Closeup view of a square wave with lower slew rate
We refer to this time delay as the slew rate. We like to think of our digital signals as transitioning from low to high immediately, but we of course know that everything takes time. Consider again the square wave of an incremental encoder, only this time zoomed in extremely close to one of its state changes. Adding external resistors to open-collector encoders is not overwhelmingly difficult to do, and many off-the-shelf controllers have them built in already, but these external resistors consume current to operate and they influence the output signal, changing its characteristics with frequency. However, the disadvantages of this interface often outweigh the ability to change encoder voltage levels. Pull-up resistor added externally to an open-collector encoder The collector could be pulled up to meet lower or higher voltage levels than the encoder operates at.
This is a useful output type if the engineer is trying to interface with a system that has different voltage levels. To interface with this device, an external resistor is required to "pull-up" the collector to the desired high voltage level. Bipolar junction transistor as used in open-collector encoders The output is named open-collector because the collector pin on the transistor is left open, or disconnected, when the input signal is high. This means that the output of a digital signal can be driven low to ground, and when the signal is supposed to be high, the output is merely disconnected. Most rotary encoders on the market are going to have an open-collector output. Typical 5 V digital square wave Open-Collector Outputs For this post we will focus on incremental encoder outputs which deliver a basic square wave. This means that for a 5 V encoder, signals will always switch between 0 V (ground) which is low or a binary 0, and 5 V which is high or a binary 1. Whether it is an incremental encoder's quadrature output, a commutation encoder's motor pole output, or a serial interface that uses a specific protocol, all of these signals are digital and have high and low states. These three output types describe the physical layer of digital communication. The answer is not always clear, so in this post we will review the three main types of outputs seen on just about any encoder: open-collector, push-pull, and differential line driver. Additionally, and sometimes overlooked, is the encoder output signal type needed. Once they know what type they need there are a laundry list of other parameters to consider such as: resolution, mounting pattern, motor shaft size, and more. An engineer specifying a sensor must decide if their application requires an incremental, absolute, or commutation encoder. When it comes to choosing an encoder for a motion control application there are a number of choices that need to be made.
0 kommentar(er)
