WELLOG ENCODERS

Encoders convert linear or rotational motion into pulses that provide information about POSITION and DIRECTION.

POSITION:

A typical encoder may use an optical method of producing pulses. Other methods using electrical contacts and magnetic sensors are also used.

Rotary encoders:

Rotary encoders use a wheel that rotates between an optical emitter (light source) and a photo sensor (detector). The rotating wheel is designed to interrupt the light path between the light source and detector. If a clear rotating wheel is used, it is coated with a material that interrupts the light path for a certain number of times per rotation. Other designs use a non transparent wheel with openings cut into the wheel that allow light to pass during a certain number of times per rotation. The detector generates voltage pulses as the optical interrupter wheel rotates. The number of pulses per rotation is a specification of the encoder. For example an encoder may produce 100 pulses per rotation.

Resolution:

The resolution of a rotary encoder is directly related to the number of pulses per rotation. A rotary encoder having 200 pulses per rotation has a greater resolution than a rotary encoder having 100 pulses per rotation.

A rotary encoder having 200 pulses per rotation may have its shaft connected to a wheel with a circumference of 2 feet. The rotary encoder is capable of resolving 2 feet into 200 pulses which is the same as 100 pulses per foot.

DIRECTION:

Rotary encoders use two optical emitters and two detectors to measure direction. The emitters and detectors are located in positions that cause them to be interrupted in a predictable sequence.

Clockwise rotation:

Output A Output B

on on

off on

off off

on off

sequence repeats…

Clockwise rotation:

Output A Output B

on off

off off

off on

on on

sequence repeats…

The two outputs are said to be at “Quadrature” with each other. When observing the output pulses, each pulse switches at the half way point of the other pulse.

WELL LOGGING:

In a well logging application, wireline cable motion is measured as the cable is played over a measuring wheel. The measuring wheel is calibrated to have a certain circumference, for example, 2 feet.

Measurement of the depth of a logging tool is possible using this method of position sensing. A two foot circumference wheel with a 200 pulse per revolution encoder provides one pulse every .01 foot.

LOGGING SPEED:

Depending on operational needs, when a logging tool is lowered into or pulled out of a well, the cable moves at a certain speed. If a tool needs to be lowered rapidly from the surface at 0 feet to 10,000 feet, then a rapid speed is used in order to reduce the amount of time for the logging operation. When the logging tool is being used to obtain formation data while being pulled up hole, the speed is generally much slower.

SPEED LIMITATIONS:

Acquisition systems used in logging applications impose a limit on logging speed. Time is required for acquisition systems to acquire, convert, and transmit data. Acquisition and conversion times are usually very short. They are usually less than 10 microseconds. Transmission of data is more time consuming depending on the transmission speed.

Example 1:

A typical acquisition system may use 6 bytes of data and a transmission speed of 38,400 baud.

The time required to transmit one byte = 1/3840 seconds = .00026 seconds

Six bytes requires .00026 * 6 = .00156 seconds.

Given other processing time requirements, a total of .003 seconds may be required.

The resulting encoder speed cannot exceed 1/.003 = 333 pulses per second.

In a system using an encoder producing 200 pulses per foot, the maximum speed = 333/200 * 60 = 100 feet per minute.

Example 2:

The WELLOG V30 Logger acquisition system uses 6 bytes of data and a transmission speed of 57,600 baud.

The time required to transmit one byte (10 bits including start and stop bits) = 1/5760 seconds = .000174 seconds

Six bytes requires .000174 * 6 = .001 seconds.

Given other processing time requirements, a total of .002 seconds may be required.

The resulting encoder speed cannot exceed 1/.002 = 500 pulses per second.

In a system using an encoder producing 200 pulses per foot, the maximum speed = 500/200 * 60 = 150 feet per minute.

SERIAL BUFFERS:

Logging systems using serial communications buffers overcome the time “overhead” caused by other processing.

Buffers allow data to be acquired and stored in memory for later use. This memory is referred to as a buffer. WELLOG uses serial data buffers in our

high-level logging acquisition software. This allows logging speeds at or near the .001 second acquisition speed of 300 feet per minute.

From the perspective of the V30 logger, the six bytes take .001 second to transmit and it must catch the next encoder cycle. Therefore encoder cycles are limited to a maximum of 1000 per second. That also equates to 300 feet per minute.

WELLOG has solutions:

Recent developments in microcontroller functionality, include quadrature counters. The integrated quadrature counter can count quadrature pulses in hardware without software intervention. The software only has to read the count rate at a frequency required by the user.

MORE INFORMATION:

For more information on encoders, contact WELLOG at info@wellog.com