If a DC motor must spin at a constant speed, one way is to measure
its speed and ajust the power on the motor accordingly. The quadrature
encoder gives the direction and allows for precise positionning.
DIDEL provides the low power optical sensor and slotted gears required
for miniature applications. See our price-list,
Quadrature encoder principle
It is easy with any processor to sample and decode the quadrature signals
and update an up/down counter. The routine needs only 10-20 instructions,
but must be executed frequently enough not to miss a transition. You can
find more explanations on page 15-16 of our PIC
The H7001 is encapsulated in a light emitting diode package, but
the chip is on the flat side, and not on the lens side.
There are four photodiodes inside which are connected to differential
amplifiers and a latch. The diodes distance is 0.15 mm, which match
slots with 0.623spacing (400 slots per inch). The circuit will work
with any slot size larger than 0.xx mm, but the spacing of transitions
will not be even. With correct slots, the interface will give a
4 time resolution.
The H7001 is packaged in two different ways, depending on if the
photodiode line matches vertical slots, as in the next figure, or
horizontal slots. There are color marks, but the best way is to
look at the orientation of the tags. Pay attention, the electrical
pinout is different for the H7001H (see below).
Voltage 1.8 to 5.5
Supply current 0.05 microAmps
Transfer time 5 to 15 microseconds
Output current high -200 microamps
Output current low 2 mA
The H7001 must be as close as possible from the slotted disk. The
photodiode is preferably an IR led with a small angle, at a distance
and position that will illuminate the two slots in front of the
H7001. We developed specially the G3360 and G348 slotted gears to
make microrobot applications easy. Having the encoder directly on
the motor axis is possible, but one should check the maximum frequency
of signals, according to the microcontroller program, since small
motors spin very fast.
The H7001 has 2 power lines (3 to 5V) and 2 signal out lines, TTL
and CMOS compatible. The light emitting LED needs an important current
(20 to 100 mA), but this current can be switched on by the microcontroller
10 microseconds before reading the H7001 outputs, and switched off
immediately after. The average power consumption is hence very low.
Several years ago, the SFH910 was sold with a 30mm 96 slots coding wheel.
Several dual channels optical encoders exist (Hewlett Packard ). Hall
encoders are available from Allegro, but it is not easy to get the magnets
that will give precise square waves, if they are required.
Mechanical encoders are easy to get from many sources, but they have an
important friction and bounces. Let us mention also the many industrial,
fully packaged optical encoders, high resolution and a high price.