Hardware manual

US 7,915,570 B2
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sources by allowing direct control of the current output. Thus
overdriving of a light source may occur when the light source
is driven with more current than would normally be appro
priate for a regular continuous operation. Due to the short
duration of the current pulse, this overdriving can be done
without damage to the light source within a range speci?ed by
the light manufacturer. Overdriving the lighting source allows
the user to obtain more illumination from the same light
source than would otherwise be possible.
For a lighting current controller with a switching power
supply, the current level may be set by a master processor
(e.g., the processor 212 and/or the FPGA 206) in the smart
camera, such as by using a DAC. As mentioned above, if no
measures are taken, stopping the switching power supply 290
for an arbitrarily long time interval may result in the discharge
of all the reactive components. In this event the lighting
current controller would need to go through the whole estab
lishment time in order to reach the ?nal value of the light
current. As a solution, a digital sample and hold circuit 856
can be implemented to sample all the control values of inter
est, such as loop voltages, and keep a memory of the loop
state. As a result, by keeping the memory of the loop state, the
key reactive elements in the control loop can be maintained or
restored to their operating/active state.
In order to integrate both of these devices, smart camera
and lighting current controller, into one device, the power
density of the lighting current controller may need to be
increased. Use of a switching power supply to provide the
control current (i.e., the current pulse) for the one or more
lighting current controllers, while making the power density
adequate (in terms of ef?ciency) may have serious limitations
as far as response time. Once the switching power supply has
been disabled for a long enough time, it may need a settling
time which may be orders of magnitude longer than some
possible strobing durations for the one or more light sources
(e. g., milliseconds or 100’s of microseconds compared to
10’s of microseconds).
The lighting controller may be able to turn off the one or
more light sources, and ensure that the control current 770
(e. g., the current pulse) can get back to the desired value of the
output current as fast as possible. This fast response time
provided by the control loop may reduce, or eliminate, any
settling time of the power supply when it starts after an
arbitrarily long inactivity interval. Thus the lighting current
controller can provide the current signal for any strobing
duration and interval that may be needed, such as indicated by
the user and/or a machine vision application.
Once the desired current pulse is established through the
one or more light sources, the values of the control variables
inside the switching power supply loop may be stable. In
other words, for a ?xed current pulse, the transfer function’s
reactive components may be charged to constant values (e.g.,
the one or more control values). Although the values of the
one or more control variables may vary (e.g., depending on
the type of the light source), once they settle into a steady state
operation they usually do not change afterwards.
The active circuit may be able to measure the one or more
control values of the control loop for the power supply once it
has reached steady state operation, and then maintain them
while the light is disconnected (i.e., when the power supply is
off). Thus state of the control variables for the transfer func
tion 850 may be stored, and the one or more control values of
the control loop may be maintained as if the one or more light
sources were connected and the control current (e.g., current
pulse) was ?owing through them. As a result, any settling time
for when the one or more light sources are reconnected may
be signi?cantly reduced because the steady state operating
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point for the one or more control values of the control loop is
held, and thus the lighting current controller would take con
siderably less time to get back to the desired control current
(e.g., the current pulse).
This can be implemented by using a sample and hold
circuit 856 that may measure the one or more control values
of the control loop (e.g., the inputs to the H(s) transfer func
tion unit 850). In some embodiments, the sample and hold
circuit 856 may store, and/or create a copy, of the measured
one or more control values of the control loop. This informa
tion may be used to restore or maintain any of the reactive
elements inside this RC circuit 730 at working levels (i.e., at
active state levels). As a result, since the “working levels”
(i.e., from the active state) now became initial conditions, the
next time the switching power supply is activated to strobe the
one or more light sources, the settling time of the switching
power supply should be reduced or even eliminated, substan
tially independent of the length of any inactivity interval.
The control loop may also use a feedback unit 882 operable
to generate a feedback signal 880. A summing unit 852 that
may receive the feedback signal 880 and the setpoint PWM
signal 869. The summing unit 852 may be further operable to
sum the setpoint PWM signal 869 minus the feedback signal
880 to generate an error signal 870. The summing unit 852
may be implemented as an error ampli?er 808 of FIG. 7B.
The control loop may also use a transfer function unit 850
operable to receive the error signal 870 and generate the
intermediate setpoint signal 884 in response to receiving the
error signal 870.
In some embodiments, the lighting current controller may
need to be initialized the ?rst time the one or more light
sources are connected to the system, such that the control loop
can settle to the needed levels (which may be unknown until
then). The initialization also may allow the lighting current
controller to generate a ?rst user and/ or application requested
current signal and any subsequent current signals with sub
stantially similar timing and current levels. In other words,
second and third user and/or application requested current
signals may be generated at the same levels and with the same
duration as the ?rst user and/or application requested current
signal.
Thus, the transfer function unit 850 may be operable to
receive the error signal 870 and generate the intermediate
setpoint signal 884. The switching power supply (e.g., the
current regulator) 860 may be operable to receive the inter
mediate setpoint signal and generate the current pulse in
response to receiving the intermediate setpoint signal and
power the one or more light sources 606.
FIG. 7B illustrates some embodiments of the integrated
lighting current controller in more detail. In some embodi
ments, the implementation may be realized using a switching
power supply, such as a single inductor buck-boost regulator
with programmable current control, and may be based on the
Linear Technologies LTC3783 PWM LED Driver and Boost,
Flyback and SEPIC Converter, but is not restricted to this
speci?c part. In some embodiments, the switching power
supply may include various elements such as a power source
862, inductor 816, transistor 812, a pass FET transistor 820,
an output capacitor 818, and a sensing resistor 824. Other
implementations of the switching supply are contemplated,
and the implementation of this ?gure is shown for exemplary
and explanation purposes only.
In some embodiments, a control voltage may be set using a
PWM generated by an FPGA (or other similar unit) 802 that
may be programmed by a user and/ or an application program.
The PWM signal then may be ?ltered by an FPGA PWM ?lter
module 804. After ?ltering, the PWM voltage may very