Section 2
Functional Description
12
FT742-SM (RS422 & RS485) Sensors
– User Manual
Note: An optional Overspeed Warning Scheme can be enabled (but is disabled by default). See Section 2.8 for
further details.
It is important that error flags are not ignored. Data associated with an error flag should
not
be processed as valid
wind data. The system should be capable of riding through rare periods when data may be temporarily unavailable.
If errors are ongoing (more than several seconds), a reset of the sensor should be applied (see Section 7.4.20 for
instructions on sending an RSU command).
It is recommended that errors are monitored and logged. If the frequency of errors has recently increased, then
inspection of the sensor for physical blockages may be required (see Section 4.1).
2.8 Overspeed Warning Scheme
During periods where the sensor detects wind speed beyond the rating of the sensor, the sensor will (by default)
indicate a general error flag status.
An additional overspeed warning scheme can be enabled: if an overspeed condition is detected (above the highest
speed rating), the error flag character will be set to: 2 (see Sections 7.4.29 and 7.4.30) unless a general error
condition is also detected and the flag will be set to 1.
To comply with legacy behaviour, this scheme is disabled by default.
The Overspeed Warning Scheme can be enabled by software commands (see Section 7.4.19), via the Acu Vis
PC software program or enabled in the factory before despatch.
2.9 Heater Setup
The sensor is fitted with an integral two-element distributed heater that can be used to prevent icing-up of the
sensor in freezing temperatures. The heater is controlled automatically by the sensor using a user programmable
‘set point’ temperature. The sensor uses a control scheme which dynamically changes the current supplied to
each individual heater element in order to maintain the programmed set point temperature.
It is important to consider the resistive losses in the cable and rate the cable appropriately. In general, power
losses in the cable should be minimised in order to maximise the available heating power to the sensor. A heater
setpoint temperature of >30°C is recommended for most applications. To change the heater set point or to disable
the heater use the Acu-Vis test software or the HT software command (see Section 7.4.15).
Since the heater circuit is thermostatically controlled, the actual power being drawn from the supply will depend
on the voltage supplied, the programmed set-point and the environmental conditions (i.e. ambient temperature,
wind speed, precipitation etc.). The maximum current that the sensor can consume is limited to 2A (nominal) by
default. The power supply must be rated to provide the maximum heater power (60W @ 30V and 2A) that the
sensor can consume. The heater requires a minimum of 9VDC for operation (contact FT technologies for further
details on the minimum heater voltage limit).
For applications requiring higher power/current ratings or modification of heater performance (including cold
environment operation) please contact the FT technical support teams for further information.
Caution - Modifications to the heater current limit are not permitted on the FT742-SM. Cables must be
suitably rated for the application. Contact FT Technologies for further information.
If the sensor is powered up and there is a possibility it has become iced, it is recommended that the sensor is
allowed to heat up for 30 minutes, followed by a User Reset Command (see Section 7.4.20) to permit the sensor
to initialise correctly without ice blockage.
2.10 Low Power Operation
The sensor is designed for typical operation at 24VDC, operating at a range of 6-30VDC, however the heater will
deactivate below 9VDC. Lower voltages reduce the overall power consumption and heater performance.
For further advice on power and heater management strategies, see Sections 7.4.15 and 7.4.16.
