An encoder (also known as a tachometer or rotary pulse generator) is an electromechanical device that converts mechanical rotation into electrical pulses. These pulses can be used to determine speed, position, and length. A typical use is for feedback in AC and DC motor drive control systems. They are also used to measure and instrument process lines to improve productivity.
- Reluctance Style (Tachometer)
A magnetic pickup is used to sense the magnetic poles on a steel rotor (gear). By varying the number of teeth on the rotor, the number of pulses per revolution (PPR) will vary.
The output of a reluctance style tachometer is a sine wave. Reluctance tachometers do not require a power supply, so their base cost is low. However, the unamplified sine wave signal is not as strong as "zero-speed" or "square wave" style encoders. Also reluctance tachometers do not sense speed at low RPM. Typically, their output does not reach an acceptable level until the speed reaches 30-50 rpm.
Sine wave tachometer signals usually operate above and below zero volts during any one pulse cycle. Sine wave output values are usually expressed as voltages "peak to peak", or in other words, a measurement from the maximum above zero value to the minimum below zero value. Contact Nidec-Avtron to replace obsolete reluctance style tachometers.
These sensors use resistors that are sensitive to magnetic fields. When a rotor with magnetic poles is sensed, a sine wave is generated which is then converted to a square wave. This pick-up is more sensitive and thus can operate from 2 to 8192 PPR depending on the model being used. They require a power supply which permits operation down to zero speed and can drive cable lengths up to 2000 feet! Nidec-Avtron's Wide-Gap magnetoresistive technology resists interference from motors and brakes (see our white paper for more details), and eliminates sensor scraping damage.
This system uses an infrared LED light source that shines through a rotating, slotted or photo-etched disk, and on to a photoelectric cell. When the slots align with the LED, light shines through to give a high output from the sensor. When they misalign, the output goes low. The signal generated is amplified and produces a square wave output. This design allows up to 32768 PPR and operation down to zero speed (see models for details). They require an external or drive power supply. Nidec-Avtron's Wide-Gap optical technology eliminates sensor damage from shock or vibration and Nidec-Avtron never uses fragile glass disks.
- Analog tachogenerators have brushes that wear out and therefore require maintenance. Also because of this brush wear, the electrical output can vary and cause process control variations. With a digital encoder there are no wearing parts other than bearings so there is no periodic maintenance. Avtron Modular encoders even eliminate the bearings!
- Analog tachogenerators are made from copper and iron, whose properties change as time and temperature change. A digital encoder's output pulses per revolution (PPR) will remain the same over both time and changes in temperature.
- Digital encoders cost less than "precision" analog tachogenerators.
- If your analog tachogenerators are BC46, BC42, or RE210 flange styles mounted on NEMA 56C face adapters, then often all you need to do is mount any solid shaft Avtron encoder with a NEMA 56C face. This method allows you to retain the same coupling and mounting bolts. It is a direct mechanical replacement. Our AV485 is a typical example.
- For an even more durable installation, consider removing the flange adapters and couplings and installing a modular encoder with no bearings, couplings, or other moving, wearing parts. Examples include: AV125, AV850, AV56, and AV85.
- If foot mounted: all of the Avtron Heavy Mill and Severe Duty (HMD/SD) encoders (such as AV485) have the same shaft size and shaft centers as BC46/BC42 tachogenerators. The mounting base dimensions are identical so no modification to the mounting holes is required. These foot mounting kits are available to match the footprint of either Avtron Classic encoders or analog tachogenerators. Simply specify the foot mount kit you need with your Avtron encoder.
- If you have a 5PY analog tachometer, the Avtron Model M925 can be used, or by replacing the stub shaft, a hollow shaft model such as HS35M can be fitted. The mechanical design of the M925 allows it to be used as a direct mechanical replacement.
- One important exception to the above procedures: Mill Duty (MD) or other motors or shafts that have sleeve or roller bearings that permit high axial movement. In these cases, the analog tachogenerator should be removed and replaced with a severe duty hollow shaft encoder such as AV685, along with the appropriate stub shaft. The tether mount of the AV685 will permit it to move with the axial thrust of the motor and give the longest life.
- If you need to maintain the analog voltage output of a tachogenerator to an older analog drive, Nidec-Avtron offers several Brushless tachometers (K660A, K661, K662) which can be combined with modern digital encoders to replace analog tachogenerators. Contact the Nidec-Avtron Customer Help Desk for assistance.
Nidec-Avtron recommends torsionally rigid, zero backlash, flexible couplings. Examples are the Thomas miniature flexible disc couplings manufactured by Rexnord in Warren, PA (814-723-6600). For customer convenience, Nidec-Avtron offers these couplings as accessories. For special high axial movement applications, spider style couplings may be used as an alternative. Refer to Nidec-Avtron's Pulse Generator Handbook for installation details.
Yes. Flexible couplings are designed only to handle small amounts of misalignment that may exist after careful installation. They will also accommodate additional misalignment that may occur as a result of temperature changes, bearing wear, etc. Refer to Nidec-Avtron's Pulse Generator Handbook for installation details.
Standard kits are available from motor manufacturers to mount almost any encoder. Nidec-Avtron offers a complete range of Mill Duty encoders including: NEMA 56C Face with 4.5" flange, NEMA FC Face with 8.5" pilot, NEMA 12.5" C-Face, PY style with 2.5" flange, and Reliance 6.75" rabbet. Adapter flanges are also available to adapt between flange sizes and permit mounting on large frame motors which lack an industry standard flange.
There are several ways of mounting encoders on new existing AC motors;
Many Marathon Blue and Black Max, GE AC, and Reliance RPM III AC motors offer a C-Face like a DC motor, which permits the installation of a modular encoder such as THIN-LINE II™, AV850, etc.
In many other cases, the hollow shaft mounted HS25A, HS35A, HS35M, M3, M4, and AV685 products simplify both the old and new motor mounting by providing an encoder that installs without requiring a flange (or flange modifications) to the motor itself. for To mount a hollow shaft tachometer, simply add a stub shaft (if one is not present) to the opposite drive end motor shaft. Nidec-Avtron offers stub shafts for many different motors, and can custom-build stub shafts as needed.
Consult the Nidec-Avtron Customer Help Desk for your options. 216-642-1230 ext1369.
If properly installed, with the correct alignment, right coupling (or no coupling at all for modular or hollow shaft models), and in a non-destructive environment, you should expect a 10 year life, on average. We recently received an inquiry from a customer seeking to replace an Avtron encoder which had been in service for over 35 years!
- "sandwich" a (modular) style encoder with your foot or face mounted encoder.
- Connect a second encoder to the optional dual shaft available on many Avtron solid shaft encoders such as AV485.
- Stack SMARTach II™ on 8.5" flanged motors for up to 4 isolated outputs.
- Stack THIN-LINE II™ encoders on a flanged motor for up to 4 isolated outputs (or more with a special shaft adapter).
- Use an extended stub shaft and mount several hollow shaft encoders on it (works best with lighter weight models such as HS35M dual output).
- If you have a motor with a NEMA FC 8.5" flange on it, then our AV850 SMARTach II™ or AV85 THIN-LINE II™ will work. This will save from 6.5" to 8" on overall length.
- If you have a 4.5" or 6.75" flange, use the AV56 or AV67 THIN-LINE II encoder. This will save 8" of space.
- If all you need is a way to monitor length or speed, then you may want to use the M185 Unipulser in conjunction with an Avtron Digital Meter. The M185 can mount just about anywhere; on a roll, shaft, or motor. See M185 instruction sheet for details.
Selecting an Encoder
All Avtron encoders are built to the same rugged standards with respect to the electronics inside. Mechanically, however, there are some differences. 70% of encoder failures are mechanical, consisting primarily of bearing and coupling failures. Eliminate failures with these designs:
- The encoders that eliminate bearings and couplings are the SMARTach II™ (AV125 and AV850) and the THIN-LINE II™ (AV56, AV67, AV85) models. These encoders are two piece devices. A rotor mounts on the motor shaft and a stator/housing mounts on a mechanical flange on the motor. Sensors in the stator/housing sense the magnetic poles on the rotor. In effect the motor bearings become the encoder bearings, and no coupling is required. The electronics are fully potted and sealed against water, oil, and other contaminants.
- Motors without machined accessory faces may be able to use any one of our hollow shaft encoders. These encoders mount directly on a motor stub shaft extension. They do have a dual bearing assembly, but eliminate the coupling and adapter flange. Select our severe duty AV685 for the heaviest bearings, best seals, fully potted/sealed electronics, and longest life.
- Nidec-Avtron's catalog and web site feature a full selection guide by application to help assist you or contact us for more information.
The dual shaft option provides a rear shaft extension and is typically used on Avtron foot mounted encoder designs such as AV485. This allows other encoders or devices to be connected directly to the rotating device.
The operating environment will dictate the style enclosure you need to ensure reliable operation. Avtron severe duty (SD), heavy mill duty (HMD), and Mill Duty (MD) encoders can handle rugged industrial conditions like paper and steel mills.
In applications where extreme low temperatures are encountered, special lubricants are used in the bearings and sensors are hand picked for optimum operation at the temperature extremes. Consult the Nidec-Avtron Customer Help Desk for details.
Explosion protection standards vary worldwide, and the applications also vary widely. The Avtron M6 encoder is rated to ATEX (European/worldwide) standards for above-ground use. Many USA customers require UL Class and Division (CI/Div) certification, or CUL (Canada) certification instead. However, larger customers and installations often site certify to UL and the M6 has been certified successfully in these installations. The M6 offers the great benefit of being one of the world's only flameproof hollow shaft encoders. This eliminates couplings and alignment issues common to shafted units. Moreover, no intrinsic safety barriers or other external devices are required. XR encoders include ATEX ratings also, with UL Class and Division ratings pending. Consult your safety certifications agency and compare your customer's requirements to the M6 and XR encoder certifications or contact Nidec-Avtron for additional help.
Replacing an Encoder
Yes, Nidec-Avtron provides a full range of encoders with electronic output, pinouts, connectors, and physical mounting that is identical to competitors' models. The big difference is the durability; Nidec-Avtron provides a full range of upgrades, from heavy duty bearings to magnetic waterproof technology that will enable an Avtron encoder to last far longer in your application. Our web site has a conversion tool to directly convert competitor's part numbers to the superior Avtron models, or contact us for assistance.
These part numbers are removable sensors for encoders. M284 sensor fits an M285 encoder and M484 sensors fit both M485 and M685 encoders. You can purchase an entire encoder (there is another nameplate on the encoder body to help you find the full part number), or simply replace a damaged sensor as your needs dictate. In most cases, the housing and magnetic rotor are extremely difficult to damage and only the sensor is damaged.
The second output can be used as an "on hand" spare for quick change over if one output is damaged. Or it can be used to supply auxiliary systems like speed readouts, computers, or process controllers.
With Avtron dual output encoders, the outputs are totally isolated. On some other brands, the second output is duplicated from the primary and upon the primary side's failure the entire encoder fails.
That depends on several factors including:
- Length of cable run
- Signal requirements at system
- Environment (temperature, abrasion, tensile strength)
- Number of conductors
- Type of signal
In general, Nidec-Avtron strongly recommends cable that provides:
- 3 or 4 twisted pairs: wire complementary signals to each pair. 16-22 gauge.
- Individual shield for each pair of wires: ground shield at one end or the other, but never both.
- Low capacitance: lower pF per foot is better. ( <50 pF/ft, <0.05 µF total cable run )
There are many different connectors available for use on encoders. The most popular are the 10 pin MS connector and the 10 pin Industrial Connector. Both offer excellent performance but have their strong points in certain applications.
The MS Connector has better moisture and vibration resistance and is better suited in outdoor applications or areas of very heavy water exposure. The downside to this connector is that the mating plug has solder terminations which are less convenient for installers.
The Industrial EPIC style connector is rectangular and has a quick disconnect feature. It is gasketed but not suitable for high vibration or severe water exposure. This connector is very popular due to the screw terminals used in the mating connector. No soldering is required so installations tend to be faster. The Industrial Connector is a great general purpose connector for the large majority of encoder applications with the above environmental considerations in mind.
Other connector styles include a cable with pigtail leads or the 10 pin Industrial Connector attached, terminal box or MS connector on a 5' flexible conduit, conduit condulet, and on smaller encoders, 10 pin MS connectors with special length cables prewired to the mating plug and European style M12 connectors. We even offer older style encoder connectors for use as replacements for classic encoders. We offer connectors and wiring schemes to replace competitive encoders so existing mating plugs may be used without modifications.
Explaining the Technology
A square wave describes one of the output choices of an encoder. If viewed on an oscilloscope, the square wave looks like this: It represents the on/off conditions of the sensing circuit. An "ON" condition is sometimes described as "HIGH" and an "OFF" condition is described as "LOW". One entire pulse, 360°, equals 180° of an "ON" condition and 180° of an "OFF" condition.
When the "ON" and "OFF" conditions are exactly 180° of the entire waveform, this is called a 50/50 duty cycle. The duty cycle can sometimes vary, and will be expressed as 50, ±X%, with "X" being the amount of variation that one could expect.
A "two-phase" encoder provides a second signal to the system it is feeding that allows the direction of rotation to be determined. This is done by offsetting one signal from the other by 90°. One signal is usually referred to as Phase A the other is Phase B. In one direction, Phase A will lead by 90° and in the other Phase B will lead. This is often called quadrature or A Quad B.
This scheme is typically used with: regenerative drives, reversing drives, position controls, and length measurements.
Direction sensing refers to the ability of an instrument or controller to determine the direction of rotation of a shaft/motor by looking at the pulses coming from the encoder. The encoder output must be a two-phase type, quadrature with one phase 90° out of phase with the other.
- That is very much dependent upon the application.
- When interfacing an encoder to a drive for speed feedback, the drive manufacturer will typically specify an optimal PPR, or a maximum frequency by maximum motor speed in revolutions per second.
- How to calculate pulses per revolution rate for a given frequency input limit and maximum motor or shaft RPM.
- RPM / 60 = RPS (revolutions per second)
Max. frequency: 10,000 Hz. Max. motor speed: 1700 RPM / 60 = 28 RPS
10,000 Hz / 28 RPS = 357 PPR nearest standard PPR to be used is 360 for this example.
- How to calculate frequency output of an encoder given the PPR and maximum motor or shaft RPM.
Max. motor speed is 3600 RPM / 60 = 60 RPS. Encoder PPR is 1024
Max. frequency output of encoder at 3600 RPM. 60 RPS x 1024 PPR = 61,444 Hz.
- In Example A, a 360 PPR encoder would work perfectly. If the desired PPR is not listed among the standard/stock items, then select the next lower value, or contact the Nidec-Avtron factory.
- When selecting an encoder for positioning accuracy, calculate the needed resolution by converting the physical accuracy required of the load (say 0.001") to the amount of rotation seen by the encoder (through the gearbox, if any) (example: 1°). Calculate the minimum PPR = 360/ (movement in degrees) x 2 = 720 PPR. Ideal PPR = minimum x 5 = 3600 PPR.
- Select an encoder between minimum and ideal values, keeping in mind any possible frequency limitations on the input device (see Example A).
- The default PPR for most new applications worldwide is 1024 PPR - high enough to give accuracy, but low enough not to overflow most recent drives and controls with too many pulses per second. 1024 PPR encoders are in-stock by most manufacturers and distributors worldwide, which makes them an easy choice.
Short circuit protection is an important feature in any encoder, especially in a multi-sectional drive control system where downtime can cost thousands of dollars per hour.
Short circuits can occur for a variety of reasons, including varying errors and cable faults. With certain types of line drivers, this can cause a complete failure of the encoder and necessitate removal and replacement. With a protected circuit, the encoder may not function if miswired, but it can be rewired and operated without replacing the encoder itself. Many Avtron encoders come with protection circuitry.
Encoders come with a variety of outputs to suit many different applications. Variables that determine what will work best include: output frequency, cable type, cable length, and drive input circuits. Some line drivers offer high power for long wires, but sacrifice short circuit protection. Others are fully protected, but cannot drive long wires, particularly at higher voltages. Nidec-Avtron's latest high power line drivers are both powerful and short circuit protected. These drivers are available on THIN-LINE II™ (AV56, AV56S, AV67, AV85, AV115), SMARTach II™ (AV125, AV485, AV685, AV850),as well as HS35M and XP45. Consult the system manufacturer to determine input requirements and then consult the encoder specification guide for your selection. You may also contact our Customer Help Desk for assistance. Some models of line drivers on older SMARTach™ models (M285, M485, M685, M1250) also greatly affect current draw of the encoder, so check the manual before specifying a particular driver.
A complementary signal is the inverse of a standard encoder output signal. When Phase A is high, the complement of Phase A, called Phase "A not" is low. When Phase A is low, Phase A not is high. Complementary signals are sometimes called "differential" because of their use with differential amplifiers. Differential amplifiers accept both the Phase A, Phase A not, and Phase B, Phase B not signals and are usually used where long cable runs exist and high noise immunity is needed. These are also known as quadrature or A Quad B signals.
Open collector outputs are an older system using a single output transistor to bring the output to zero volts, then a "pull up" resistor wired to V+ to bring the voltage to the high level. They produce square waves like modern line driver outputs, as long as the cable is not too long. Open collector outputs can be replaced by modern line driver outputs; be sure to remove any external or internal pull-up resistors remaining in the circuit.