Alt-BEAM Archive
Message #05994
To: alt-beam@egroups.com
From: "Hyndman" hyndmanm@cadvision.com
Date: Tue, 7 Sep 1999 23:19:01 -0600
Subject: [alt-beam] Re: FREE Eletronic Samples (NOT SPAM)
>Ok... I found this VERY interesting site... that sends Free samples of
eletronic >components... Sharing this info with you... I only ask to some
of you... what kind of >components would be most used on our circuits?! :-)
(They have a limit of 8 or >something...) E-mail to me if you don't want to
do it to the group! Thanks... Oh... >and i don't have anything to do with
them... I only want some (the most usefull) >components >;-) LINK:
http://www.maxim-ic.com/Products.htm
<http://www.maxim->ic.com/Products.htm>
I know a lot of people would be jumping up and down to get "free"
components sent to them. It's basically scamming the buisnesses. They give
out free samples for people (mainly buisnesses), they try them out and see
if the product is to their liking. Just because it's there doesn't mean
it's a free ride.
Devin
5995 Tue, 7 Sep 1999 22:57:11 -0700 [alt-beam] PS HEAD V2 BEAM Wilf Rigter
Prodded by Justin, here is an updated and tested version of the Power_Smart
head circuit complete with a description of operation.
The Power_Smart Head V2 - 9/99 - wilf rigter
NOT A BICORE
The basic BEAM head is a tried and proven design with a substantial number
of working units posted at various websites. However the conventional bicore
head circuit is difficult to adjust to operate over a wide range of light
conditions and is notorious for high power consumption. The switching losses
are significant, because the bicore is continually reversing the motor
current. In fact, current required for a motor in a bicore head is almost
double that needed with the motor connected straight to the power supply.
Moreover the circuit consumes power even when the head is locked on and not
moving.
HIGH/LOW/OSCILLATE
The Power_Smart (PS) head , as the name implies, was designed to address
these problems while maintaining the same simple "freeformable" circuit
layout and similar components. The PS circuit uses what could be called a
modified bicore oscillator but which I will call the High/Low/Oscillator
(HLO). This is in fact a textbook CMOS oscillator with a voltage
controlled Pulse Width Modulation input. This PWM control input is simply a
1M resistor connected on one side to a voltage source (in this case the
midpoint of two photo sensors ) and connected on the other side to bias
point of the oscillator timing components (U1-pin2). Two photo sensors use
reasonably well matched photodiodes or light depended resistors to form a
voltage divider (or half bridge), with the voltage at the midpoint close to
1/2 Vcc when the light on each sensor is balanced. This 1/2 Vcc voltage is
near the input switching threshold of standard 74HC240 (not HCT) logic. The
control range was designed so that when the photo sensors are balanced,
the oscillator produces a square wave with 50% duty cycle. When unbalanced,
the duty cycle changes between 0% or 100% (full reverse to full forward).
The two complementary outputs of the HLO are connected to the inputs of a
bipolar pulse generator.
A CHANGE OF PHASE
The HLO is followed by the Nv/Nu (2N1) bipolar pulse generator. The 2N1 uses
two capacitors and one resistor. The produces positive going AND negative
going pulses with a pulsewidth about half the period of the HLO output. For
the duration of the pulse width the output of the 2N1 is in phase with the
resistor signal. If that signal is longer than the pulse width, the 2N1
output changes to the inverse of the resistor signal. Since the resistor
input signal and the 2N1 output are connected through a driver stage to the
motor, it is the difference (out of phase) between these two signals which
cause the motor current and rotation. Therefore a steady state signal
applied to the 2N1 resistor input, when the sensor bridge is greatly
unbalanced, will cause continuous rotation. When the sensor bridge is
balanced and HLO oscillator produces a 50% duty cycle (squarewave), there
is no difference signal developed between the 2N1 input and output and the
motor does not rotate. When a HLO output is a pulse width modulated
oscillation, the 2N1 circuit produces a pulse train phaseshifted with
respect to the input signal with a pulse width difference which is
proportional to the sensor bridge imbalance. This difference signal is a PWM
signal the causes slow rotation of the motor when the sensor bridge is
slightly unbalanced.
TWO NEURONS IN ONE
The Nv/Nu (2N1) circuit in one sense combines the functions of both types
of neurons. Normally a Nu integrates a signal applied to the resistor with
the capacitor grounded. A Nv differentiates a signal applied to the
capacitor with the resistor grounded. The Nv/Nu stage applies signals to
both the capacitor and the resistor. A second capacitor and second inverter
is used to provide positive feedback to overcome the common problem of
linear inverter high frequency oscillation as a Nu input voltage passes
through the linear region. The driving outputs also act as virtual grounds
for the respective capacitor and resistor. (note that the earlier power
smart head used just one 240 stage for the Nv/Nu neuron resulting in bursts
of high frequency oscillations especially when using the preferred 74AC
logic) .
HUNTING/SEEKING/TRACKING
When there is a strong light source off-center from the photo sensor's line
of sight, the midpoint of the sensor bridge is strongly unbalanced. This
causes the PWM control input to saturate (full forward or reverse) and the
oscillator output no longer pulses but stays high or low depending if the
control voltage is higher or lower than the linear region. In this saturated
state, the motor is driven continuously left or right, turning the photo
sensors towards the light source . This provides a feedback servo loop
driving the motor to balance the light level on the sensors until the
control voltage is restored to 1/2Vcc. While the oscillator is saturated,
all power is continuously (efficiently) applied to the motor without the
switching losses common in Bicore designs.
BUFFERIN/BUFFEROUT
The output of the HLO and the Nv/Nu stages are buffered with 2 parallel
inverters each and these can directly drive a small gear motor of the BG
Micro type. If more power is needed then I recommend using a complementary
mosfet h-bridge to source and sink up to 1A with low losses. With a heavy
duty motor and the more powerful mosfet h-bridge, a PS head can be used to
position a camera or other device (ie an electromagnetic BB gun : you'll
shoot your eye out!).
ONE MORE TIME
To recap, the HLO generates a High or a Low output when the photo sensor
bridge is unbalanced and Oscillates when each photo sensor receives roughly
the same light. The complimentary outputs of the HLO connect to the inputs
of a Nv/Nu stage which generates pulses when the HLO oscillates. The motor
driver consists of a buffered bridge connected to one HLO output and the
Nv/Nu output. The bridge drives the motor in one direction or the other when
the photo sensors are unbalanced. When the photo sensors are slightly
unbalanced, the motor driver produces narrow pulses (reduced power) to nudge
the head position towards the balanced condition. When both photo sensors
are equally illuminated, the HLO oscillates width 50% dutycycle square wave
output. At this frequency and duty cycle, the Nv/Nu stage generates pulses
which are precisely in phase with the HLO output. Therefore the bridge
outputs are the same polarity, the motor is stopped and the brake is on.
This is a low power standby mode which requires just 1 ma of supply current.
240 or not 240
Since only 6 inverters are needed for the PS head, it is also possible to
make a PS head using only one 74HC04 chip and MOSFET h-bridge or for small
motors, use just a single 74AC04 chip. For a 2DOF PS head just duplicate the
circuit and add another motor.
AN EYE FOR AN EYE
I recommend installing red and green LEDs connected in reverse parallel
across the motor to indicate direction of motor rotation providing visual
feedback.
X MARKS THE SPOT
Although not shown in the schematic, an auxiliary IR LED can be mounted on
the head to provide an active IR SPOTLIGHT. The LED current can be also be
synchronously Pulse Width Modulated to increase IR brightness while
reducing overall power consumption. When using the IR SPOT, make sure the
head is far from reflective walls, etc since the head will lock on any
nearby object reflecting the IR beam.
That's it for now: but remember feedback is always welcome here.
enjoy
wilf
<>
Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411
Attachment: SM_HEAD2.gif
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