♪ Turn my motor on ♪
We're demonstrating an isolated
H-bridge motor control
with non-contact TMR
angle and current sensors.
With the full H-bridge,
we can reverse the motor
or generate AC.
So here we're ramping
the speed up and down
and reversing the motor.
A magnetic brake
provides some loading.
The PWM frequency is about 500 hertz,
which is the B4 (B above middle "C")
tone you're hearing on start-up.
The current sensor is bipolar,
so it measures
positive or negative current.
Let's look at
closed-loop motor control.
Right now we're running open-loop...
...so of course the motor slows down
when we apply a load.
This switch turns on closed-loop
proportional-integral speed control,
which adjusts the motor voltage
to keep the speed
close to the 2000 RPM set point
as mechanical loading
decreases...
and increases...
We could just as easily control
the motor current
to keep torque--
rather than speed--constant.
We used an off-axis
angle sensor arrangement
with a ring magnet.
Here's the magnet we used.
And we used an NVE Web application
to determine
the optimal sensor position.
Alternatively, we could use
an axial angle sensor configuration--
in this case the sensor
is on the other side of the board--
with a diametric disk magnet
like this.
Another Web application
checks the sensor position.
The current sensor uses
a trace on the top side of the PCB.
And we have a Web app for that, too.
Based on the predicted field
at ten amps,
we selected an SM228
150 oersted magnetometer
for current sensing.
There are other trace configurations
or bus-bars for higher currents,
and we have more sensitive magnetometers
for lower currents.
Here's the top-level schematic.
I²C allows multiple peripherals on
a low pin-count microcontroller
but SPI versions of both sensors
are also available.
The controller board has
an 8-pin micro,
four MOSFETs,
and four channels of isolation
in three MSOP-8 isolators.
Two microcontroller signals
control the left and right halves
of the H-bridge.
The isolators allow referencing
the high-side gate signals to
the floating MOSFET source pins,
plus they level-shift
low-voltage microcontroller outputs
to six volts
to drive MOSFET gates.
These isolators
have low-impedance outputs
to directly drive MOSFETs,
so there are no MOSFET drivers.
The isolator inputs
on each side of the H-bridge
are connected in series,
which ensures
two MOSFETs on the same side
can't be ON at the same time.
The IL600CMTI isolators are
the world's smallest isolators,
with the highest
common-mode transient immunity
in the industry.
Complementing the ultraminiature isolators
are the world's smallest DC-DC convertors
to float the high-side gate power.
Commodity regulators
boost the output to the six volts
needed to drive the MOSFET gates.
The demo power supplies are:
five volts for the Arduino board,
display, and DC-DC convertors;
A 12-volt motor supply;
and a bench supply
for the magnetic brake.
Here's the code for the
closed-loop control:
This works with any Arduino-type board.
We read the angle sensor,
calculate RPM,
and read the current sensor.
This is the proportional-integral
speed-control code.
Here's where we display
the information.
This is a summary of the main things
we used in the demo:
NVE Smart Sensors include a
GMR or TMR sensor element,
Digital Signal Processing,
and an I²C or SPI interface.
SM-Series Magnetometer specs include:
accuracy;
resolution;
and sensitivity.
ASR-Series Smart Angle Sensor specs include:
0.1 degree resolution;
0.2 degree repeatability;
and wide mechanical tolerance.
Click, e-mail, or call us
for more information,
or to buy sensors and accessories.
