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USING SHAFT ENCODERS IN MOTION CONTROL

OPERATION
Encoder Operation
An encoder is a rotational transducer that converts angular movement into digital impulses. It usually consists of cylindrical body, flange, shaft and output connector. An internal transparent disk with photographically printed radial markings is rotated and interrupts an infra red light source produced by an LED. An optical receiver (phototransistor) on the other side of the disk converts the light beam switching into electrical pulses. The pulses are then amplified and squared before sending out to another piece of equipment. The output pulses can be used to determine angular displacement, direction of rotation, speed of rotation or angular acceleration. 
 


Encoder Types
There are two main types of optical encoders. Incremental encoders can measure speed and direction of rotation but can only measure shaft movement relative to the last position, by counting the number of pulses. Absolute encoders can measure position at any point in a full rotation, regardless of the previous position.

Construction
Incremental encoders have a disk with two radial bands of markings. These are called Channel A and B. The two bands are printed 90 degrees out of phase. This method enables the user to determine the direction of rotation and is known as quadrature. There is also an optional band called Z for zero output. This only has one marking per rotation and is used to reference a zero position or starting point.

Absolute encoders also have multiple bands, each band representing one bit of the position reading. There are more of them compared to incremental encoders and the markings are arranged in a binary code. Incremental encoders are lower in price, however, they will not know their absolute position after power supply has been removed.

INCREMENTAL ENCODER DISK

 ABSOLUTE ENCODER DISK

 

CHOOSING AN ENCODER
The choice of an encoder depends on many factors. These are the main points that must be considered.

Incremental/Absolute
Incremental encoders are lower in price, however, they will not know their absolute position after power supply has been disconnected and reapplied. Incremental encoders are well suited to speed sensing or applications where position data loss during power down is not important. Incremental encoders are suitable for most applications.

Number of turns
Incremental encoders can rotate and produce pulses for as many turns as necessary. The main problem is the receiving instrument being able to count and store a very large number. Basic absolute encoders with binary or gray code output can only read position within one rotation. These are not suitable for multiturn applications.

Series
Most of the Eltra incremental encoders are available in two styles, the EL and the EH series. The functions are the same although the EL series have slightly better specifications and are marginally more expensive. The EL series often have a wider power supply range, better sealing, higher pulse/rev counts and higher output frequency.

Size
The most critical factor is physical size, usually determined by the body diameter or flange size. This determines if you can mount it onto your machine or in a confined space.

Mounting Style
Most encoders have a simple flange and a shaft. This is the lowest cost option but may involve making brackets and shaft couplings. Hollow shaft encoders may make mounting easier. Also, an encoder with servo mounting flange may be useful where the encoder must be easily adjusted to a precise position to align a zero index or an absolute encoder zero position. Some models are available with servo mounting flanges. This allows the angular position of the encoder flange to be adjusted to align the zero index or absolute zero position to the machine.


FLANGE MOUNTING


HOLLOW SHAFT MOUNTING

Pulses per rev
Some ELTRA incremental encoders are available with upto 10000 pulses/rev or down to 2 pulses per rev. The higher the pulses/rev count means the resolution and accuracy are greater. However, choosing the highest pulse/rev count is not always the best solution. Higher pulse/rev count means higher cost and a higher output frequency for a given speed. If your shaft speed is very high, the output frequency may be too high for the encoder or receiving instrument to handle. Most ELTRA encoders are limited to 100kHz output frequency. Another problem that can occur with high pulse/rev encoders is vibration. Vibration in the machine may be transmitted to the shaft which may produce output pulses when no motion is occurring.

Higher pulse/rev counts are usually only available with large body diameters. Small compact encoders have limited pulse/rev counts. Values include 360 degree fractions, decimal numbers, 200 or 1000 factors and binary values. This list is constantly being updated so check with our office for updates. Typical pulse/rev values for ELTRA incremental encoders are as follows, although, availability depends on model and body diameter.

2-3-4-5-6-8-9-10-11-12-14-15-16-20-22-24-25-30-32-33-35-40-45-50-60-64-70-75-80-90-100
113-120-125-130-140-150-160-175-178-180-190-200-222-225-240-250-254-256-290-300
314-350-356-360-392-400-500-512-534-540-600-650-754-800900-1000-1024-1068-1100
1256-1440-1500-1599-1600-1800-1920-2000-2048-2500-3000-3600-4000-5000-9000-10000

For absolute encoders, the maximum resolution available is 8192 positions in one rev (13 bit).

Quadrature
ELTRA incremental encoders have two output channels, A and B. One channel leads the other by 90 degrees phase shift. By looking at both rising and falling edges of the outputs of both channels and ANDing the two signals, it is possible to get 4 times the encoder resolution. This allows you to obtain very high resolution from a low cost encoder (eg. a 1000 pulse/rev encoder can provide 400 pulse/rev resolution). This can be done by computer or PLC software or by commercially available dedicated ICs.

Direction
Another useful advantage of two output channels is the ability to sense direction of rotation. This cannot be done with single channel encoders. These can only be used for speed sensing or position sensing where motion occurs in one direction. All ELTRA incremental encoders have dual channel outputs.

Direction sensing is achieved by generating separate pulse trains for CW and CCW direction. A pulse train is produced by checking for falling edges on A or B pulses when alternative pulses are high. For position measurement using a PLC for example, one pulse train adds to a count register and the other subtracts from a count register.

Output Style
Depending on model, incremental encoders are available with several different electrical output styles. Choice of signal depends on receiving instrument and cable distance. Line driver outputs with complimentary outputs can be used with longer cables as noise spikes can be cancelled.

NPN
Uses an NPN type transistor and an internal resistor pulling up to the power supply rail. The output is an active voltage.

 
NPN Open Collector
Uses an NPN type transistor but without an internal pull up resistor to the supply rail. The output is passive so a separate power supply can be used.

 
PNP
Uses a PNP type transistor and an internal resistor pulling down to zero volts. 

 
PNP Open Collector
Uses a PNP type transistor but without an internal pull down resistor to zero volts.

 
Push Pull
A problem with NPN and PNP type outputs is the high output impedance. This can be solved by a complementary output allowing better switching to zero and positive supply rails. 

 
Line Driver
This output style has two complimentary outputs per channel allowing better transmission in noisy environments and long cable lengths. The receiver can process the signal, eliminating noise spikes. 

 
PTC protection
A positive temperature coefficient resistor can be added to the output of a NPN or PNP encoder, protecting it from output short circuits. 

 

Signal Connection
Three main types of output connection are available although this depends on encoder model. The basic is flying lead which is usually 1.5 metres long. Extra cable length can be ordered. Connectors are available on larger encoders. These are 'J' type or MS3102 Mil Spec connectors. The mating plug is supplied with the encoder. Connectors add to the price of an encoder but can make installation easier, particularly when long cables are involved.

 

MIL SPEC CONNECTOR 

Smaller encoders do not have sufficient space for connectors which are often supplied with flying lead only. If using line driver output, a connector will require more pins (10 instead of the usual 7).

Connector Direction
ELTRA encoders allow the cable or connector to exit the encoder body in the radial or axial position. Larger encoders have a connector mounted on a 45 degree block so connector or cable outlet can be radial or axial. This should be decided at time of ordering and may be critical when an encoder is installed in tight spaces.

 

AXIAL OR RADIAL CONNECTOR 

Sealing
Depending on model, encoder environmental protection ranges from IP54 to IP67. There are two areas where water or dirt can penetrate the encoder. These are the shaft and output connection. Shafts are sealed using a synthetic rubber seal on the shaft. This sometimes increases friction on the shaft, resulting in heat generation at high shaft speeds.

Shaft loading
Small encoders have limited axial and radial shaft loadings before damage is done to the bearings. Larger encoders can withstand higher shaft loads.

Power supply
ELTRA encoders run on a DC supply and have internal voltage regulators. The EL series will operate on between 5 and 28V or 5V only if a Line Driver output is required. The EH series will operate on between 8 and 24V or 5V if a line driver output is required. Never connect AC mains voltage to an encoder! They are all low voltage DC powered.

Motion Type
Most encoders are designed to measure angular movement. If linear motion must be measured, a cable, belt or rack and pinion system can be used to measure linear motion. Alternatively, ELTRA supply encoders specially for linear motion measurement.

INSTALLATION
Disassembly
An attempt at disassembly of an encoder breaks the seal and voids warranty. Also, the encoder contains precision aligned optical components and any adjustment of these components may cause the encoder to fail.
 

Coupling
The coupling between the encoder shaft and the machine should be made using a flexible coupling, not a rigid one. Any misalignment between the two shafts will put the encoder bearings under stress and cause damage over time.
 

 
 
Shaft Loads
Check that axial and radial shaft loads do not exceed the specifications. High loads will limit bearing life.
Do not apply excessive shaft loads when installing.

Shock
Avoid mechanical shock to an encoder, such as dropping. Internal precision optical components will be dislodged and the encoder will fail.
 

 
Power Supply
Encoders run on low voltage DC supply. Never connect to mains AC supply. Check that the DC supply is within the range of the encoder supply.
 

Environmental
Check that the encoder is not going to be exposed to water, oil or dust unless it has adequate sealing. Check that the encoder will not be exposed to corrosive chemicals that will corrode the aluminium flange.

Cable
Use screened cable for all encoder signals. Insulate any unused output wires. Do not coil unused portions of encoder cable. Excess cable should be cut off or stored without coiling which increases inductance. Do not short any output wires to power supply or other output wires.

APPLICATIONS
Encoders can be used to measure position, displacement, speed and direction of rotation.
Here are some typical examples.

Servomotor position feedback
Speed sensing on a belt weigher for conveyor belt
Backstop position display
CNC tool position feedback
Reverse direction sensing on large pumps
Position feedback on rotary hydraulic actuator
Position sensing for telescope or satelite tracking
Portable measuring instruments
Precision vehicle speed measurement

ABSOLUTE ENCODERS

Gray Code
Absolute encoders usually have Binary output or Gray Code output. Binary is often used on the smaller lower cost models. The problem with binary output is than several channels may change state at a given time. Small differences in electronic gain between channels could result in errors in when reading position at this point.

BINARY OUTPUT

GRAY CODE OUTPUT

For this reason, Gray Code is often the preferred output. It is achieved by shifting the binary number right by one bit and exclusiveOR the result with the original binary value. This does not have more than output bit switching at a time. It is not difficult to convert Gray Code back to Binary by using software in a look up table or exclusive OR gates. For large numbers of encoder channels, a lookup table approach may be simpler and not have long calculation and bit shifting times.

Binary Value 

Shifted Value 
Gray Code Value 

A

B

A xor B 

0000 

0000 

0000 

0001 

0000 

0001 

0010 

0001 

0011 

0011 

0001 

0010 

0100 

0010 

0110 

0101 

0010 

0111 

0110 

0011 

0101 

0111 

0011 

0100 

1000 

0100 

1100 

1001 

0100 

1101 

1010 

0101 

1111 

1011 

0101 

1110 

1100 

0110 

1010 

1101 

0110 

1011 

1110 

0111 

1001 

1111 

0111 

1000 
 

Multiturn Absolute encoders
A problem with absolute encoders is the ability to read positions only within one rotation. Multiple rotations cannot be detected. This has been solved by incorporating precision gear reduction stages on multiple optical disks in the encoder. The ELTRA EAM multiturn absolute encoders allow position measurement with upto 8192 (13 bit) position per turn resolution and 4096 (12 bit) revolutions.

TESTING ENCODER OUTPUTS
We often have many customers working on site wanting to do a quick check of encoder outputs without specialised equipment. There are several test devices available to test encoders. These include oscilliscopes, encoder testers, digital counters, PLCs, computer interfaces and even connecting encoders to servo and stepper drives. Digital multimeters are not very helpful for detecting pulses and the reading will be constantly changing as the encoders shaft is rotated.

A simple and low cost method is to work out what the output should be and connect high intensity LEDs to the outputs (via voltage dropping resistors). Be careful to include the correct resistor to avoid damaging the encoder. As the shaft is rotated, the LEDs will flash. You will also be able to detect the phase difference between the A and B channels. Most of the ELTRA encoders have a maximum output of 50mA per channel (20mA for line driver) so if you aim for 10mA LED current this should be safe.

This is not as thourough as using an oscilliscope or an encoder tester but it is often sufficient to check that output channels are working. Red high intensity LEDs work best due to brightness and low current drain.




AUTOMATED MOTION SYSTEMS PTY. LTD. - WESTERN AUSTRALIA