In the automation industry, some people don't pay enough attention to encoders , let alone encoder signal cables. Phrases like "twisted pair shielding," "the thicker the better," and "ground, ground, and ground again, ground any empty space" seem simple, but these vague or even incorrect understandings of cables often lead to interference in the field, starting with a misunderstanding of a simple cable and its connection. In fact, encoder signal cables are highly technical, involving fundamental physics and materials science. The more basic the concept, the closer it is to physical principles. Understanding these principles requires combining fundamental theory with laboratory experiments and field verification, rather than relying solely on product manuals.
Heidenhain encoders from Germany have always had strict requirements for signal cables. I cannot provide a definitive answer here, as I lack sufficient laboratory facilities; I can only deduce this from the principles of electromagnetic wave signals. This is based on a custom encoder cable I commissioned twelve years ago from a foreign cable manufacturer. This manufacturer already had cables, but based on my understanding of Heidenhain cables, I made specific requests for encoder-specific customization, valuing their laboratory facilities to test and provide the parameters I required, ensuring standardized experimental parameters. We have used this cable for 12 years, in numerous projects of varying sizes, and many knowledgeable peers have used and approved it. This foreign company has also successfully transitioned into the industrial control field, producing robot cables. As the saying goes, "Know yourself and know your enemy, and you will never be defeated." In this article, we discuss: we need to first understand what encoder signals are, what the characteristics of the cable are, and where interference might originate, in order to analyze and develop countermeasures based on the complex interference environment on-site. Here, I have selected some vague or even incorrect understandings of encoder signal cables in the industry, hoping to stimulate discussion and reference for those truly practicing in the field. Comments and debates are also welcome.
1. What are encoder signals?
There are many types of encoder signals. This section only discusses the most commonly used incremental pulse signals and digital serial signals (SSI, etc.), with electronic switching frequencies below 800kHz. Other types of bus signals, single-cable technology cables, and industrial Ethernet cables are not discussed here. (I haven't fully grasped them yet either.) The encoder signal discussed here is a square wave. However, the conductive characteristics of cables are calculated and designed based on electromagnetic waves. A square wave is not a single-frequency electromagnetic wave. According to Fourier decomposition, a square wave is a superposition of electromagnetic waves of many frequencies. The diagram below illustrates that a square wave is composed of at least N=19 different frequencies of electromagnetic waves. (N=1 represents only one frequency of electromagnetic wave.)
Therefore, the signal transmitted on the encoder cable is a combination of electromagnetic waves ranging from lower to higher frequencies. The frequency characteristics of electromagnetic waves are as follows: Extremely Low Frequency (ELF) below 3kHz; Very Low Frequency (VLF) 3-30kHz; Low Frequency (LF) 30-300kHz; Medium Frequency (MF) 300-3MHz; High Frequency (HF) 3-30MHz; Very High Frequency (VHF) 30-300MHz (TV channels 1-12); Extra High Frequency (UHF) 300-3GHz (TV channels 13 and above); Ultra High Frequency (SHF) 3G-30GHz. Light is also an electromagnetic wave. Many characteristics of high-frequency electromagnetic waves are the same as those of light that we are familiar with.
When electromagnetic waves pass through the interface of different media, they undergo refraction, reflection, diffraction, scattering, and absorption. Electromagnetic waves propagate within a conductor medium both along the direction of the conductor and along its diameter. They are refracted at the outer surface of the conductor, radiating into another medium (similar to light encountering water; this could involve entering another conductor medium or spatial radiation), and reflected back into the conductor medium for continued propagation (similar to light reflecting off water). The repeated reflections of various wavelengths create a chaotic scattering effect (similar to fog). Impurities within the conductor absorb electromagnetic wave energy, generating heat and causing wave absorption (similar to the principle of an induction cooker or microwave oven). Electromagnetic waves propagate both within conductors and outwards from the conductor interface. At low frequencies, electromagnetic waves are primarily transmitted through tangible conductive bodies. The reason is that in low-frequency electrical oscillations, the mutual change between magnetism and electricity is relatively slow, and almost all of its energy returns to the original circuit without enough energy to be radiated outwards. When the frequency of electromagnetic waves is high, the proportion of outward radiation gradually increases. In high-frequency electromagnetic oscillations, the mutual change between magnetism and electricity is very fast, and it is impossible for all the energy to return to the original oscillation circuit. Therefore, electrical energy and magnetic energy propagate into space in the form of electromagnetic waves with the periodic changes of electric and magnetic fields. Energy can be transferred outwards without a medium, which is a kind of radiation.
When higher-frequency electromagnetic waves reach the interface of a conductor, part of them refract and radiate away from the conductor, while the other part is reflected back into the conductor as if it hits a mirror. This reflected high-frequency electromagnetic wave superimposes with the next wave moving outward, resulting in a concentrated movement on the conductor surface. Therefore, high-frequency electromagnetic waves exhibit both the "skin effect" (moving along the conductor surface) and the "radiation effect" (leaving the conductor surface). Higher-frequency electromagnetic waves are also more easily absorbed by impurities in the conductor and attenuate rapidly.
When the surface of a conductor carrying high-frequency electromagnetic waves is a sharp interface, the shape of the tip, even after reflection, may still point outwards, increasing the chances of multiple radiations. Waves moving along the surface are more likely to radiate outwards, while fewer waves are reflected back. This is the tip radiation effect of high-frequency electromagnetic waves. This is also the principle behind antennas for transmitting high-frequency radio waves.
An LC high-frequency oscillator circuit is an oscillator circuit that uses an inductor (L) and a capacitor (C) to form a frequency-selective network, used to generate high-frequency sinusoidal signals. The two ends of the inductor (L) are either a spiral coil or a long metal wire, while the two ends of the capacitor (C) are either two non-contact metal conductors or a sharp, diverging metal tip connected to ground. A sharp, diverging metal tip connected to ground more easily forms an oscillating antenna. Under certain energy, frequency, and circuit open configurations, the LC high-frequency oscillator circuit can transmit electromagnetic waves into space (transmitting antenna) or receive electromagnetic waves from space (receiving antenna).
In summary: Electromagnetic waves are a form of energy, sinusoidal waves. High-frequency signals are conducted along the surface of a conductor. High-frequency signals also have a proportion that radiates into space. When a metal conductor has a long section with sharp corners, high-frequency signals are more likely to radiate at the tip or receive external high-frequency electromagnetic waves (interference).
Second, what is the encoder signal frequency?
The encoder signal frequency we often talk about is the frequency of a square wave, also known as the electronic switching frequency (switching characteristics from 0 to 1). However, the frequency transmitted in cables should be calculated based on electromagnetic wave frequencies, that is, electromagnetic wave frequencies composed of more than 19 electromagnetic wave frequencies. Each electromagnetic wave frequency has a multiple coefficient relationship with the square wave frequency. Even if the square wave frequency is not high, the signal still contains some high-frequency electromagnetic wave components, which will become apparent in long-distance transmission or in the presence of high-frequency interference.
The rising and falling edges of a digital square wave signal follow the high-frequency characteristics of electromagnetic waves, while the smooth high and low levels follow the low-frequency characteristics. The frequency of a digital signal is not the same as its electromagnetic wave frequency; higher-frequency electromagnetic waves exist within it. If our digital signal is only below 300kHz, due to the steep rising and falling edges of the square wave, there are still many high-frequency electromagnetic waves present, which is the main reason for the occurrence and reception of various electromagnetic interferences. Similarly, any switching voltage, rising sharply from a low level to a high level, is also a surge combination of electromagnetic waves of various frequencies.
Third, what are the electrical characteristics of the encoder cable?
Since encoder cables transmit electromagnetic waves of various frequencies, the calculations are not based on Ohm's law for DC (or low-frequency) current and voltage, but rather on the capacitance and inductance of AC signal electromagnetic waves. Furthermore, as previously mentioned, higher-frequency electromagnetic waves have the characteristic of propagating along the surface of metallic conductors.
Inter-line capacitance: With sufficiently high-frequency electromagnetic waves and longer cable lengths (more noticeable after 30 meters), the larger surface area between the two core wires within the cable creates inter-line capacitance. High-frequency electromagnetic waves travel through capacitance. Inter-line capacitance also forms between signal core wires and power lines, as well as between signal core wires and the shielding layer. High-frequency electromagnetic waves traveling from one signal core wire to another through this capacitance are called "crosstalk interference," such as crosstalk between phases A and B. Electromagnetic noise traveling on the shielding layer travels through inter-line capacitance to the signal core wire; this is called "external high-frequency electromagnetic interference." Inter-line capacitance is measured in power volts (pF) per meter in the laboratory.
Signal interference is caused by the transmission of high-frequency signals through inter-line capacitance. The performance of inter-line capacitance changes abruptly as the cable insulation ages, and some aging cables that have been used for a long time may suddenly start losing signals.
Inductive characteristics of a conductor: A sufficiently long conductor will exhibit inductance due to the surrounding electric field that conducts electromagnetic waves. This inductance results in a delay in the electromagnetic wave signal, measured in nanoseconds (ns/m). When the surrounding electromagnetic field is disturbed, this inductive delay effect becomes more pronounced, leading to more uncertain signal delays. Because different frequencies of electromagnetic waves have different effects on inductance, the superimposed square wave signal is already distorted.
This inductive characteristic will be more pronounced if the encoder signal line is wound. Therefore, encoder cables should be routed as straight as possible.
