CRS F3, User Manual

Page: 59 / 128

Chapter 5: Installation 45
99-05-31
System Features
Absolute Encoders
Principles of Operation
Absolute encoders are used in the F3 robot. Their design is based on using a
low resolution gray scale disk in conjunction with a high resolution
incremental disk to detect absolute position within one turn.
Multi-turn data - the number of turns made by the encoder shaft - is
maintained by electronic counters within the encoder. When controller power
is not available, a set of three (3) nickel-metal-hydride batteries located in
the fibre drawer at the base of the robot provide power to the encoders. The
batteries ensure that the multi-turn data is kept valid if the arm is moved
when power to it is off. The batteries are trickle-charged when the controller
is on.
The encoder also has an internal capacitor, which keeps the multi-turn data
valid for up to 10 minutes, providing ample time to change batteries in the
fibre drawer if needed.
Important Encoder Characteristics
•
Single turn resolution - 11 bits or 2048 counts.
•
Maximum number of turns is 8192 (13-bit count).
•
Absolute position.
•
Typical active battery voltage (controller on) - 4.5 V.
•
Typical resting battery voltage (controller off) - 3.6 V.
•
Battery alarm-on voltage - 3.1 V to 3.4 V. (See below.)
•
Battery alarm-off voltage - 3.2 V to 3.6 V. (See below.)
•
Battery error voltage - 3.0 V. (See below.)
•
Low current consumption in battery mode - 150
µ
A max.
•
A minimum of 6 continuous weeks in battery mode before encoders go
into a state (i.e. loss of robot calibration) where they require reset of
multi-turn data.
•
Minimum of two days of charging with controller on after one week or
longer in battery mode.
•
Encoders should be reset with the robot in the calrdy configuration. The
calibration markers are also installed while the robot is in this
configuration. This facilitates re-homing.
Encoder Battery Issues
The encoder signals an alarm if the collective battery voltage drops below a
threshold that varies between 3.1 V and 3.4 V. This alarm will be detected
and reported by controller startup diagnostics. The alarm is turned off when
the voltage is restored to a level above a threshold between 3.2 V and 3.6 V.
Users should heed the warning and either keep the controller on
continuously for at least 72 hours, or replace the batteries if they have been
in use for at least 1 calendar year.