Product manual
Table Of Contents
12
1047_0_Product_Manual - June 18, 2010 2:20 PM
Technical Section
General
The PhidgetEncoder Highspeed 4-Input can be used with a wide assortment of mechanical and optical encoders.
The encoder should be of quadrature output type, indicating that there will be two quadrature output channels
(usually labeled A and B) and a third output channel (only on some encoders) to signal when the index pin (a
reference point for zero position or a complete revolution) has been reached.
The PhidgetEncoder Highspeed 4-Input is able to read four encoders simultaneously. Encoders are not powered
up until all initialization of the device is complete. It is possible to enable some or all encoders, depending on how
many of the channels are being used. This can also be used to reduce power consumption when certain encoders
are not needed.
The PhidgetEncoder Highspeed 4-Input has the added ability to time the duration between a group of quadrature
changes. The time is returned in microseconds. This time value can be used to calculate velocity and acceleration
with very high precision.
If the number of quadrature counts per revolution is unknown for a particular encoder, this value can be determined
by using the index signal. In addition, it is possible to monitor how many counts have occurred since the last
index. The index signal is an output only on certain encoders. Refer to the encoder’s description to check if this
third output channel exists or not. If the encoder does not have this signal, it is still possible to use it with the
PhidgetEncoder Highspeed 4-Input, but an event for the index will never get triggered.
ThemaximumrateofthePhidgetEncoderHighspeed4-Inputisspeciedat250,000countspersecond.Inyour
application, this number relates directly to the number of revolutions per second you wish to measure, and the
numberofcountsperrevolutionspeciedforyourencoder.Ifyourencodermeasures1000countsperrevolution,
then the limit on measurable revolutions per second is 250, or 15,000rpm.
Quadrature Encoder Fundamentals
Quadrature encoders are common, using two output
channels to dictate both a change and the direction of
change. In an encoder system, two parallel mechanical
switches or optical slots are offset slightly. This way, as the
slots passs by the sensor, the staggered output indicates
both the number of pulses that have occurred (the change in
position) as well as which output channel is leading the other
(direction of change).
Choosing Encoders
Both mechanical and optical encoders are available, with optical encoders dominating at > 100 counts per second.
Review the data sheet for the encoder you are planning to use to ensure it is
compatible with the PhidgetEncoder Highspeed 4-Input. Compatible encoders
should operate from the +5VDC power provided by the PhidgetEncoder, and use
a single wire for the A, B, and optional Index channels. Some types of encoders
will use two wires (differential) for each channel - these are not compatible.
Compatible encoders are often advertised as being ‘single ended’, and will have 4
or 5 connections.
Absolute encoders will not work with this device.
Mechanical encoders are effectively just two switches, and often have a push
button switch on the shaft. This push button switch can be wired into a digital input on the PhidgetEncoder.
Mechanical encoders do not have to be connected to +5V.
Warning: The PhidgetEncoder Highspeed 4-Input incorporates a 10 kOhm pull-up resistor on each line from the
encoder input connector. If your encoder is mechanical, these pull-up resistors eliminate the requirement to add
your own external pull-up resistors.
Some optical encoders will have a simple photo-transistor / open-collector output. The 10 kOhm pull-up resistor
may have to be augmented with a stronger parallel resistor if your optical encoder datasheet calls for it. Some
open-collector outputs will not be strong enough to pull this resistor to ground. These encoders are not compatible