Manufacturers of industrial equipment try to keep system wiring neat-and-tidy by running all system cables—both motor drive power and instrument signal wiring—in a common cable harness. That’s serious mistake number one. Fast-rise pulses carried by motor power cables can capacitively-couple noise currents into adjacent conductors. Noise current is proportional to cable length, capacitance between power and signal cables, and the voltage rise rate. For a given cable, noise current Inoise becomes:
Inoise = C x dv/dt,
where C is capacitance between motor-powering
and signal-level cables, and dv/dt represents rise rate, in volts/second.
Megahertz-frequency noise currents as high as 1.5A can cause malfunction in sensitive circuits. In fact, position-feedback signals from the motor’s own encoder can be compromised by injected noise currents. In an instrument or automated system, numerous additional circuits are candidates for noise-induced malfunction. Equipment builders have no control over the rise-rate of the servo amplifier, but they do have access to noise-curbing techniques. This article discusses servo amplifier noise mitigating solutions.
The first step toward interference reduction lies in good housekeeping. Put distance between motor power cables and signal wiring. Run the signal conductors and power conductors in separate cable bundles, spaced well away from each other. Use shielded cables for both signal and power circuits. And follow this article’s guidelines—not always intuitive—for handling shield connections. And in the worst case, an Edge Filter in series with amplifier and motor will save the day.
Sources of Problems
Motor power cables don’t just couple noise currents into adjacent cables: they inject unwanted noise currents into all conducting surfaces, including the machine frame. Machine-frame currents travel the path of least impedance, which can create insidious and difficult-to-diagnose equipment malfunctions.
The use of shielded cables is a no-brainer for equipment involving multiple signal cables. Key to noise reduction is proper care in handling shield connections. Make sure the shield encloses the maximum possible amount of signal wiring. Connect the shield to the amplifier ground terminal only, not to the sensor circuits at the opposite end. (This connection rule is well recognized and minimizes the potential for signal circuit ground loops). Shielding motor power cables, discussed next, is a different matter and departs from what might seem intuitive.
The starting point for power cable noise reduction is to use shielded cable to “contain” or confine noise. The shield encloses the power cables in what amounts to a Faraday cage. Noise currents are coupled into the shield via cable-to-shield capacitance. The shield provides a return path, to the servo amplifier’s power semiconductors, for the noise currents. Owing to its low impedance at the noise current’s megahertz frequencies, the shield in effect short-circuits alternative—and equipment interfering—noise return paths. (We will revisit shield connections after discussing the servo motor’s own contribution to noise).
The servo motor’s own drive coils provide capacitive coupling to the motor frame. (The motor becomes “hot” at megahertz noise frequencies). In absence of any alternative, the motor’s internally produced noise current would find its own circuitous return path via the equipment frame. In accordance with Murphy’s Law, the noise current will inevitably thread its way through sensitive equipment, otherwise assumed to be entirely unrelated to motion control circuits.
Machine vibration and programmed motion can produce wide variations in the path for megahertz noise currents, leading to intermittent or difficult-to-diagnose malfunctions.
Not only does a shielded motor power cable “contain” capacitively coupled cable noise, it will provide the low-impedance return path for the motor’s own noise current. Consequently, and in violation of conventional wisdom for avoiding ground loops, the cable shield is connected to both servo amplifier power supply ground, Figure 1, and to the motor frame as well.
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Figure 1: The shield returns noise current via the shield’s low impedance path to the switching circuit source; the cable shield connects to both motor frame and amplifier’s ground terminal.
The shield should envelop (cage) as much of the power cable as possible, and its connections to amplifier and motor should be kept short. Avoid connecting with tight-radius bends that add impedance, which can be significant at noise frequencies, to the current path.
For motors with integral encoder and Hall sensors, harmful coil-to-sensor noise coupling can occur. Capacitance between motor coils and encoder circuits is the danger. Although the degree of noise coupling for a given motor can’t be altered, it pays to select a servo motor that keeps internal signal cables well segregated from power circuits. Pay special attention to the way power and signal wires are shielded and separated as they pass through motor-frame bushing to its connection box.
Worst-Case Noise Condition
Noise currents coupled into the cable shield and motor frame reach a maximum when the leading and trailing edges of all three U, V and W drive waveforms coincide. (Leading edges of the waveforms are in phase, anyway). This waveform coincidence occurs at zero load, Figure 1, when the motor is stationary. Trailing edges also approach when the motor is “holding” and draws only a light load.
The calculations of Figure 1 determine worst-case noise for ten feet of shielded power cable. Motor PWM drive pulses have approximately 0.5V/ns rise rate. Cable-to-shield capacitance is measured at 2.5 nF. Motor coil-to-frame capacitance is about 0.5 nF. From Inoise = C dv/dt, cable current amounts to about 1.5 A peak. A noise current of this magnitude can certainly cause significant misinterpretation of a sensitive logic circuit’s ONE and ZERO values.
In contrast to Figure 1’s systematic noise reduction, Figure 2 highlights the interference-prone “before.”
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Figure 2—Unshielded amplifier power cable runs in same bundle as unshielded acoustic sensor cable. Power cable injects noise current into sensor cable, and noise currents complete the return path from acoustic processor to the amplifier via the machine frame, causing further interference
This poorly designed system lacks a safe return for path both motor and cable noise currents. For illustration’s sake, there’s also an acoustic sensor whose signal cables run in the same bundle as motor power cables. Coupling between power cable and acoustic-signal cable adds noise that undermines acoustic data integrity. Besides compromising the acoustic processor’s circuits, the interfering noise current must also complete its own return path (from acoustic processor), to the servo amplifier’s power semiconductors. In doing so, the current may well return via the machine frame, creating disturbances elsewhere in the system.
Off-Line Servo Amplifier
Offline servo amplifiers incorporate an internal DC power supply that permits direct operation from 50/60 Hz AC power. No separate DC power supply is needed, nor a bulky and expensive isolation transformer. Such amplifiers typically run on 120 or 240VAC, which means no-load DC operating voltages as high as 300V. Owing to their limited reservoir capacitance, offline servo amplifiers also tend to undergo appreciable voltage droop from no load to full-load. Nonetheless, they must operate at both voltage extremes.
From the discussion of Figure 1, the maximum noise coupling occurs when both leading and trailing edges of PWM waveforms are at or near coincidence, that is, at motor standstill. At standstill, the motor draws close to zero power, which frees the DC supply voltage to rise towards its unloaded peak value. Maximum noise coupling therefore occurs at the highest level of the amplifier’s internal operating voltage.
Offline servo amplifiers, like their separately-powered alternatives, use PWM switching frequencies in the 10 kHz to 20 kHz region. With pulse rate fixed, and operating at high voltage, the amplifiers must switch faster for comparable MOSFET or IGBT dissipation. Faster switching (rise rate) increases the potential for noise coupling.
Figure 3 presents a simplified view of the offline amplifier’s internal DC power connections.
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Figure 3: For noise-sensitive applications, an edge filter’s noise reduction can augment shielded-cable’s protection.
Because the DC power supply “floats” relative to ground, its common ground cannot be connected directly to the amplifiers frame. Instead, capacitor C2, which is selected for low impedance at noise frequencies, links the power-supply common and the amplifier’s ground terminal. The amplifier end of the cable shield should be securely grounded to this terminal
Due to an offline amplifier’s combination of higher PWM rise rate and absence of direct connection between cable shield and DC common, the design may require further noise-reduction measures. Accordingly, the next step is to interpose an Edge Filter, Figure 4, between the amplifier and its power cables.
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Figure 4: The edge filter is connected between amplifier and motor power cable, and located physically close to the amplifier.
The edge filter, which provides connections for cable-shield continuity, should be located as close to the amplifier as possible.
The edge filter uses a passive combination of inductance and capacitance to reduce the rise rate of motor-drive pulses from about 0.5V/ns to roughly 0.2V/ns. Since noise current is directly proportional to pulse rise rate, the peak noise current calculated in Figure 1 would drop from 1.5 A to 0.6A. This compromise is achieved with minimal energy dissipation in the edge filter.
Servo amplifiers are inherently noise producers. Noise is minimized by separating signal cables from power cables, and by using shielded cables for both. Although the motion-system designer can’t change servo amplifier rise rate, noise can be minimized by keeping motor power cables short, shielded, and properly terminated. Select a servo motor that segregates power and encoder cables. Use an edge filter in very noise-sensitive applications.
About the Author
Dean Crumlish is Senior Application Engineer at Copley Controls Corp, Canton, MA, www.copleycontrols.com . He can be reached at firstname.lastname@example.org.