Choosing the correct industrial encoder is easy when you know the answers!
There are many reasons why you may need a rotary encoder, but what questions should you ask to determine which one is the most appropriate? Here are seven questions that can help you make the best decision:
- Do I need an incremental encoder or an absolute encoder?
- What output do I need?
- What resolution does the application call for?
- How will I mount this encoder?
- How will I connect the encoder to my controls?
- What environmental and mechanical stresses does the encoder need to withstand?
- Does this encoder need safety or hazardous location approvals?
This is a good starting point to determine the right encoder characteristics for your application. Let's go through each question and explore the many options you have when selecting an industrial rotary encoder.
1. Do I need an incremental encoder or an absolute encoder?
Incremental and absolute encoders can be used for speed, direction, and position. The absolute encoder will retain your position after power loss and an incremental encoder will not. The incremental encoder usually needs to perform a “homing” sequence after a power loss.
Absolute encoders have an option of 16 bits (65536) per turn which provide higher resolution than incremental encoders. Incremental encoders are generally less expensive and the output needed is a square wave or sine/cosine for counting, speed, and direction. Absolute encoders are normally used for continuous position and possess other attributes such as speed, scaling, preset, and fieldbus functions.
2. What output do I need?
For incremental encoder output, there are open collector (OC), push-pull, line driver, and sine/cosine. The open collector (OC) has lower leakage current and less voltage drop than push-pull, but push-pull output has better immunity and slew rate. Push-pull can be NPN (sinking) or PNP (sourcing), and therefore has better flexibility in matching an input of your controller or counter. The line driver has a high level of immunity (higher than open collector and push-pull) so this should be used for longer cable runs. The sine/cosine is used for speed and position computation. Some motor drive and safety systems have sine/cosine inputs.
For absolute encoder output, selecting a specific fieldbus is necessary. There are many to choose from such as Ethernet (EtherNet/IP, TCP/IP, PROFINET, Powerlink), CANopen, PROFIBUS, DeviceNet, SSI, AS-Interface, and Parallel. The fieldbus is usually chosen as a preference, to match existing hardware, or is mandated by the company (that the equipment is for).
3. What resolution does the application call for?
The incremental encoder can have 50,000 pulses per revolution (ppr) and an absolute encoder can have 16 bits (or 65536) per revolution (turn). To determine the resolution, the circumference of the rotating part (whether it’s a pulley, gear, measuring wheel, or cable-pull) will need to divide by the pulses per turn of the encoder. For example, if you had a 200 mm circumference measuring wheel and an incremental encoder with 5000 ppr, the resolution would be 0.04 mm.
The resolution also depends on the precision of the mechanical and electrical components being used as well as the needed resolution to solve a problem or to meet the process demand. In certain instances, some controllers cannot handle the frequency response of a high-resolution incremental encoder, so a lower-resolution encoder would need to be chosen. The specification of the controller/counter input response time would need to be compared with the maximum speed (RPM) of the application along with the pulse count of the encoder.
4. How will I mount this encoder?
The encoder can be selected with a solid shaft, hollow shaft, or recess-hollow shaft. The hollow shaft and recess hollow shaft are quick and easy to mount. But if there is movement or “run-out” in the shaft, the solid shaft with the appropriate coupling would be the better solution. A coupling can have a radial offset of +/- 1.5 mm, an axial offset of +/- 1 mm, and can have an angular error of 5 degrees. This may be enough flexibility to help prevent overloading the bearings of the encoder. There are magnetic encoders that have targets that mount as a hollow shaft and the “sensor” is mounted next to the magnetic target. These types of magnetic encoders eliminate mechanical wear, but there are tolerances between the magnetic target and the sensor that need to be observed.
5. How will I connect the encoder to my controls?
The encoders can be selected with cable or connector. If you choose the connector option, you will need a mating connector or a cordset (both connector and cable). This will give you a connection point to the encoder but does not necessarily get you all the way to your control cabinet. Depending on the distance, you may need a junction box, conduit, and cable tray. Care should be taken when routing encoder cable to reduce the influence of "noise". To reduce this influence, the cable should have a braided shield around the wires and the wires should be twisted pair. The encoder cable should be in a conduit with only low DCV cables and keep AC and high-power cables separated. If there are cable trays with AC and DC cables, there should be a grounded metal divider between the DC and AC cables. If the encoder cable does need to cross an AC cable, it should cross perpendicular (90 degrees to each other). The shield should be connected to one earth ground location in your control cabinet (so it has a star topology).
6. What environmental and mechanical stresses does the encoder need to withstand?
Not all encoders are created equal. The less expensive encoders do not have the same environmental (IP) protection and mechanical resistance as the higher priced harsh-duty designed encoders. To answer this question completely, the questions “What resolution does the application call for?” and “How will I mount this encoder?” need to be answered. The answer to these questions helps determine how much movement (“run-out”) there is and the speed of the shaft. These values, shaft speed and run-out, help in determining the shaft loads and angle offset that the encoder bearing will be exposed to. The vibration of the machine should also be measured by using an accelerometer.
For the environmental aspects, the amount of moisture, duration of moisture, chemicals, and cleaning regimen need to be verified. The IP54 rating is good for most applications with minimum moisture exposure, but if moisture is present for a longer duration, then you should select IP65 or higher. For high-pressure washdown, the encoder should have an IP69K rating.
7. Does this encoder need safety or hazardous location approvals?
The safety approval is related to a person’s safety and hazardous location approval is for an area with possible explosive gas or dust. The safety incremental encoders usually have sine/cosine output, which would be processed by a safety speed monitor or safe drive. Usually, safety encoders have SIL3 (EN 62061) and PLe (IEC 13849) type standards.
There are many hazardous approvals, i.e., Ex (ATEX), FM, UL, CSA, NEMA, SIL2 (IEC 61508), and SIL3 (IEC 61508).
There are Zone and Division methods for the time duration of the hazardous material (explosive gas or dust) being present. The Division method is usually used by North American companies and the Zone method is usually used by European companies. Having both Zone and Division approvals is becoming more important.
Division 2 and Zone 2 is when ignitable mixture is not normally present, Zone 1 is when ignitable mixture is present intermittently, Zone 0 is when ignitable mixture is present continuously (for long periods), and Division 1 is when ignitable mixture is present intermittently and continuously (for long periods). A qualified person will need to analyze the safety and hazardous location to determine the proper classification.