System Integration, Servo & EMI

Motor overheating is the leading cause of insulation degradation in servo motor windings. Accurate temperature monitoring — beyond simple binary PTC trip points — requires a continuous analog measurement model. The Steinhart-Hart equation provides an empirical model that converts thermistor resistance to temperature with the accuracy needed for predictive load management and cooling control.

Torque motors are frameless, high-pole-count brushless motors designed for direct drive without gear reduction. Position feedback for torque motors requires absolute encoders with sufficient resolution to provide commutation data across the large number of pole pairs, while achieving the axis accuracy required by the application.

Velocity control in servo systems is computed from position encoder data — differentiated over time, filtered, and fed back to the velocity regulator. The differentiation method, filter design, and feedback rate determine the achievable velocity bandwidth and low-speed smoothness. This article covers the four principal velocity estimation methods and their performance tradeoffs.

Absolute encoder position data is transmitted via synchronous serial protocols. BiSS-C, SSI, and SPI each handle clock generation, data framing, and error detection differently. The choice of protocol affects maximum achievable update rate, cable length, error detection capability, and compatibility with existing servo drive hardware.

Safe Torque Off (STO) replaces the mechanical redundant contactor as the hardware safety function for servo drives. STO implementations are rated by FMEA of the power stage circuitry. Current system design trends integrate STO, encoder feedback, and safety-rated fieldbus communication into a single drive-encoder safety system without a dedicated safety PLC.

Encoder interface cables carry differential position signals that are highly susceptible to noise, crosstalk, and impedance discontinuities. Correct cable selection — including characteristic impedance, conductor gauge, shielding type, and jacket material — determines whether the encoder achieves its specified noise immunity and signal quality at the required cable length.

Cable shield termination and ground loop prevention are the most common sources of encoder signal noise in servo motor installations. This article defines the correct shielding method for motor and encoder cables, identifies the conditions that create ground loops, and describes practical mitigation techniques.

Incorrect grounding in servo motor systems produces fault currents, EMI, and encoder signal noise that degrade position feedback quality. Understanding the functional role of each ground type — earth, chassis, and signal — is prerequisite to designing a reliable servo installation.

Single-ended encoder signals degrade over cable lengths that are entirely acceptable for differential pairs. When cable length and data rate require transmission line modeling, unterminated lines produce reflections that corrupt position data. This article provides the engineering criteria for determining when termination is required and how to implement it for RS-422 encoder outputs.