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The goal was to make a compass sensor
compatible with the RCX and have it only take one sensor
input port to take a compass reading. We decided to use
a Dinsmore analog compass part that has two outputs that
need to be combined to determine the direction the
compass is pointing.
With our design, it takes two readings
by the RCX to get one complete reading from the compass
sensor. Software then must combine those readings
together to determine the angle the compass is pointing.
We had to do it this way because of power demands by the
Dinsmore part and power limitations of the RCX.
We wanted to add hardware around the
Dinsmore sensor to combine the two outputs into one RCX
input. We first tried to put all the "inside/outside
crossings" logic into hardware, but we found out the
hard way that the RCX can only provide about 10mA of
current. It turns out we can only power 1/2 the Dinsmore
compass sensor at a time.
Rick Sammartino (RickSam from the
Mindstorms website) who has been helping me all along,
had and idea of reading one curve then the other. This
fit well with powering one compass sensor then the
other.
The Dinsmore compass sensor has two
outputs that need to be combined to know the direction
you are pointing.
To determine the outputs' values when
pointed a particular direction, draw a verticle line at
the angle you want the compass to point. The vertical
line intersects with the two output curves only once per
vertical line. These intersections are the two output
values you can expect when the compass is pointing in
that direction. As you rotate the compass, you are
basically sliding your vertical line left or right on
the graph.
As you rotate the Dinsmore compass,
there are two angles where the output values are the
same. One is above the 2.5V center line, and the other
is below the 2.5V center line. We'll call these the
upper crossing and the lower crossing. Knowing these
voltages are important to understanding the compass
output.
At any given orientation, one of the
Dinsmore compass outputs is either above the upper
crossing, or below the lower crossing while the other
compass output is between the crossings (unless you are
at one of the two crossover points). The output that is
between the crossings is follows a fairly straight line,
while the output outside the crossings does not.
The trick is to determining the
direction the compass is pointing is to determine which
output is outside the crossings, and then use the output
that is inside the crossings to know the angle.
We designed and debugged a "two reading"
circuit shown in the schematic. The circuit has four
major parts: RCX power supply, compass output select
state machine, compass power and output select, and RCX
sensor readings hardware.
We get the power we need to run the
electronics in our sensor by setting the RCX sensor port
type to light sensor. This makes the RCX output 7 volts
at a maximum of about 10 mA of current. Every 3
milliseconds the RCX drops the power for about 1/10 of a
millisecond to take a sensor reading.
The cables that connect sensors to RCX
sensor inputs can be oriented one of two ways: cable
pointing towards the IR port, or away from the RCX port.
There are two terminals on an RCX sensor port. The RCX
always sends power out one terminal and back in on the
other terminal. When we connect a sensor with the cable
pointing toward the RCX's IR port, the power travels
down one wire and comes back the other. When we orient
the cable the other way, the power flow is reversed.
Reversing the power can be bad for electronics, and they
certainly won't work as planned.
To allow the user to connect their
sensor cable with either orientation we use something
called a diode bridge to make sure the power always
flows the correct direction. A diode bridge is make up
of 4 diodes.
When the sensor cable is hooked to the
sensor input with one orientation, power travels into
our circuit through D3, and back to the RCX through D6.
With the cable the other way, power travels in through
D4, and back through D5. The ends of D3 and D4 that are
hooked together always provide the power, and the ends
of D5 and D6 that are hooked together always return the
power.
As mentioned before, every 3
milliseconds the RCX turns off the 7 volt power to the
sensor to take a reading for 1/10th of a millisecond.
This means that our electronics are unpowered during
that 1/10th of a second time period. 1/10th of a
millisecond (1/1000th of a second) may seem fast to us
humans, but this is pretty slow in electronics terms, so
we need a way to save up some power to run the
electronics while the power is off. We use a capacitor
C1 to store up power for these power out periods.
A capacitor acts like a water tower. It
has pumps that fill it up, but it takes time. As the
tower fills up with water it gains energy, because
gravity pulls down on the water. As people use the
water, the gravity pulls the water down and into their
houses.
Capacitors work in much the same way.
While the RCX is sending power to our sensor, the other
electronics in the sensor use the power, and capacitor
C1 charges up until it is full. When the RCX shuts off
the power, the rest of the electronics draws its power
from capacitor C1. The electronics can function normally
until capacitor C1 runs out of power.
The size of the capacitor (measured in
farads) you use depends on how much power you need and
for how long a time. The larger the capacitor, the
longer it can survive a power outage. We need our
electronics to survive longer than the 1/10th of a
millisecond power outages.
Our sensor needs to provide two
different readings to software in the RCX to know what
direction we're pointing. This means the software needs
to tell our sensor to stop reading one compass output,
and start reading the other compass output. The software
signals this to our sensor by turning off the power for
a longer period of time. We need the electronics to stay
alive over this longer period of time so it can remember
which compass output it is currently on.
These power outages can last in the 10
to 20 millisecond range. Capacitor C3 is large enough to
provide power to the other electronics for this long a
power outage.
The 7V power provided by our power
supply is called VCC (everything has to have a name ;-).
The place where the power returns to the RCX is called
ground. In electronics ground has its own special symbol
that looks like a triangle pointing down mad up of
horizontal lines. Since our sensor is a rather
complicated RCX sensor we do not run lines all over the
page showing power and ground hookups. Instead we show
local hookups using VCC and ground. You just have to
imagine that there are wires (lines) leading from the
power supply to the various electronic components.
The Dinsmore compass needs to have its
power very stable to give reliable readings. We use an
LM317 voltage regulator U1 to provide the same voltage
to the Dismore compass, even if the power from the RCX
fluctuates. The voltage regulator and its companion
resistors and capacitor provide 4.5 volts (4.5V) to the
Dinsmore compass. The Dinsmore compass is supposed to
run on 5V, but the RCX cannot provide enough power at
that voltage, so we run the Dinsmore compass at 4.5V to
save power.
Our sensor and the software need to
always be in sync about which compass reading is being
sent by the hardware, and which reading the software is
receiving. Our sensor remembers which reading it is
currently sending using a digital memory element called
a flip flop.
In the schematic the flip flop is
component U2A. Our flip flop has 5 inputs and two
outputs. The two most important inputs are the D input
and the CLK input. These two inputs work together. The D
input is for inputting data. The CLK is a clock input,
which is used to tell the flip flop "forget what you
were remembering and remember what is on the data
input."
The Q output is the value that the flip
flop is remembering. The NOT Q (a Q with a bar over it)
is the opposite of the value being remembered. If Q is
outputing a 0, then NOT Q is outputing a 1. If Q is
outputing a 1, NOT Q is outputing a 0.
The two remaining inputs S (for set) and
R (for reset) can be used to force the flip flop to
either a 1 or a 0, without having to put the value on
the data input and clocking it into the flip flop. We do
not use these in our sensor.
The flip flop is used to remember which
compass output we are sending to the RCX. Each time the
RCX tells us to switch to the other compass output, the
flip flop needs to change to the opposite of the value
it currently has. If it is currently remembering a 0, it
needs to remember a 1. If it is currently remembering a
1, it needs to remember a 0. The NOT Q output is the
opposite of the currently remembered value, so we hook
the NOT Q output to the data input. When we apply the
remember clock, the flip flop changes value.
The software in the RCX turns power off,
and then on to tell the flip flop to change its value.
We use operational amplifier U5C and some resistors and
capacitors to detect power toggling (being turned off
and on.)
To amplify is to make something larger
from something smaller. Your car stereo has an amplifier
in it that makes the small radio waves, or small
magnetic changes on tape large enough to wiggle speakers
so we can hear it. How much you amplify something is
called gain.
An operational amplifier (op amp for
short) is a general purpose electronic amplifier that is
easy to use. It has two inputs and one output. An op amp
subtracts one of its inputs from the other and sends an
amplified version of the difference to it's output. An
operational amplifier has very high gain, which means it
can take small differences nad make them very, very
large. We can control the gain of an operational
amplifier by adding or subtracing some of the output
from the input.
We use an LM324 quad operational
amplifier chip in our schematic. It has four operational
amplifiers on one chip that all run off the same power
supply.
So how do we use an operational
amplifier to detect power toggling? First we must think
back to the power supply explanation. The power supply
uses diodes to make the power from the RCX travel in the
correct direction, and we use a capacitor to store power
for power outages.
To detect power outages we use a second
pair of diodes D1 and D2, that act just like diodes D3
and D4. Power comes in D1 or D2, and goes out D5 or D6.
In this case though we do not use a capacitor to cover
for power outages. This means that the power coming
through D1 and D2 goes on and off when the RCX turns
power on and off.
The flip flop remembers new values when
the clock input goes from a 0 (no power) to a 1 (power).
It is called an edge triggered input. The change from a
0 to a 1 has to be fast for the flip flop to notice it.
The RCX's change from no power to power is too slow for
the flip flop to notice. To make a faster rising change,
we use the subtracting nature of an operational
amplifier and it's ability to change values very
quickly, by using the op amp's maximum gain.
We run the voltage from D1/D2 to the
positive input (pin 10) of U5C. We run a reference
voltage (one that does not change) to the negative input
(pin 9) of U5C. The operational amplifier subtracts the
refernce voltage from the D1/D2 voltage. When the
reference voltage is larger than the D1/D2 voltage, the
op amp puts out a very low voltage (almost 0V). When the
D1/D2 voltage is larger than the reference, the op amp
puts out it's maximum voltage. As the D1/D2 voltage goes
from lower than the reference to even just a little bit
higher than the reference voltage, the op amp output
voltage rapidly goes to its maximum output voltage
because we are using the op amps maximum amplification
capabilities. Using an op amp with its maximum gain like
this is called a voltage comparator.
Using U5C as a voltage comparator makes
our power toggle detect signal rise fast enough to clock
the flip flop.
We generate the voltage reference by
using two resistors. Resistors are electronic components
that resist the flow of electricity. In resisting the
electical flow power is lost through the form of heat.
When the power is lost, there is voltage from one end of
the resistor to the other. This loss is called a voltage
drop.
We want our voltage reference to be
somewhere between 0V (the lowest voltage) and 7V (out
highest voltage). To do this, we our 7V from our power
supply into one end of a resistor, and we hook the other
end of the resistor to ground (0V). The incoming voltage
stays at 7V, and the outgoing voltage stays at 0V, so
there is a 7V drop across the two resistors. The voltage
where the resistors meet in the middle is based on
resistive the two resistors are. If the two resistors
are of equal resistance (measured in ohms), half the
voltage drop is due to the first resistor and half due
to the second, so the voltage in the middle is 3.5V.
Our reference voltage for power toggle
detect is generated using a voltage divider implemented
by resistors R3 and R4. The middle voltage in our
divider is 7V (VCC) * 100,000 (R3's resistance) /
(100,000 (R3's resistance) + 220,000 (R4's resistance))
or 2.1V. This voltage reference is used again for other
purposes.
The Dinsmore compass is actually two
sensors in one. Each sensor has its own power
connections, so we can power each one seperately. To
satisfy RCX power limit constrainst we can only power
one half of the Dinsmore part at a time. When one half
is powered, the other half has to be unpowered. In
electronics we can use transistors to turn things on and
off. We use two MOSFET transistors to control the power
to each half of the Dinsmore compass. MOSFET transistors
are very good for controlling power because they consume
very little of the power they control. Power goes into a
MOSFET transistor in the source input. Power traveling
though the transistor is controlled by the gate input.
The power comes out of the MOSFET through the drain
output.
In our sensor, the source power is
provided by U1, the Compass Voltage Regulator. The drain
output is connected to a half of the Dinsmore compass to
deliver power (or not ;^) The gate inputs of our two
MOSFET transistors are controlled by by our Compass
Output Select State Machine flip flop. Remember that our
flip flop has two outputs Q and NOT Q, and that NOT Q
always has the opposite value from Q. So if Q is on, NOT
Q is off and visa versa.
This sounds exactly like the way we need
the power delivered to the two halves of the Dinsmore
compass. We hook Q to the gate of one of our MOSFETS,
and NOT Q to the gate of our other MOSFET and only one
half of the Dinsmore compass is powered at a time.
Each Dinsmore compass output is a
voltage that varies based on the direction the compass
is pointing. Unlike our digital flip flop who's output
voltages can only have two values either on (high
voltage) or off (low voltages), the Dinsmore compass is
analog which means a whole range of voltages.
The Dinsmore has two analog output
voltages, and we have to choose one of them to send to
the RCX. Electronics provides us with a wonderful thing
called an analog switch for just this purpose. An analog
switch has two inputs, and an output. One of the inputs
is called the control. When the control input is driven
so the switch is on, the analog input is sent to the
analog output. If the control input is driven to turn
the switch off, the analog output is disconnected from
the analog input.
We use two analog switches to route the
Dinsmore compass outputs to the RCX. There is one analog
switch for each compass output. We connect the Q and NOT
Q outputs from our flip flop to the control signals on
our two analog switches. We also hook together the
outputs of the analog switches which is sent off to the
RCX. When Q is on, it turns on a MOSFET that powers one
half of the Dinsmore compass. The output of that half of
the compass is fed into an analog switch, who is also
controlled by Q, so the output is fed onto the RCX.
When Q is off, NOT Q is on, so the other
MOSFET is turned on, the other half of the Dinsmore
compass is powered, and the other analog switch is
turned on, so the second compass output is fed to the
RCX.
We're most of the way through our sensor
explanation, so I hope you are still with us.
The remaining part of our sensor feeds
our selected output to the RCX.
The Dinsmore compass output gives us
less than one degree of resolution. Resolution refers to
how big or small the smallest detectable part is.
The Dinsmore part can only provide 4mA
of current. To make sure we do not draw too much power
through the compass, we run the selected compass output
through an operational amplifier that interfaces to the
RCX. This guarantess that we don't draw too much power.
It also allows us to better interface to the RCX's
resistance based sensor reading technque.
Diodes D1 and D2 combined with D5 and D6
are how the RCX actually reads the sensor values. The
RCX sends a small amount of power through either D1 or
D2, our operational amplifier combined with a 2N2222
transistor only lets some of that through and back to
the RCX through either D5 or D6.
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