Servocables: Combining power and feedback while mitigating noise
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One of the golden rules in wiring servomotors is to separate the power and feedback cables. Otherwise, power conductors near feedback conductors degrade data transmission by inducing noise in the form of electromagnetic interference (EMI) and crosstalk. But like a lot of equipment, servomotor-driven machines are becoming smaller to cut costs. With space inside such machines decreasing, snaking multiple cables through them is becoming increasingly difficult — and adding more cables increases the risk of failure due to flexing.
It’s increasingly common for drive and OEM manufacturers to switch from dual cables to single cables for connecting power and feedback to servomotors. Cables that combine power and feedback reduce cost and boost equipment performance. Plus they let designers use narrower cable tracks.
That’s why servo cables, sometimes called composite or hybrid cables, are becoming more popular. These cables contain bundles of copper wire acting as conductors that transmit power, as well as conductor pairs that send power, temperature, and feedback signals to and from sensors.
Properly specified hybrid servocables supply sufficient electrical power to the motor at the right voltage — 480, 600, or 1,000 V for example. They also transmit feedback signals without garbling the data, even though noise-inducing power conductors run side by side the signal conductors. Hybrid servocables simplify connecting encoders and other feedback devices to servomotors and controllers. Finally, servocables meet regulatory guidelines and withstand the machine’s environment and modes of motion.
Four features to prevent signal loss
Like traditional feedback cables, hybrid cables transmit analog or digital signals to controls from feedback devices such as temperature and position sensors, encoders, and limit switches. The trouble is, many servomotors operate like variable-frequency drives (VFDs) to regulate speed, though with added controls and functions for position and torque commands. Therefore, hybrid servocables must carry signals through regions subject to EMI and crosstalk from the servodrive’s noisy power conduction, the same issue that plague VFDs.
Hybrid servocables are more compact than standard cables to wire a motor-powered design. Just how much depends on gauge sizes for power cables and feedback pairs which, in turn, depends on the application. Integrated servocables also reduce the number of connectors on motors so they can be smaller.
Such servodrives shape input through pulse-width modulation (PWM) at the cost of EMI in the feedback conductors, high leakage (from the conductor shields to ground), and the risk of overvoltage in the power conductors. Insulated-gate bipolar transistors (IGBTs) switch on and off over just a few nanoseconds. Sometimes, narrow PWM pulses travel along the cable before the capacitance of the motor and cable discharge the last pulse. Then the next drive pulse transmits at a higher voltage, resulting in voltage overshoot. High cable and motor inductance can slow charging and exacerbate the issue.
In addition, if a motor’s phases become imbalanced, a potential can form between the drive output and ground, which forces common-mode currents through the cable’s ground wire or shielding from the drive to the motor and back. Using unshielded cable makes common-mode noise even more detrimental because it goes back to the drive through feedback devices in unpredictable ways.
Servocables mitigate the effects of electrical noise with conductors that have the least possible capacitance as well as the following four features.
- Twisting.
Many controllers track the feedback of devices in differential mode and monitor differences between twin input signals to generate suitable output. These controllers work by inverting the polarity of one input and adding it to the other. When a pair of feedback conductors is twisted together, common-mode noise and other noise signals on the conductors match. Therefore, the controller cancels the effect of noise when it sums input signals. Without twisting, noise affects the conductors differently, causing mismatched noise signals the controller can’t sum to cancel. Untwisted conductors also form a loop antenna that collect crosstalk signals.
- Shielding.
Braided shielding wraps around each conductor run to cover 85 to 100% of each bundle and mitigate noise. The maximum surface area a braided shield can cover is 90%. In contrast, spiral shields cover to 98%.
Usually, shielding is made of bare or tinned copper, though metal-coated Mylar is an option that lasts longest in servo applications that flex cable millions of times. Servocables with multiple signal and power conductors incorporate triple shielding: Around individual conductors, around twisted pairs, and around the entire cable bundle.
Shielding is woven around conductor strands at an angle compatible with the cable’s bending radius. For example, conductors can herniated out of wide braids, and smaller braiding angles withstand vigorous flexing. That said, shielding woven at larger angles is easier to attach and properly ground through end connectors. Ultimately the braiding angle is a compromise to satisfy both flexing and connection requirements.
Another shield feature is thickness. The drive’s high-frequency output generates high-capacitance leakage currents that flow over the shield and motor housing to ground. These currents determine the cross section of braided shields and shield connections because the shields mustn’t overheat from current flowing through them. (Shields that are too thin fail.)
- Insulation.
Reducing cable capacitance reduces voltage overshoot, because there’s less capacity for the cable to build potential between drive pulses. So it’s helpful to decrease cable capacitance, which can be done by decreasing conductor diameter, increasing insulation thickness, and decreasing the insulation’s dielectric constant. The first two specifications are set by design requirements. Therefore, to further protect paired feedback conductors from the noise of power conductors and other feedback pairs, several servocable designs include low-dielectric polypropylene insulation. Common PVC insulation has a dielectric constant of five or more (though it’s normally not rated because fillers make inconsistent dielectric). Polyethylene insulation has a dielectric constant of 2. Polypropylene insulation has a constant of about 1.5 — approaching 1.0 constant of air.
- Solid terminations.
Even connectors must include additional connections for shielding to attenuate noise.
Reactive noise-induced currents don’t help motors generate torque. They flow to ground as current leakage via cable shields and metallic motor parts. They can also flow through the motor shaft’s ball bearings and damage races with micro welds. In traditional designs, standard eight-pin connectors without shielding link cables, including those designed to common standards.
In contrast, some servo cables connect to shields’ terminals through large, round connectors that ground copper braiding through a 360° metal contact.
Engineers should properly ground currents induced by noise by using electromagnetically compatible (EMC) cable glands to secure the cable to equipment while attaching the cable’s ground wire to the panel-bus ground terminal.
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