Stevens Shaft Encoders and Floats

A shaft encoder is an electro-mechanical device used to convert the angular position of a shaft or axle to an analog or digital electrical signal. These devices are used in many applications including liquid level measurement. Part of the mechanical aspect of this device for level measurement utilizes a float and counter-weight attached to a line or tape placed around a pulley attached to the encoder’s shaft.

As the level changes, the float moves up and down and, thereby, rotating the pulley and the attached shaft – generating an electronic wave form for both rotating direction and amount. By converting shaft rotation into electronic signals, encoders are used to electronically monitor the position of a rotating shaft. There are two main types of encoders for liquid level measurements are absolute and incremental.

Absolute encoders provide a binary “word” for each position. Each bit requires a separate optical channel. The resolution is equal to the number of output bits. Absolute encoders constantly retain the correct position for one revolution. Therefore, the main advantage is that the output signal is not affected by a power shut-off. When power returns the encoder recognizes what position it is in based on the voltage measurement reference. Whereas incremental measurements rely on a referenced position pointer. Therefore, if power is shut off to an incremental encoder, the reference is lost and incremental pointer resets to zero.

Incremental (relative) encoders provide a contact or pulse for each increment of shaft movement. Usually this consists of two optical quadrature channels to enable the determination of the direction of rotation. The incremental encoder has a lower cost than the absolute encoder due to the limited number of channels, and the encoded position is not limited in revolutions.

Advantages of Float-operated sensors for water level measurements:

• Float-operated systems are easy to understand and troubleshoot.
• Most encoders offer good temperature stability.
• Various electronic technologies can be used including digital incremental and digital absolute (encoders); analog absolute (potentiometers and Linear variable differential transformers); or digital absolute (synchros).
• The float is protected in a stilling well and sensor is not in direct contact with the water. Therefore the risk of damage is low from debris flow or fouling.
• Highly accurate with large sized floats.

Limitations of Float-operating sensors in water level measurement:

• Requires a stilling well to assure stability and reliability of the float-operating system.
• Rapid changes in water level may result in the cable / tape line becoming disengaged from the float-operating sensor’s pulley.

	Encoder	Accuracy	Operating Temperatures	Ranges	Input	Output Signal	Torque	Dimensions W x H x D
	SDI-12 (stand alone)	.01 ft w/12" pulley	-40 to 70 C	-999.99 to 999.99 ft.	12 VDC	SDI–12	0.2 in. oz	6.4 x 5.9 x 5
	SDI-12 (A/F Attachment)	.01 ft w/12" pulley	-40 to 70 C	-999.99 to 999.99 ft.	12 VDC	SDI–12	0.2 in. oz	4.8 x 4.8 x 3.5
	PG III	300 pulses/ shaft rev	-40 to 70 C	500 pulse/sec	5-12 VDC	Up-Down pulse	NA	5 x 5 x 5
	Type A/F (stand alone)	400 transition /rev	-40 to 70 C	15/rev/sec	5 VDC	Quadrature	NA	5 x 5 x 5
	Type A/F (chart recorder attachment)	400 transition /rev	-40 to 70 C	15/rev/sec	5 VDC	Quadrature	NA	2.45 x 2.2 x 1.3
	Position Analog Transmitter (PAT)	0.03% (FS)	-40 to 70 C	0.8 to 105 ft.	13.4 – 40.2 VDC	4-20 MA, VDC	0.3 – 0.6 in. oz	5 x 5 x 5
	Syncro	.01 ft w/18" pulley	-55 to 85 C	0 – 999.99	110 VDC	Voltage	NA	5 x 5 x 5