Encoders in motion control applications: Noise and signal distortion considerations

How to ensure clarity of the signal from your encoder, and avoid excessive electrical noise.

An encoder is an electro-mechanical transducer that converts mechanical rotary motion into digital signals for the control of machinery. The encoder produces a square wave signal as the shaft rotates. Speed, position, servo feedback, etc., can be determined through proper processing of this signal. As the electrical signal leaves the encoder, it is free of electrical noise. However, by the time the signal reaches its intended counter, PLC, etc., it may be degraded and may not be clean enough for the system to work properly.

To ensure clarity of the signal from your encoder, and avoid excessive electrical noise, there are several options and installation considerations to take into account.

The encoders’ cables are an important consideration, with cable length, termination, and connections all playing a part in keeping a signal ‘clean’. There are also additional methods to reduce noise, and this article will focus on strategies and ways to reduce noise and signal distortion to ensure the signal from your encoder remains clean and uncorrupted.

Electrical noise

A common cause of signal degradation is electrical noise. The longer the cable run, the more induced noise the cable picks up. If this noise becomes excessive, miscounts will occur. Electrical noise causes miscounting because the receiving device cannot tell if an input signal is a valid encoder signal or noise.

Figure 1: Electrical noise on an encoder signal.

Normally there is sufficient input signal conditioning, or filtering, to take care of this problem. However, filtering at the input of the receiving device will reduce the speed at which the system can operate. Years ago, most counters had high frequency limitations of between 5 and 20 kHz. Speed is now the name of the game, and these frequency limitations are simply not acceptable in today’s production environments. Electrical noise generated by AC power, electric motors, fluorescent lighting, relays, and many other sources can cause a plethora of problems in electrical systems. For the encoder in your system these problems can range from simple miscounting to a complete servo system lockup.

Electrical noise typically enters a system as one of two types: radiated and conducted. Radiated noise propagates through the air, while conducted noise finds an electrical path onto the encoder cables from ground loops, power supplies, or other equipment connected to the system.

Differential signals

One method to alleviate the problem of electrical noise is using what is called differential signals. With differential signals, the output from the encoder is transmitted as two signals that are exactly 180° out of phase with each other. This is also called complementary signals, because one signal is the complement, or mirror image, of the other. As long as the two signal conductors are next to each other, any noise picked up by the cable will have equal and in-phase components on each conductor in the cable. Using differential input circuitry, the input will recognize only the difference between the signals: as one signal line is in a high, or logic 1 state, the complement is at a low, or logic 0 state. The differential input circuitry will accept this as a legitimate signal, and the in-phase noise products are simply ignored.

To use this type of noise immunity, the encoder must have what we refer to as a line driver output circuit. However, having the line driver output circuit is only half of the equation. Transmitting the signal in differential form is not enough; it must also be received in differential form. To accomplish this, the receiving device must also have a differential input circuit, or what is commonly called a line receiver input.

Many people believe that by specifying the differential output on the encoder, their noise problems will simply go away. However, without the proper line receiver input circuitry, it is a waste of money, and may even be worse from a noise standpoint. If the differential output of the encoder is not properly terminated, ringing and other spurious oscillations will appear on the signal lines.

Differential output circuitry on most encoders operates over the voltage range of 5-28 VDC supply voltage. The older standard for differential signals (also known as RS-422) called for 5 V operation. By raising the voltage in the system, a much better signal to noise ratio results. In a 5 V system with 3 V spikes, the spikes are nearly as great as the desired signal amplitude. In the same setup with the voltage increased to 24 V, for example, it is easy to see that the same 3 V spikes can be easily ignored. However, it is important to remember the input circuitry must also be able to handle this higher voltage.

Cable considerations

Cable lengths

All cables have small amounts of capacitance between adjacent conductors. This capacitance is a direct function of the cable’s length, and tends to round off the leading edge of the square wave signal, increasing rise times. It can also distort the signal to the extent of causing errors in the system.

Signal distortion is not usually significant for lengths less than 9 m (or 1000 pF). To minimise the distortion, use low capacitance cable (less than 100 pF per metre), in the shortest length possible for the application. To minimise distortion for cable lengths in excess of 10 m, use differential line driver outputs, along with differential type receiver circuitry.

Also, a low capacitance twisted-shielded pair cable should be used whenever using differential signals. For high frequency applications (>200 kHz), this type of cable may be needed for all lengths.

Cable termination

Proper cable termination is vital with differential signals. With an unterminated configuration, signal reflections can occur, resulting in severely distorted waveforms. If signal distortion occurs, try parallel termination, which involves placing a resistor across the differential lines at the receiver end of the line. The parallel termination resistor value (R_T) should match the characteristic impedance (Z₀) of the cable, typically 70-150 Ω. This permits higher frequencies to be transmitted without significant distortion.

Figure 2: Parallel termination to eliminate signal reflections.

It is usually better to select a value for R_T that is slightly larger (up to 10% larger) than Z₀, as over-termination tends to improve signal quality better than under-termination.

Unfortunately, low valued resistors can increase the power dissipated by the line driver, and reduce output signal swing. In this case, a capacitor should be placed in series with the resistor. The capacitor value should be equal to the round trip delay of the cable divided by the cables cable’s Z₀. Round trip delay is equal to two times the cable length multiplied by 5.6 ns/m.

Figure 3: Adding a capacitor to improve signal strength.

C_T ≤ Round Trip Delay / Z₀

Example of capacitance calculation

Cable length = 30 m
Signal velocity = 5.6 ns/m
Z₀ = 120 Ω
C_T ≤ (30m x 2 x 5.6ns/m) / 120 Ω
C_T ≤ 2,800 pF

Note that the RC time constant of this type of termination can reduce the system frequency response.

A parallel termination resistor value larger than listed above can often provide adequate reduction of signal reflections, and still maintain adequate frequency response, and low power dissipation. Experimentation is often required for each application consisting of long cable runs and high frequencies.

Cable connection

It is important to connect cable shields to ground on the instrument end (counter, PLC, etc.). Always properly ground the motor/machine on which the encoder is mounted. Also, ground the encoder case with the following recommendations:

1. DO NOT ground the encoder case through both the motor/machine as well as the cable wiring
2. DO NOT allow the encoder cable wiring to ground the motor/machine exclusively. High motor/machine ground currents could flow through encoder wiring, potentially damaging the encoder and associated equipment.

Summary of methods to reduce noise

There are several methods that can reduce noise in an encoder’s electrical signal:

1. Route power and signal lines separately.
2. Twist and shield signal lines, and place signal lines at least 30 cm from other signal lines and from power leads.
3. Maintain signal wire continuity from the encoder to the controller/counter (i.e. avoid junctions or splices).
4. Provide clean regulated power to the encoder and associated equipment (±2%).
5. Ensure all equipment (motors, drive, shaft, etc.) is properly grounded.
6. Connect the encoder cable shield to ground at the controller/counter end, leaving the end near the encoder disconnected.
7. If possible, use differential line driver signal outputs with high-quality twisted, shielded pair cable. The complimentary signals greatly reduce common mode noise levels, as well as signal distortion resulting from long cable lengths.

If you follow the recommendations given here, you will see a reduction in noise and signal distortion from your encoder.

Top image credit: istock.com/Supersmario