The encoder count (position) is read once during each unit time event. The quantity [x(k) - x(k-1)] is formed by
subtracting the previous reading from the current reading. Then the velocity estimate is computed by multiplying
by the known constant 1/T (where T is the constant time between unit time events and is known in advance).
has an inherent accuracy limit directly related to the resolution of the position
sensor and the unit time period T. For example, consider a 500-line per revolution quadrature encoder with a
velocity calculation rate of 400 Hz. When used for position, the quadrature encoder gives a four-fold increase
in resolution; in this case, 2000 counts per revolution. The minimum rotation that can be detected is therefore
0.0005 revolutions, which gives a velocity resolution of 12 rpm when sampled at 400 Hz. While this resolution
may be satisfactory at moderate or high speeds, for example 1% error at 1200 rpm, it would clearly prove
inadequate at low speeds. In fact, at speeds below 12 rpm, the speed estimate would erroneously be zero much
of the time.
provides a more accurate approach. It requires a position sensor that outputs a fixed
interval pulse train, such as the aforementioned quadrature encoder. The width of each pulse is defined by motor
speed for a given sensor resolution.
can be used to calculate motor speed by measuring the elapsed
time between successive quadrature pulse edges. However, this method suffers from the opposite limitation,
as does
. A combination of relatively large motor speeds and high sensor resolution makes the time
interval ΔT small, and thus more greatly influenced by the timer resolution. This can introduce considerable error
into high-speed estimates.
For systems with a large speed range (that is, speed estimation is needed at both low and high speeds), one
approach is to use
at low speed and have the DSP software switch over to
when the
motor speed rises above some specified threshold.
7.1.1 EQEP Related Collateral
Foundational Materials
•
Real-Time Control Reference Guide
– Refer to the Encoders section
Getting Started Materials
•
C2000 Position Manager PTO API Reference Guide Application Report
Expert Materials
•
CW/CCW Support on the C2000 eQEP Module Application Report
7.2 Configuring Device Pins
The GPIO mux registers must be configured to connect this peripheral to the device pins.
For proper operation of the eQEP module, input GPIO pins must be configured via the GPxQSELn registers for
synchronous input mode (with or without qualification). The asynchronous mode should not be used for eQEP
input pins. The internal pullups can be configured in the GPyPUD register.
See the
GPIO
chapter for more details on GPIO mux and settings.
Enhanced Quadrature Encoder Pulse (eQEP)
464
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
Страница 2: ......