Installation, Programming and Maintenance Manual
4
Chapter 3: 955QD Overview
Chapter 3: 955QD Overview
A new method of interfacing magnetostrictive transducers offers customers an interface as common as analog
with the speed and accuracy of pulsed type signaling. The Gemco 955QD LDT provides quadrature output
directly from the transducer to the controller (see drawing below). The output from the transducer can be
wired directly to any incremental encoder input card, without the need for a special converter module or PLC
interface card designed specically for use with a pulse output magnetostrictive transducer.
3.1: Quadrature Output
The quadrature output provides absolute position data in engineering units. This means that the need for the
calibration constant (wire speed) programming has been removed, thereby eliminating the possibility of having
an improperly calibrated system. The output signal wires are driven by differential line drivers, similar to the
drivers used in most magnetostrictive pulse type transducers, providing a high degree of noise immunity.
A unique feature of this transducer is a “burst” mode of operation. An input on the transducer triggers a
data transfer of all the incremental position data relative to the transducer’s absolute zero position. This can
be used to achieve absolute position updates when power is restored to the system or anytime an update
is needed to re-zero or home the machine. Additionally, another input to the transducer can be used to
establish a “zero” position for the transducer.
3.2: Signal Connection Application Note
Overview
This application note will attempt to clarify the input and output signals of the 955QD quadrature probe.
Inputs
The quadrature probe has two inputs, the “zero” and “burst” inputs. These inputs are “single ended”. That
is, the connection for each input consists of only one wire, the corresponding signal wire. For these (single
ended) inputs, the signal is measured with reference to the power supply ground, which is sometimes referred
to as “common”.
The quadrature probe is available with 24VDC level signal thresholds or TTL level thresholds. The
signal voltage level required to activate the input for the +24 VDC level signal is proportional to the power
supply voltage that the customer is supplying to the probe. This level is approximately 41% of the power
supply voltage. For example, if the power supply voltage powering the probe is exactly 24VDC, the threshold
voltage would be approximately 9.84 volts.
