What is meant by Rotary Incremental Encoder Index Pulse “gating”?


qd145_sealed_open

Gating refers to the width of the Z (index) pulse relative to the A and B channels. With ungated devices the edges of the Z pulse have no hard relation to A & B.

Gating to channel A, requires the Z pulse to be high once per revolution and only when A is high.

Gating to channels A&B high requires the Z pulse to be high once per revolution and only when A AND B are high.

Gating to A creates a Z pulse roughly 180 Electrical Degrees in duration while gating to A&B creates a Z pulse roughly 90 Electrical Degrees in duration.

Gating to a smaller duration increases the precision of the Z pulse, but also requires the motion system to be able to read the Z channel at a faster rate.

Gating is sometimes required by certain drive/amplifier/controller manufacturers.

QD145 Z (Index Pulse) Specifications:

incremental-encoder-gating1
Ungated Z
Z-A/A-Z min = 0 Electrical Degrees
Z-A/A-Z max= 225 Electrical Degrees
Z width min = 180 Electrical Degrees
Z width Max = 540 Electrical Degrees

Z Gated to A
Z width min = 135 Electrical Degrees
Z with max = A true

Z Gated to A&B
Z width min = 45 Electrical Degrees
Z width Max = A&B true

incremental-encoder-gated1

Ungated Z
Z true over A&B> 45 Electrical Degrees
Z-A&B/A&B-Z min = 0 Electrical Degrees
Z-A&B/A&B-Z max= 315 Electrical Degrees
Z width min = 180 Electrical Degrees
Z width Max = 540 Electrical Degrees

Z Gated to A
Z width min = 135 Electrical Degrees
Z with max = A true

Z Gated to A&B
Z width min = 45 Electrical Degrees
Z width Max = A&B true

Any optical encoder related questions can be posted here or sent to info@quantumdev.com .

For more information on optical encoders visit the main Quantum Devices Inc. Web site at http://www.quantumdev.com.

.

.

Bearing Encoder vs Modular Encoder – Making the right choice.

QD-200

The following is a comparison of Bearing encoders vs Modular (Bearingless) encoders for use in Brush less DC Motors.

Construction

Bearing encoders have a shaft that is centered to the body of the encoder on a bearing system.

Modular encoders have a shaft that does not use bearings so the shaft/disk is separate from the body of the encoder.

An examination of traditional bearing less encoder misconceptions:

“No Bearing to wear out.”

The modular design relies upon the motor bearing and shaft tolerances to hold the necessary air gap. Motor bearings have typically lower tolerances than that of Encoder bearings.  This places the responsibility of holding air gap tolerances on the motor bearings.   In encoders with bearings, the encoder bearing holds the air gap. Any excessive endplay or TIR is either held in place by the encoder bearing as the entire encoder assembly moves, not just the disk. The bearing in the encoder is essential for holding the air gap between the disk and sensor.

“Unlimited RPM

While a bearingless design would allow for unlimited RPM in theory, in practice all encoders have an upper frequency response, which ultimately limits their top speed.   The higher the line count, the less RPM required before the frequency limitation is hit.

“Fewer Parts means the  encoder costs less.”

The initial encoder may be cheaper, but in order to hold tolerances, a higher quality bearing must be used in the motor that the encoder is being mounted on.  There are also many “infant death” failures for modular encoders due to the precision that is required to set the disk to sensor air gap.  This increases overall cost of using modular encoders in a design.

The Bottom line

In the end, the decision comes down to whether or not you should take on the burden and responsibility of holding encoder air gap and alignment.

If the application is cost critical, and you feel confident you can handle the technical challenges and efficiency needed to keep from incurring labor costs that make a modular purchase prohibitive, you may be better off with a modular encoder.

If you want the speed and ease of installation, with the added benefit of having the encoder manufacturer hold air gap tolerances for you, a bearing encoder is the right choice.

.

.

Calculating Output Frequency for Rotary Encoders

dscn5852

Here are three handy calculations for incremental rotary encoders:

Frequency  = (RPM X Line Count/60)

Use this formula when line count of the encoder and RPM are known.  The answer is in Hertz (Hz) or cycles per second.  Keep in mind that many encoders and drives will have a Maximum frequency limit that you should not exceed.

.

Line Count = (Frequency X 60/RPM)

Use this calculation when Frequency and RPM are known.  Can be useful for determining the highest resolution encoder you can use without over driving the Maximum output specifications on the encoder or equipment it is being connected to.

.

RPM = (Frequency X 60/Line Count)

Use this formula when frequency and Line count are known. This is useful when you are trying to determine the speed of rotation.

.

Quantum Devices Inc. has a calculator for finding RPM, Encoder output Frequency,  or Line count (Resolution) when given the other two variables.

You can check it out here : http://www.quantumdev.com/Calculator.html


.

.

Quantum Devices is a manufacturer of rotary encoders.  Incorporating QDI’s patented sensor technology, our QPhaseTM Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations.

The Quantum Devices main web site can be found at www.quantumdev.com

.

.

Interfacing to 26C31 Line Drivers in QDI Rotary Encoders

Encoder Electrical Structure
The internal electrical structure of the QDI series of encoders consists of a photo interrupted light source that is picked up by a common mode photo detector array.

“Common mode” means that there are two channels that are at exact opposite states in any position.  The advantage of common mode sensing is that light and noise interference does not create false triggering, but is instead filtered out.

Besides having common mode signals the incremental channels are also set up in an interlaced fashion. High-resolution channels need to be interlaced to provide a greater amount of signal strength and noise immunity at analog levels.

At this point the signals from the sensor are at small analog voltage levels. Depending upon the line count of the encoder these analog voltage levels may or may not be amplified before being digitized by a comparator.

The purpose of the comparator is to “square off” the signals to create digital signals. The comparator converts these Digital signals into single ended (Referenced to ground) signals.  Besides being single ended, these digital signals are still not robust enough to be used directly by most applications.

After the comparator, the signals sent to a Line Driver.  The line driver splits each signal back into two complementary signals, and also provides the ability to deliver much more current.

The output of the line driver is then electrically connected to flying leads or an interface connector of the encoder, making it the final interface device inside the encoder that is electrically connected to the outside world

Damage to the line driver can occur if the amount of power the line driver is delivering is greater than its specification. The amount of power is directly related to the resistive, inductive, or capacitive loading of the line driver.

DS26C31 Output Schematic

ds26c31-outputs1


Areas of concern that may contribute to Line Driver Failures

If the flying leads of the encoder are shorted to other leads or if the leads are shorted to the supply voltage, or ground. Excessive current may be allowed to flow through the output channel of the line driver. This current generates heat that the IC is not able to dissipate and damages the internal junction. This junction may fail open or closed effectively creating a locked Hi or Lo Electrically.  Proper termination of both used and unused leads before power is applied to the encoder is essential to prevent unintentional shorting.

If the output channels of the encoder are loaded too heavily (greater than 100 ohms) then the Line Driver IC may not be able to dissipate the heat from the internal junction and can damage the line driver.

If the encoder is operated in ambient temperature exceeding specifications there is a possibility for damage to the encoder outputs as the difference between the encoder temperature and the ambient temperature plays a key role in the ability for the Line driver to dissipate heat.  If there are any uncertainties as to the environment that the encoder is to be used in QDI engineering should be consulted for recommendations.

External Voltage that is applied to the outputs of the encoder may cause damage. This can come into play if a line driver is set up to be used in an open collector style application and pulled up to a voltage higher than VCC.  You can recognize this situation if you are using a pull up resistor with a power supply that is something other than the power supply for the encoder.  A pull up resistor is typically more effectively used with an open collector type output, where as line driver outputs should be terminated to signal ground or to complementary signals in a differential fashion.

Interfacing to a line driver output with a power supply different than the encoders. Using a separate isolated supply to pull up an output or to interface to the encoder can cause signal amplitude irregularities, noise problems and line driver failure.  The signal ground for the encoder should be electrically tied to the power supply common of any systems that are interfacing to the encoder.