Engineers designing servo and stepper systems tend to focus on the obvious: the motor, controller, amplifier, and sensors. Signal and power cables, though no less important than the components they connect, are often overlooked until the project's end, or worse, handed off to an electrician without proper training. The result: lower-than-expected accuracy, frequent failures, low immunity from electromagnetic interference (EMI), and adverse effects on neighboring equipment.
Despite their apparent simplicity, cables are highly engineered components. They are designed and manufactured to let other components operate at peak levels. Cable features of note include insulation breakdown voltage, shielding (for individual conductors or pairs), jacketing (to protect the cable from mechanical, chemical, and environmental influences), and ampacity (the maximum current a conductor can carry before exceeding its temperature limit). Other considerations include a drain or grounding wire (used with foil shields), binding tapes, embedded steel-support wires, and fillers (to give the cable a uniform circular, cross-section).
Designers must consider several things when choosing cable for servo and stepper systems. Operating conditions — temperature, moisture, chemical exposure, abrasion, flexing, and expected life — are key. Other criteria, such as proper insulation type and thickness, depend on working voltages. The number of conductors and current ratings, in contrast, are often specified by the motor and drive manufacturer.
Some applications need separate feedback and power conductors. Others can make do with a single composite power and feedback cable. This is a decision based, in part, on how much interference (inductance) is acceptable between conductors and between the cable and its surroundings.
If the area between conductors is too great, alternate paths for the signal will be found. Twisting the cable pairs (up to four turns per inch) decreases the likelihood of this sort of coupling. Additional measures to prevent coupling include the use of shielding.
Some applications require stationary cables, such as systems where the motor and drive are fixed relative to each other. In these situations, cable trays and conduits are frequently used to route the wiring. Other applications, like those involving a motor constantly moving in relation to the overall system (such as a robot arm), might require significant cable flexibility.
Inside and out
Two types of material are used in the conductor insulation and jacket, and each plays a different role in the cable's structure. One type electrically isolates individual conductors, or cable pairs. By comparison, jacket material provides the cable “skin” and protects the conductors, insulation, and shield from the environment, mechanical impact, and chemically aggressive substances. While some products, such as conventional hook-up wire and consumer-product power cords, have only a single layer of insulation that acts as a physical protective jacket, most industrial grade cables contain both. Jacket material is the major source of friction within moving tracks, and selection is tantamount to system success or failure.
The environment, including ambient temperature and heat created by conductor current, determines the insulation material's maximum operating temperature. In general, temperature ratings can be interpreted as the maximum conductor temperature safely sustained by the insulation. However, if any cable (even a feedback cable) is routed near a heat-generating machine, ambient temperatures will influence the operating temperature.
A typical, nominal rating for power and control wiring is 600 V and relates only to conductor insulation. It is the maximum working voltage that can be applied between the conductor and any adjacent part (such as another conductor), shield, or conductive object outside the cable. The Underwriters' Laboratory, UL, does not recognize jacket insulating properties as part of the voltage rating. Instead, they are considered a mechanical protective and binding element of the cable.
Conductor insulation and jackets made from the most common insulation material, Polyvinylchlo-ride (PVC), are suitable for many motion-control applications, including continuously flexible (flex) cables. Machine tools, robots, pick-and-place equipment, material handling equipment, and cable tracks are just a few examples. A typical PVC jacketed cable has a static temperature range of -30° to 70° C. A flexing requirement narrows the lower temperature range to about -5° C. Multiple conductors also reduce the thermal area for dissipation, and thus, reduce power for a given wire size.
Another type of insulation material, Ethylene Propylene (EP), is less resistant to benzene and various oils, but has excellent UV and ozone resistance. Polyurethane (PU) is rugged, extremely flexible, and protects against acids, alka-lines, solvents, and hydraulic fluids. Applied to many continuous-flex cables because of its flame retardant characteristics, it also has a temperature range wider than PVC. On the downside, PU jackets and insulation are difficult to cut, strip, and terminate, especially by hand. Combining PVC insulation for individual conductors with a PU jacket eases cable fabrication and maintains excellent protection.
A fourth material, Neoprene (Polychloroprene or PCP), performs well under extremely harsh conditions. It remains flexible to -40° C, and does not melt under high heat. In addition, these jackets do not age when exposed to sunlight and oxidation, and are resistant to abrasion, crushing, and cutting.
Lastly, Teflon allows for high temperature conditions (greater than 100° C), but can be difficult to work with due to high memory and tensile strength. In fact, manual cable fabrication of Teflon is more difficult than the other types considered.
A cable contains a single or multiple insulated conductor, usually copper, that carries current. Conductors can be solid (one copper wire) or stranded — with several smaller, twisted strands comprising the composite conductor. Bare copper with a high number of fine strands can also provide maximum flexibility, especially in continuous-flex cables.
Two main factors in selecting a conductor's size are its location and the presence of other conductors. A conductor located inside the enclosure of a heat-generating machine must be larger than a conductor exposed to the open space in an air-conditioned facility. According to specification NEC 75°C shown in the Recommended conductor current chart, the conductors in a cable connecting the motor to the drive shall have an ampacity of no less than 125% of the motor's full-load current.
Shielding can be applied over individual conductors, pairs, and entire cables and is normally covered by a jacket. The combination of an overall shield and twisted wires helps reduce electromagnetic radiation emitted from the cable. In addition, a shield prevents external radiation and electrostatic fields from entering the circuitry, which are disruptive to normal signal transmission. Avoiding radiation and electrostatic discharge are especially critical for cables carrying feedback and other low-level signals.
Feedback and resolver cables often have several levels of protection against electrical interference. First, individual pairs are twisted to reduce electromagnetic radiation from analog and digital signals. Then, each twisted pair is placed inside a shield to reduce crosstalk between adjacent pairs. Finally, a ferrous-type, overall shield provides top-level protection against electromagnetic and electrostatic interference and reduces emissions in critical applications.
Power cable shields are required to contain EMI. Often, motion controllers drive various types of steppers and servomotors with high-frequency switching currents to minimize losses in power semiconductors. Of concern is dv/dt, the ratio of the switching signal's rise or fall time to the voltage magnitude. Large and steep dv/dt switching currents produce high levels of interference around the power cable and must be shielded to 30 MHz in industrial machinery.
A properly grounded overall shield provides additional shock protection. If the power cable's insulation is damaged and the conductor is exposed, it will likely short circuit to the grounded shield and trip a circuit breaker or fuse before harming end users.
Choosing connectors affects both the selection of cable style and overall reliability. With motors, either the cable must be terminated inside the connector, or an inter-cable connector must be supplied. In a gantry system (typical for cutting applications and electronic assembly equipment), the two axes of motion, X and Y, require an interconnecting cable assembly that traverses the moving tracks.
A typical servosystem includes a power supply (not shown), a servomotor with a built-in position sensor, and controller with a built-in amplifier. The controller receives the feedback signal through a feedback cable, and the motor receives power through the power cable.
Bend but don't break
In many motion control applications, the motor, feedback device, or both move relative to the controller and require special, high-flex cables. High and continuous-flex cables are a combination of conductors, insulation material, shields, and a jacket that can withstand mechanical impact. A typical continuous flex cable has many fine, bare copper strands covered with extra flexible PVC insulation and a PU jacket.
Bend radius requirements determine the service life of a continuous flex cable. The smaller the radius, the shorter the life. The minimum allowable cable bend radius is specified as a factor N multiplied by the cable outside diameter — for example “12 × cable diameter” — where N = 12. Properly selected and installed continuous flex cables have a life expectancy of several million cycles. Special flat cables have also been developed to decrease the limit on the dynamic bend radius.
Any cable can fail prematurely when not properly terminated. One scenario is using hand tools to make electrical and mechanical connections between the conductor and contact when machine crimping is not available. Further, many stepper and servomotor drives come with terminal blocks for power cable termination. Alternately, a large number of stepper systems come with IDC connections. These must be limited to static applications, as motion will compromise the connector.
Proper shield grounding is required to reduce emissions, increase immunity, and prevent personal injury from ground currents. A safe practice is to bond shielded motor cables to the drive's back panel with metal cable clamps.
On the opposite end of the spectrum, a bad practice, is installing cable that is “just long enough” because it puts unnecessary stress on cable termination points and can form extremely sharp bends that reduce cable reliability. On the other hand, excessive cable length increases overall system costs and can degrade performance. Long cables degrade feedback signals, generate more heat in power connections, and encourage crosstalk, due to the cable's resistance, inductance, and capacitance. Excessively long, coiled power cables reduce drive voltages at the motor terminals and act as antennas, which radiate electrical noise interference.
For more information, call Danaher Motion at (866) 993-2624 or visit www.danahermotion.com.