Xylem H-335 - Manual (Page 6)

1-2 Introduction H-335
1.2 Theory of Operation
The H-335 has a single axis tilt sensor, signal processing circuitry, 14-bit analog to digital converter and a
microprocessor. The sensor is a ceramic based electrolytic tilt sensor which provides high resolution and
very good linearity. The sensor is constructed as a hermetically sealed ceramic cavity with electrodes
partially submerged in a conductive fluid. The ratio between submersed areas of the facing electrodes is
proportional to the inclination angle. The electronics provide an AC sensor excitation together with a
precision, ratiometric peak-hold detector. Because scale factor of electrolytic sensors is sensitive to
temperature, the H-335 has a built-in digital temperature sensor. The microprocessor uses a math
polynomial to correct for temperature induced effects and any non-linear characteristics of sensor.
1.3 Problems with Winch Mechanism and Existing Measurement Systems
Tainter gates are typically controlled with a motor operated winch mechanism. The gear box for the winch
has a “sundial” scale and pointer which shows the current gate opening. Often the mechanism also has a
selsyn transmitter which allows the gate position to be monitored in a nearby control room. The sundial and
selslyn driven scales are usually marked in units of “feet”. Unfortunately most winch mechanisms have no
provisions for monitoring the gate position with a data logger or SCADA (System Control and Data
Acquisition) system. In the past, users have tried using incremental shaft encoders connected to the winch
with a sprocket and chain. This technique has the following shortcomings:
The radius of the winch drum changes as the cable winds up, giving a non-linear readout.
The lift cables stretch as the gate is raised off the sill.
The backlash (hysteresis) in the drive chain between the winch drum and encoder can be large.
Each installation requires custom mechanisms, enclosures and setup.
The winch shaft is not accessible on some gearboxes.
The H-335 overcomes many of the problems associated with measuring the position of a tainter gate by
directly monitoring the angle of the gate structure. Backlash and non-linear effects of the winch drum and
cables are completely bypassed.