by Scott Hewitt, President, Sick Stegmann, Inc., Dayton, Ohio
Resolution is one of the most critical factors that must be considered when selecting an encoder and probably the least understood. Resolution is often described in terms of lines, pulses, and counts. Incremental encoders provide a fixed number of square pulses on each channel per shaft rotation, and each unique number of pulses requires a separate code disk. Typically, each pulse has a nominal 50% duty cycle.
Common line counts, the number of lines on the code disk, include binary numbers such as 1024, one of the most common used in incremental encoders, multiples of 360, or even hundreds. For position control, the line count is normally selected to give some meaningful number of pulses per degree of rotation or unit of linear travel such as in ball screws or rack and pinions. Incremental encoders come with single channel output (A) or dual channel output (A/B), with or without complementary signals (A-not and B-not), and with or without an index/marker pulse (M and M-not). Unless the encoder’s electronics multiplies or divides the basic signals, the number of lines equals the pulses per revolution (PPR). Avoid specifying “counts per turn” or “counts per revolution” because counts also may refer to pulses. If counts refer to voltage transitions, “counts” can be two times the pulses for a single channel or four times the number of pulses for dual channels. Make sure you know the number of pulses per revolution needed and communicate what is truly needed to the encoder supplier.
Not every PPR that you need is readily available. However, several manufacturers offer resolution selectable with DIP switches or external software programs. These encoders may be more expensive than standard encoders, but if the PPR you need is not available as a standard, these encoders are less expensive and easier to purchase.
Older systems use encoders with open-collector outputs. The encoder signals must be connected to a supply voltage or ground inside the controls, depending on whether the controls source or sink current. Open-collector outputs are generally limited to cable lengths of 30 feet or less.
The most common type of encoder output is the differential line driver (DLD). It has an integrated circuit to drive various ranges of voltage and levels of current. These line drivers can handle signals over hundreds of feet of cable. They also provide complementary signals for noise immunity, provided that cables comprise twisted wire pairs with shields. The most common line driver chips include the 3487 (5V), the 7272 (5/30V), and the 4469 (5/15 V, with high current capability).
The 3487 chip is the most economical device and should be used for 5V only systems. The 7272 is widely used, but costs more than the 3487. Also, not all 7272 chips are created equal. Some have high voltage drops and may not work reliably with 5V supply voltages. In any event, the 3487 and 7272 are generally limited to 40 mA or less of current. The 4469 should only be used where wiring mistakes are easy to make and cause the chip to draw excessive current. They are nearly indestructible, but they are the most expensive. Most encoder manufacturers recommend the proper chips to use. Otherwise, request the information.
Cable costs can be a significant part of the expense associated with encoder installation. It’s tempting to cut costs in this area, but cable and connector assembly problems cause a significant number of field failures. It’s relatively easy to inspect and troubleshoot cable assembly quality issues, but there are several more subtle things that are much more difficult to detect.
Cable length, application bandwidth requirements, which is a function of resolution and rotation speed, and driver type must be considered when selecting a cable. Higher bandwidths mean shorter cables. Differential line drivers can support longer cables than can open collectors. For applications requiring differential signals (A and A-not) for noise immunity, the signals must be transmitted on twisted pairs with an overall shield. Normally, shielding over each twisted pair is not necessary, except in high-noise applications.
US manufacturers typically recommend connecting the shields only at the power supply to avoid ground loops. European companies normally recommend connecting shields at both the machine and the power supply to avoid any “antenna” effect that can influence the encoder or the power supply level. Few seldom recommend that shields be connected at the machine only, but there have been applications where precisely this arrangement has resolved noise problems. In any event, avoid running signal cables in the same conduits as motor power cables or power cables for other machinery.
Encoder connectors are another consideration. Most US-standard encoders use mil-spec (MS) or similar connectors, with 6, 7, and 10-pin being most common for incremental encoders. By comparison, most European encoders use M23 (23 mm) connectors.
Such connector-to-cable connections require hand assembly, and each solder joint is an opportunity for a loose connection, a solder bridge, nicked wire insulation, or other problems. Contact materials, size, tolerance, and plating or flashing thickness (such as for gold contacts) can all affect performance as well. For example, in high-vibration applications, contact flashing can wear through and cause problems that are difficult to detect.
Many sensors use relatively low-cost M12 (12 mm) connectors, and these assemblies are relatively economical. Such cables are produced on automated equipment, make extremely reliable connections, and cost less due to over-molded construction. Encoder users looking for these same cost savings have demanded M12 connectors for their encoders, and many manufacturers have obliged. But take care in selecting the correct cable assembly for the encoder because it may not be a low-cost, off-the-shelf solution. Most M12 cable assemblies use 5-pin connectors, which cannot be used for 6-channel incremental encoders that need 8 pins. M12 eight-pin connectors and cable assemblies are readily available, but make sure they use twisted pairs and have the necessary shielding.
Encoder enclosure ratings generally follow NEMA or IEC specifications. The most common ratings follow the IEC IP (ingress protection) ratings. For industrial applications, encoders follow IP65, IP66 or IP67 requirements. This means that they can be exposed to liquids; dripped, splashed or sprayed. Resist the temptation to pick an encoder with the highest rating because it’s difficult to know what is really required in some applications. Rotary encoders have a shaft and shaft seal and will not let liquids enter the encoder unless exposed to a high-pressure spray directly on the shaft, or the encoder is installed with the shaft vertical so liquids can pool on the shaft and seal area. For most applications an IP65 or IP66 rating is suitable. Beware of the IP67 rating, as it may be based on static testing (non-rotating shaft).
When you follow the NEMA ratings, ask the encoder manufacturer for the NEMA rating and likewise for IEC IP ratings. Numerous cross-reference charts are available to convert NEMA to IP and vice versa, but the ratings have no direct correlation, so be aware of any deficiencies when following this practice.
Bearing Loads and Ratings
The actual axial and radial loads on an encoder shaft are rarely known. Ideally, avoid mounting belt sheaves directly to encoder shafts or otherwise loading it unless it is specifically designed for such a situation. For most applications, it is quite easy to install the encoder so the actual loads on the shaft are acceptably low. Correct mountings, shaft to coupling alignment, and flexible couplings instead of hard connections will minimize the loads on encoder shafts.
For applications equipped with measuring wheels or belt sheaves directly mounted on the shaft, determine if the encoder can tolerate the load, or use some type of mechanical isolation. Encoder data sheets usually specify allowable radial and axial loads. For radial loads, ask the manufacturer where these loads can be applied on the shaft. The ratings are not always based on the end of the shaft. Sometimes it’s the middle of the shaft or a specific distance from the bearing.
Bearing ratings (usually lifetime in cycles) also are specified on data sheets. Determine if these bearing lives are based on the maximum allowable shaft loads. They may not be. Ask the manufacturer when in doubt.
Rotary encoder shafts connect in one of two ways. The most common is to attach the encoder shaft to the machine shaft with a suitable coupling. The other is to mount an encoder with a blind or through-hollow shaft over the end of the machine shaft. Coupled shafts can isolate the encoder from runout, shocks and vibrations from the machine shaft. But the encoder needs a mounting bracket with a suitable coupling, and they need more space for mounting.
Hollow shaft mounting requires no bracket or coupling, only a suitable point for the anti-rotation arm to be mounted. However, hollow shaft encoders are often more expensive than shafted encoders. Also, all machine shaft runout, shock, and vibration are transmitted directly into the encoder. If the machine shaft diameter is larger than the shaft bore of the encoder, adapter shafts can be used. However, the tolerances of the adaptor must be suitable so that it does not produce additional runout. For applications with high loads where space constraints do not allow for shaft mounting, hollow shaft encoders are available with internal flexible connections to isolate the encoder bearings and optics from the loads. However, these components are significantly more expensive.
A suitable coupling for mounting encoders should be flexible enough to allow for some minor angular and parallel misalignment of the encoder and machine shaft. However, they must be torsionally stiff so no relative movement develops between the encoder and the machine shaft.
It is tempting to simply follow the coupling data sheet for allowable angular and parallel misalignment. Many times the coupling can tolerate much more than the encoder can. But any misalignment imparts loads on the encoder shaft, and these loads can be significant. Such loads can lead to premature bearing failure, while being well within the load rating of the coupling itself.
Most encoder shafts are 10 mm/ 0.375 inch diameter and smaller, and many machine shafts are significantly larger than this. Generally, the machine shaft diameter should match the encoder shaft, or an adaptor should be used. Avoid a large mismatch between the shaft diameters when a using coupling. The coupling needs to accommodate the larger shaft, which produces an extremely stiff coupling. Even a small shaft misalignment can create a large load that will prematurely fail the encoder bearings.