It’s all in the outputs
Selecting the right digital encoder requires attention to the component’s output channels and electrical output.
Choosing the correct number of channels and the correct output type for an encoder can be the determining factor in whether or not a feedback system will function properly. However, this is one of the most common problems customers have when selecting encoders. The thing to remember is that it’s the receiving device that determines the correct configuration of the encoder’s outputsWithin linear or rotary encoders, there are two basic types of electrical output formats: Incremental or absolute. Absolute encoders have a unique digitally coded output, or address, for every distinct position of the encoder. Incremental encoders will output a count or pulse at periodic predetermined intervals.
There are two important considerations when selecting the outputs of an encoder. The first is the number of output channels. The second is the electrical output characteristics of the encoder channels. To make this determination, it is very important to understand the requirements of the receiving device.
Receiving devices may consist of PLCs, motion controllers, high-speed counters or any other device designed to accept encoder inputs. Some receiving devices have selectable inputs, which will provide more flexibility in selecting the appropriate encoder.
Rotary incremental encoders have three basic choices for the number of channels. A single channel has a single “A” channel that will output a set number of pulses per revolution. The single channel encoder will give feedback that can determine rate, velocity or acceleration. A single channel encoder cannot give directional feedback and is therefore limited from many bi-directional applications.
A quadrature encoder has two channels, typically referred to as channel A and channel B. Each channel of the quadrature encoder will have the same number of pulses per revolution offset by 90º electrical. This quadrature relationship provides directional information to the receiving device. Many receiving devices also have the capability of using a quadrature encoder’s signals to perform edge counting to increase the resolution two or four times, sometimes called quadrature counting. This can increase the effective system resolution by up to four times.
The index or “Z” channel of an encoder happens once per revolution of the encoder. The index is also sometimes called a reference, marker or homing pulse. One common function of the index is as a home position locator. Every time the index is triggered, the receiving device knows where the encoder is within its rotation. The index is normally 180º electrical and gated to either channel A or channel B.
Some rotary incremental encoders have additional auxiliary channels for motor commutation. Encoder commutation channels can replace Hall Effect sensors in a brushless DC motor for motor control and timing. The pole count of the encoder commutation tracks needs to match the motor pole count. Encoder commutation for DC motors is more accurate and repeatable than using Hall Effect sensors. The typical encoder commutation accuracy is ±1º mechanical. The higher accuracy commutation outputs will allow the motor to run more efficiently.
After selecting the correct number of channels for an encoder, the next task is selecting the electrical output type of the encoder channels. When you’re designing a system, you can choose from differential line driver (HV), push-pull (PP), and open-collector (OC) and pull-up (PU) output types.
The first output type is differential line driver, which will have two connections for each channel (Figure 4). It provides differential output, or complementary signals, for better noise immunity, especially at higher voltages. Noise immunity is obtained by what is called “common mode rejection.” Each channel is split into two signals out of phase by 180 degrees to create a mirror image. The differential between these allows for a comparator circuit to identify the noise present on the signal and cancel it out, allowing for a clean cycle to be seen for each output. This type of output can work at voltages up to 28 VDC.
Differential line driver is the preferred output type for longer encoder cable lengths because of the inherent noise immunity. Differential line driver also meets RS-422 standards when operated at 5 VDC. An encoder should have differential line drive outputs when the receiving device is set up to receive differential signals.
The second output type is push-pull or “totem-pole” type of output circuit. This is a combination of sinking and sourcing outputs. When the output is in logic state high, current will source to the receiving device load. When the output is in logic state low, current will sink from the load.
The downside of push-pull is that it doesn’t have the noise immunity inherent in a differential line driver. When the output is high, the noise on the DC power supply (ripple, voltage, spikes, etc.) may show up on the output of the encoder. When output is low, immunity is equivalent to that of a NPN open collector setup.
A push-pull output type can replace a PNP transistor output in some applications. Some encoders have a 5 volt regulated push-pull output, so the encoder will have a 5 volt signal when the supply voltage to the encoder is higher than 5 volts. (PNP output is not the same as a NPN open collector, since PNP output requires a pull down resistor and is a current sourcing type output.)
The third output type is NPN open collector. NPN open collector is a current sinking output type and requires a pull-up resistor external to the encoder. Typically, the pull-up resistor is built into the receiving device. Open collector is useful for doing what is called level shifting. Level shifting occurs when the encoder is pulled up externally to a different voltage level. For example, the encoder can be powered with 5 volts, and the output can be pulled up to a 24 VDC level.
The fourth output type is pull-up. It is the same as the open collector, but, as the name implies, it contains the pull-up resistor internal to the encoder. If your controller doesn’t have a load, the encoder output will have to be supplied at the correct voltage.
A pull-up encoder output places the load on the encoder side so the output matches that expected by the controller. The load is usually a 1.5KΩ or 2.2KΩ resistor depending on your controller’s needs. This is useful for customers who don’t need the level shifting capabilities of an open collector and don’t want to add external pull-up resistors to the feedback system.
Your encoder’s output must be compatible with the device receiving its signals, so check your controller’s reference manual first for information on signal type and acceptable operating voltage levels.
This article was contributed by Encoder Products Company.