Churchill Nano Link IP67, Техническое руководство, Страница: 27 / 42

Page 25
sends data to continuously-powered outstations. However, if the outstation reports that it uses digital outputs, a
Nano_Link
base-station will automatically send data to it as well as reading from it.
Obviously, the power consumption in this mode is higher than in normal power-saving mode.
7.1.7 Power-save mode, analogue averaging (S2.6…S2.8 = 011)
This mode is similar to the other power saving modes described above, but makes provision for ‘noise’ on the analogue
inputs.
All other modes take a snapshot of the analogue level at the time it is transmitted to the base-station. If there is any
variation in the input signal (for example, if a depth transducer is affected by waves on the surface) the level sent may
be in error, depending on the height of the wave at the instant the reading is taken.
The analogue averaging mode minimises this by applying power to the transducer 35 seconds before the outstation is
next due to be called. It waits 5 seconds for the transducer to stabilise, then takes continuous readings at one second
intervals. Thus when the outstation is called it will have taken 30 samples, and it returns to the base-station the average
of these. The effect of any fluctuations due to waves on the surface will thus be minimised, at the expense of a slightly
higher overall power consumption.
7.1.8 Power-save mode, Rotation Sensor (S2.6…S2.8 = 111)
This mode should only be set if a 7062-1 magnetic field sensor is connected to analogue input 2.
The magnetic field sensor is a very sensitive resistive bridge which is unbalanced when a magnetic field is applied. It is
sensitive enough to detect the earth’s magnetic field, so the output varies from a maximum when the sensor points
north to a minimum when it points south.
When the outstation is configured for use with a rotation sensor it will power the analogues at regular intervals for
250ms then read analogue input 2. It compares the new reading to the last. If the new reading is greater than the last it
knows that the sensor is rotating from south towards north. Conversely, if the new reading is less than the last it knows
that the sensor is rotating from north towards south. Therefore it knows that when the ramp changes from positive to
negative the sensor has passed north, and when it changes from negative to positive the sensor has passed south.
From these sample readings the outstation calculates the total of the number of revolutions and the time per revolution
(in seconds). These are stored in Count 1 and Count 2 respectively (when in this mode, digital inputs 1 and 2 cannot be
used for totalising pulses).
To calculate the ramp direction the outstation must sample the sensor at least twice in each half-revolution. It therefore
uses the time per revolution to define the sample rate. If it finds there are less than 4 samples per half-revolution or
more that 8 it will recalculate the sample rate to achieve nominally 6 samples per half-revolution. If it was sampling at
4 per half-revolution and the speed suddenly doubled it would only take two samples in the next half-revolution, but
this still is enough to correctly track the movement. The sample rate will then adjust to ensure it is able to track future
speed changes. If the speed increases by a factor of more than 2 in a half-revolution there is a possibility that the
system could temporarily lose synchronisation.
If the outstation detects that the rotation speed has reduced to less than one fifth of the previous rate it will assume that
the sensor has stopped rotating and will raise an alarm. For example, if it was rotating at 1 revolution per minute it will
raise an alarm if no changes are detected for 5 minutes. The rotation alarm can be read at the base-station in the alarm
flag position allocated for
Bus_Link
fail (since this is not normally used on a
Nano_Link
outstation).
The sampling period automatically adjusts in the range 1…60 seconds, to try to keep the sampling rate in the range
8…16 samples per revolution. There must be at least 2 samples per revolution, so the maximum rotation speed that can
be tracked is 4 seconds per revolution, and the minimum is 512 minutes per revolution.