As if the selection of manufacturer weren't enough, many encoders offered in today's marketplace are highly configurable. Resolution, shaft size, seal options and connector variations all confront the design engineer with serious choices. Output type is equally important. In this paper, we'll cover some of the more typical applications and common output types.
Encoder 101: Selecting the Right Output Marketing Contact: Allen Chasey Marketing ManagerDynapar Corp. allen.chasey@dynapar.com678-817-4945
thFebruary 28 , 2008 In today's world of high-tech automation, the design engineer must consider a myriad of variables when specifying the right encoder. Incremental vs. absolute output, resolution, and of course physical form factor are key parameters that most every engineer recognizes as top design elements. However, selection of the output chip or "driver" is just as important in many applications. At best, an improperly selected encoder output for the intended application can provide unreliable performance in the form of nuisance-trips, reduced throughput, or diminished production efficiency. A Variety of Choices As if the selection of manufacturer weren't enough, many encoders offered in today's marketplace are highly configurable. Resolution, shaft size, seal options, and connector variations all confront the design engineer with serious choices. Output type is equally important. In this paper, we'll cover some of the more typical applications and common output types. Application Considerations Two of the biggest application considerations when selecting the proper encoder output are noise and cable length, which in many cases are inter-related. Longer cable runs by nature are more susceptible to noise as they become larger "antennas" and pick up noise. In cases like this, the type of cable used is paramount to a properly functioning system. Shielded, twisted-pair cable is a must, preferably of the low-capacitance variety. A good quality cable will have a low capacitance-per-foot rating, helping to ensure the square-wave pulse leaving the encoder doesn't become "saw-toothed" after traveling the distance to the controller end. Another consideration is current drain. High-current line drivers can quickly become a liability in battery-powered applications where battery life is critical. However, many battery-powered systems are designed to be portable, so in many cases cable runs are in terms of inches rather than feet, so a high-current line driver may not be required. Of course, all of these considerations are a moot point if the controls being used won't accept the encoder output chosen. If a motor drive is set up with differential line receivers, then it doesn't make much sense to select an encoder with open collector outputs, even if the cable run is only a few feet. Open Collector One of the most prevalent and simplest kinds of output for incremental encoders is the open collector output (See Figure 1). As the name implies, the collector of the output transistor (internal to the encoder) is floating "open" so a pull-up resistor must be used to pull the signal to the supply voltage. The active output then pulls the signal down to switch the output pulses on and off, creating the familiar square wave. Many manufacturers will offer customers the choice of providing their own pull-up resistor, or will include it internally with the encoder. The main advantage to the open collector is its simplicity and therefore inherent low-cost. The open collector output is a current-sinking only device (sinks current from the load), and therefore suitable for shorter cable runs, typically below 30 feet in length. Since the open collector output cannot source or "push" current, it is unsuitable for longer cable runs or high-noise environments. As a result, the open-collector output is not suitable for use with differential or complementary encoder signals (A, A-not, B, B-not). However, if short cable runs and a low-noise environment are application characteristics, the open-collector output is probably the more economical choice. Line Drivers As the name implies, this kind of output chip sources or "drives" current down the line. Unlike the open collector outputs, line driver chips actively force the output high or low, therefore being able to both sink and source current from the load (See Figure 2). The main advantage of the line driver is its ability to push higher current through the cable, enabling longer cable runs. Although a line driver can be used in a single-ended format (i.e. push-pull output), they are most commonly used with complementary or differential signals. The use of the differential signal, when used with shielded, twisted-pair cabling, is paramount when using longer ... [download for more]
