Page 430 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

SKIN DESIGN 405

which decodes the order of sensor addressing. The second part is a sensor detec-
tion circuit, which amplifies and filters signals from the light detectors.
The addressing scheme is organized as follows. Each sensor module has a
counter that keeps track of which sensor is being addressed currently. The counter
is incremented by a clock, causing selection of a new sensor. When needed, the
counter is set to zero by a long pulse from a pulse discriminator. In the earlier
system, pulses longer than 10 µs are considered zero reset pulses; pulses shorter
than 10 µs increment the counter. This addressing scheme allows one signal line
to address a practically unlimited number of sensors.
Besides its serial nature, an obvious drawback of this scheme is that it does
not allow random addressing. When picking a particular sensor, all sensors with
addresses lower than this sensor will be selected. Note, however, that this is not a
serious drawback, because by the nature of the skin all sensors must be addressed
in turn in each cycle of sensor polling. The order in which sensors are addressed is
immaterial, and so the advantages of serial addressing outweigh its disadvantages.
The sensor module circuit implemented in Ref. 134 is shown in Figure 8.5. In
brief, it operates as follows. The Sensor Select signal from the Sensor Interface
is first “cleaned up” by triggers IC8b and IC8c and is then passed to the “Clk”
input of the 8-bit counter that keeps track of selected sensors. The function of
the pulse discriminator IC6 (a dual one-shot) is to choose the time of resetting
the counter.
In the pulse discriminator, when the Sensor Select line is “low,” the one-
shots’ outputs “Q” are low, and the 8-bit counter is not reset. As a pulse arrives
on the Sensor Select line at time T a , the output “Q” of the one-shots IC6a is
triggered high. If the Sensor Select line stays high longer than 10 µs, IC6a will
time out, causing its output to go low at time T b . This triggers IC6b, and its
output “Q” goes high, resetting the counter. If, on the other hand, Sensor Select
signal goes low before IC6a times out, no reset pulse is generated and the counter
increments normally.
The infrared diode (LED) light is amplitude-modulated and then synchronously
detected, to increase the system immunity to other light sources. This scheme
allows operation on several “channels”: For example, light transmitted by an
LED on the robot link X will not be sensed by a detector on a link Y even if
directly illuminated by it.
The output byte ‘Out’ of IC7 controls analog multiplexers that switch optical
components in the sensor circuit. The least significant four bits are connected to
the analog multiplexer IC2, which selects signals among the 12 preamplifiers on
the skin. The analog signal is first high-pass filtered by IC1a to remove noise due
to the ambient (room) light, then passed to the synchronous detector ICb, which
demodulates the transmitted signal, and then low-pass filtered by a three-pole
Butterworth filter composed of IC1c and IC1d. The IC1d output is then passed
to one of the input channels of the Sensor Interface Board via a resistor, which
provides short-circuit protection for the IC1d’s output.
The setting time of the Butterworth filter is about 0.25 m, which determines
the overall scheme’s response time. A higher bandwidth filter would settle in less