64SPU-1 Noise Protection - for added
system reliability
With three drive motors connected to a single
battery supply, and the battery ground connection shared
through the motors, motor speed controllers and
64PSU-1 (or other) drive
controller there is always the possibility of electrical
noise from the motors affecting the DIY servo system operation. This is most
likely to happen in systems using higher powered motors
and/or inexpensive lower quality noisy motors.
Steps can be taken to reduce motor noise related voltage
spikes within the system such as fitting snubbing capacitors
to motors, making sure ground return lines are robust,
avoiding ground wiring loops and physically separating the
low voltage electronics from the high current motor drives.
However some electrical noise, especially that present on
the electrical ground lines, can be very difficult to remove.
If this noise if heavy enough it can affect
the smooth operation of the microcontroller IC
on the 64SPU-1. Symptoms can be intermittent black-outs of
the 64SPU-1 or encoder read errors and interrupts to the
servo drive operation.
The
BFF I2C Isolator card is a simple and effective way
to fully isolate the 64SPU-1 from the noisy end of your DIY servo
drive system and to help trouble-free operation of the
SPU. It is connected on the low voltage logic lines leading
from the 64SPU-1 card to the MD03 motor controllers and
allows the 64SPU-1 to be operated from a completely separate
(clean) 5V power supply from that powering the MD03 speed
controllers. When fitted there is no electrical path between
the MD03's SDA, SCL, 5V and 0V lines and the 64SPU-1. A
typical system layout with the BFF I2C Isolator
card fitted is shown below.
IMPORTANT -
Although the isolator card will protect the 40SPU-1 from the
effects of electrical noise, electrical conditions can still
arise in which the I2C comms can seize with the I2C Isolator
card fitted. This can happen, for example, when heavy vibration effects
are configured (eg runway vibrations) and the MD03
controllers attempt to drive very fast reversing movements
against the inertia of the platform and/or heavy unbalanced
load. On my platform for
example configuring heavy runway vibration then running the
platform unloaded (ie against the full unbalanced load of
the bungee spring) can cause problems for the MD03
controllers and the I2C Isolator card. So the MD03's and the
I2C comms can be affected by heavy and erratic motor drive
conditions - take care not to configure too heavy runway
vibration effects in your DIY motion platform and avoid
running it in a heavily unbalanced condition.
The BFF I2C
Isolator Card uses the
Silicon Labs Si8400 I2C Isolator chip to
provide the electrical isolation. Two-way data
communications between the 64SPU-1 and the MD03 speed
controllers are maintained across the card so allowing the
64SPU-1 to continue to both send motor speed and direction
commands and read-back data from the controllers whilst
remaining unaffected by noise on the lines.
An on-board voltage
regulator allows the isolator card to provide the 5V supply
for the MD03 logic inputs from its 9V DC input supply.
Alternatively the motor controller 5V supply can be provided
separately. In either case the 64SPU-1 logic supply (5V and
ground) should NOT be connected to the 5V and logic ground
of the MD03 speed controllers.
If you wish to use the BFF I2C Isolator in
your system please read
the data sheet
carefully.
The BFF I2C Isolator card is now available to
buy -
see the order page here.
If you use the isolator card remember to
ensure that your feedback pots or encoders do not provide a
separate electrical noise path back to the 64SPU-1. If they
are fitted to motor or gear box drive shafts make sure they
are electrically isolated from the metal of the shaft.
Otherwise any ground noise on the motor shaft will travel
back along the pot 0V line to the 64SPU-1. MA3 encoders are
not so much of a problem as their casings and shafts are
electrically isolated from their electrical connectors.
IMPORTANT –
WHEN THE ISOLATOR CARD IS
FITTED TO YOUR SERVO DRIVE DO NOT
POWER DOWN THE 64SPU-1 CARD WHILST THE SERVO SYSTEM IS
MOVING.
ALWAYS STOP
THE DRIVE FIRST BY
CLOSING THE PID SERVO CONTROLLER SOFTWARE ON THE PC
BEFORE
CUTTING THE POWER TO THE 64SPU-1 CARD.
ON SYSTEM SHUT-DOWN POWER
DOWN THE ISOLATOR CARD BEFORE
POWERING DOWN THE 64SPU-1 CARD.
With the isolator card
fitted the 5V 40SPU-1 supply no longer provides the MD03 5V
logic supply. The logic elements of the MD03 controllers are
powered by the I2C Isolator card and will therefore remain
active until the I2C Isolator card power supply is removed.
Closing the BFF PID Servo Controller software before
powering down the 64SPU-1 card will allow the card to
instruct the MD03 controllers to stop driving before it is
made inactive.
Powering down the
I2C Isolator card will cut the logic supply to the MD03's
and stop them driving (so long as the 5V is supplied by the
voltage regulator on the I2C Isolator card).
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