- Hello, everyone.
My name is Matthew Bon,
I'm an applications engineer here at Digi-Key Electronics,
and today I'm gonna introduce you to
Maxim Integrated's Smart Force Sensor Reference Design.
The Smart Force Sensor Reference Design
is a unique human-machine interface
that uses strain gauges to detect the mass
and relative position of an object on the face plate.
This technology offers similar performance
to the capacitive sense technology
you see in most smartphones and touchscreen devices today.
However, because it uses strain and force gauges
as opposed to capacitive touch sensing,
it has several advantages:
mainly, that you can construct the face plate
out of thicker and more durable materials,
and that your user can use gloves
or other objects while operating the device.
Here we see the Maxim Integrated
Smart Force Sensor Reference Design in action.
As you can see, it's powered by
a simple micro-USB connection.
At the top part of the LCD panel,
we can see the different mass values being displayed
for each sensor, as well as the total mass
on the face plate itself.
You can see as I apply more force with my finger,
that the display changes to indicate
that more force is being applied to that corner load cell.
Other features of this reference design include status LEDs,
as well as a reset and calibration button.
Now that we've seen this reference design in action,
let's take a peek under the hood
and see how this reference design really works.
As you can see on this block diagram
of the reference design,
we have four weak stone bridges,
which represent the four strain gauges
on our reference design.
When a force is applied to the face plate,
each one of these weak stone bridges
will exhibit a differential voltage,
which will be read in by the MAX11254 Sigma-Delta ADC.
This ADC will then convert these voltages
to digital values, which it'll write out
via the SPI bus to the STM32 microcontroller.
This microcontroller will then determine
from those four values
how much mass is sitting on the face plate
and where that mass is
in relation to the four corners of the board.
Once it's determined that, it'll write its output
to the LCD panel.
Now, I know what you're thinking:
this microcontroller already has three embedded ADCs.
Why do we need to use the other one?
Well, the reason for that is performance.
In this case, mostly resolution.
The resolution on this guy is 24 bits,
whereas the resolution in most embedded microcontrollers
is on the order of 10 to 14 bits.
So, assuming that we have a voltage span of 1.8 volts,
let's do a quick comparison.
Let's assume that you get a real-world value
of 20 bits of resolution out of this guy,
once you factor in noise and all the other stuff.
That means, with your 20 bits of effective resolution,
the smallest signal change that you can measure
is 1.7 microvolts.
Now, let's compare it to the microcontroller.
Assuming that you can get 12 bits of resolution
out of the embedded ADC of a microcontroller,
the smallest signal change that you can measure
is 440 microvolts, which is much larger
than the 1.7 microvolts you got with this ADC.
And when you're dealing with sensors
as sensitive as strain gauges and stuff like that,
those extra bits of resolution really matter.
If you're interested in finding out more
about the MAX11254 ADC,
Maxim Integrated offers a separate evaluation board
just for that part,
which you can find a link to
in the description of this video.
Of course, even the best ADC on the planet
is still garbage without a decent voltage reference.
The MAX6071 voltage reference gives an amazing
initial voltage accuracy of 0.04%
and a absolutely incredible 1.5 ppm
per degrees C temperature coefficient.
What that means is that voltage reference
is going to stay very, very close
to 1.8 volts over its entire temperature range.
Likewise, that's why they use the same voltage reference
to power all of the strain gauges on the reference design.
Other parts to highlight include the MAX17651 3.6-volt LDO
and the MAX8574 LCD Boost converter,
which provides 19 volts to the LCD backlight.
Overall, the Maxim Integrated Smart Force Sensor
Reference Design provides a great jumping-off point
for you to start your own human-machine interface design.
This reference design is available for purchase at Digi-Key,
as is all the other parts highlighted in this video,
and you can find links to all of them
in the description below.
I hope you all found this video informative.
Thank you all for watching.