Function; Operating Principle - Datcon DT920 User Manual

1. Function

.
The apparatus DT920 is a pulse-input tachometer, which can be installed in explosive areas
classified as Zone 1.

2. Operating principle

.
Contact signal transmitters, or any inductive proximity sensors in accordance with the standard
DIN 19234 (NAMUR) can be connected to the input of the apparatus. The pulses, coming from
the signal transmitter, arrive at a repeater output after galvanic isolation, providing opportunity
for a processing independently from the device. The proportionality factor between the input
frequency and the displayed value is programmable. The apparatus has 4 galvanically isolated
limit-value outputs, which are also programmable. A special service provided by this apparatus
is a continuous inspection of the input line concerning the short circuit or the breaking of the
line. In addition, the apparatus has a 4-20 mA scaleable analogue output.
The apparatus works with a microprocessor, and it is programmable with the 4 pushbuttons on
the front cover. The values of the parameters are maintained even after the apparatus has been
switched off, as they are stored in an EEPROM.
The apparatus uses the following algorithm to create the displayed values from the incoming
pulses:
Rotation speed = ((f / P1) * P2) / P3
f = frequency coming from the signal transmitter [1 / minute]
P1 = number of teeth of the signal transmitter gear (programmable parameter)
P2 = number of teeth of the power transmission ratio multiplier gear (programmable parameter)
P3 = number of teeth of the power transmission ratio divider gear (programmable parameter)
The basic version is equipped with the pulse input and the display only.
The following additional functions are available for the tachometer DT920:
•
Limit value output(s), maximum 4
•
4–20 mA scaleable analogue output
The Manufacturer may modify the program of the apparatus in accordance with customer
needs, or in order to ensure technical improvement. Such modifications do not have any im-
pacts on the requirements of intrinsical safety.
Figure 1 presents the block diagram of the apparatus:
NAMUR
NAMUR
input
Us
Power
supply
4
[1 / minute]
Display
Keyboard
CPU
Figure 1. Block diagram
4–20 mA
Analogue
analogue
output
output
Digital
Limit
contact
outputs
outputs