide but has just 2 layers.
- The main part of the board is a DOGM LC display from Electronic Assembly
with 2x16 characters.
- Below the LC display there are located three so-called "optical buttons"
that have been implemented for the very first time there. So to speak, this is
the first official reference implementation of this button concept. More details on
these buttons can be found on the dedicated
optical button project page.
- On the reverse side of the board are mostly just some analog components
needed for the optical buttons.
A single measurement unit is intended to measure two high-side currents and one voltage
within a single electrical circuit. The topology of the measurement setup is made in a way,
where a common measurement wire is being attached to the plus pole of the battery,
the voltage measurement wire is being attached to the minus pole of the battery,
and two current measurement wires are attached to the two shunt resistors located at the
plus pole of the battery. Notice that this setup does contain a small flaw (see
The measurement units are basically self-contained small embedded systems.
Here are a few comments:
- The board is a multilayer board with 4 layers and an overall dimension
of 44mm by 42mmm which has been specially designed to fit in a Bopla
housing that has been found useful for this application (see photographs).
- From point of view of their basic architecture, they are very similar
to the Plane Sensor for Inertial Measurement.
In fact, the micro controller firmware and the communication infrastructure
is significantly based on this technology.
- The communication between a measurement unit and the central unit is based on
a robust RS485/422 electrical level and does also include some forward error
correction for the transmitted data.
- The heart is formed by a PIC16LF76 micro controller.
- The analog-to-digital conversion is being done by three independent
Analog Devices AD7792 16 Bit Sigma-Delta converters. Two for the current channels
and one for the voltage channel. The AD converters are set up to
deliver samples at a rate of 10Hz which is ok for that application
(it could also be set to a higher rate).
- The analog-to-digital converters are galvanically isolated from the
micro controller and hence the remaining PowerMonitor. This is an important
requirement for the special current measurement technology that has been
implemented here (see further below).
The galvanic isolation is not realized by opto couplers, but by so called
iCouplers from Analog Devices. These state-of-the-art isolation devices
are more reliable and consume less power.
- The power supply for the analog-to-digital converters is realized with a
galvanically isolated DC-DC converter. In particular, a C&D Technologies LME0505SC
(5V/5V) has been used. Although this does not seem to be the best solution because this
device does formally consume almost half of the power needed by a whole
measurement unit, it has been chosen for the sake of simplicity, as there was no
experience with building up such a specialized device in a much more efficient
- For the connection of the measurement unit to the central unit a rather
expensive and hermetically sealed and water-proof connector from Tajimi Electronics
has been chosen. Together with the hermetically sealed housing, the measurement
units can also operate in a flooded environment, if needed (hopefully not ;-)
The four measurement cables that can be seen in one of the pictures are also
being fed out of the housing so that it remains sealed.
PowerMonitor at work
The following picture, despite of somewhat poor quality, is showing the PowerMonitor
at work in its final application including a simple front panel.
In the moment shown, its operational state is in the so-called "interactive mode"
where the yellow buttons can be used for walking through the measurement channels
for the top respectively bottom line. The middle (green) button can be used for
entering a menue where various settings and adjustments can be made.
As it can be seen, because of the limited display capabilities only two measurement
channels can be visualized at once.
Employed Current Measurement Technology:
There need to be added a few more words regarding the methodology of the
The defacto-standard for doing this is by using precisely trimmed shunt
resistors that are placed into the current path. The voltage appearing
across the shunt is proportional to the current flowing through it
(just following Ohm's Law) and can be measured rather easily.
However, shunt resistors are per definition uncool and completely unsexy
especially when talking about high-power electrical systems.
They get warm at higher currents and hence energy is being wasted.
Some voltage is being dropped too.
As a possible alternative there has been evaluated another current sensing
technology based on hall-effect sensors. Allegro Microsystems is offering
several devices that seemed quite promising. Another advantage of this
technology is the inherent galvanic isolation from the electrical circuit
that is under measurement.
However, as it turned out, these devices cannot (yet) be used for systems
that require a wide range into the hundreds of amperes while still allowing
a precise measurement of a few milliamperes. Therefore this had to be
dropped, unfortunately. But I'm sure that this technology will be pushed
as well and will be improved over the next years.
Another similar technology are so-called current transducers. This is an
older technology that even allows to measure the current flowing through
a cable without the need to open/cut this cable. This is an almost optimal
solution. However, there have been made no in-detail evaluations here, because
these devices seem to require a lot of energy to operate and are more likely
suitable for even higher power-class applications that also do not require
a precise measurement in the range of milliamperes.
So finally the bitter pill had to be taken and the shunt solution has been
implemented. But here was another challenge regarding the requirement for a
high-side current measurement in junction with potentially up to 60V of
input voltages. While this is a well-known problem, the industry is
providing special amplifiers for dealing with shunt measurements by
offering a so-called common-mode input range of up to 60V. However, these
devices do have a significant drawback: They are by the order of a magnitude
less precise than their counterparts that are not suitable for such a
high common-mode input voltage.
In order to deal with that issue a very simple but effective method has
been introduced: A galvanically isolated power supply is being generated
that will be connected to the electrical circuit that is to be measured.
This power supply can be also derived from the circuit to be measured,
of course. Then the ground of this "new" power supply can be attached
to the high-side shunt and all high-voltage common-mode problems disappear
like magic. Hence ultra-precise operational amplifiers can be used in the
path for measuring the voltage across the shunt. In this particular case,
the shunt from the more or less rough environment is being directly
connected to the very fragile and ultra-precise AD converter without any
There is also planned the preparation of a white paper that is describing
this technology in more detail.
This project is basically done and is already working fine in the
targeted application. However, as usual there are many things that could
be improved. Here are a few of them:
- Complete Objective Performance Analysis
Of course, the data delivered by the PowerMonitor has been checked against
a regular multimeter for its correctness. Do not ask for the exact precision that
has been finally achieved, as it as not been written down that time :-(
But the tests - especially the current measurement tests - have not been
made with tons of Amperes, as this is not trivial....
So it would be interesting to know what the PowerMonitor can achieve
over its full formal capabilities.
- Improvement of the Display Capabilities
Instead of the rather poor LC display that is being currently used, some
larger one could be used. Possibly some OLED/PLED or TFT screen.
This would allow a more extensive and more "verbose" and intuitive
- Implementation of Logging Capabilities
It would be really interesting to log or trace the measurements over
a longer time (hours, days, months, ....) in order to have the capability for
a later analysis of the power system and its possible optimization.
- Environmental Hardening of the Central Unit
Although the central unit is usually not exposed to the rough environment
as the measurement units are, it could be made mechanically more bullet-proof and
also more (or at all) hermetically sealed.
- General Re-Make of the Central Unit
In view of enhancements such as logging and better display capabilities
it seems to be sensible to migrate to another processor or micro controller
system with more capabilities - possibly running Linux or uCLinux.
- Improvement of the Measurement Unit
The measurement unit could be improved by throwing away the currently used
DC/DC converter and replacing it with a solution specifically designed
for this application. The currently used converter has a rather high power consumption
of 10mA which is very likely caused by its cheap design. Currently, a complete
measurement unit is drawing somewhat over 20mA @ 6V. So this is a significant
Furthermore, the measurement unit is containing a minor design flaw as it
is sharing a single measurement cable for the two shunts of the two current
measurement channels. This is, however, not good practice because attaching
the measurement cables to shunts is usually a critical matter. Every
microvolt is counting there. And, in fact there have been observed minor
current measurement anomalies because of this flaw when both channels are
being used. This issue could be solved by providing two independent power
supplies - one for each shunt. Alternatively, two measurement units can be
used instead of just one, or the minor error is being tolerated (as it is
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