Where Motor Temp & Current Monitoring Fits in a Robot Axis

In an industrial robot axis, motor current and temperature monitoring spans three physical locations: the servo drive PCB, the motor body and the junction or terminal box. The monitoring chain closes the loop between electrical loading, mechanical torque and thermal limits, so that over-current, over-temperature and abnormal duty cycles can be detected early.

Current feedback used for FOC or servo control is mapped to the Servo Drive topic. Here the focus is on the monitoring channels that supervise phase and DC-link currents, motor winding and housing temperatures, and connector hot spots. These channels may reuse the same shunts and sensors as the control loops or be implemented as separate, slower paths for diagnostics and safety.

The section positions shunts, isolated amplifiers, NTC sensors and ADC or monitoring ICs in a typical mains–to–motor chain. This makes it clear which signals belong to real-time torque control and which belong to protection, derating and lifetime monitoring, avoiding overlap with servo drive power-stage design or encoder feedback AFEs.

The result is a clear map of where motor current and temperature monitoring should live in a robot axis, and which boards, connectors and harness segments need dedicated sensing and diagnostics.

What Needs to Be Monitored — Protection, Derating and Health

Motor current and temperature monitoring in industrial robotics serves three practical objectives: protecting the drive and motor against abuse, enforcing thermal derating limits, and building a credible picture of long-term health. A single monitoring chain is expected to cover fast events such as short circuits and stalls, slower heating in windings and housings, and cumulative stress over thousands of duty cycles.

Protection focuses on detecting hard faults and overload conditions before they damage insulation, bearings or power semiconductors. Typical cases include phase-to-phase or phase-to-ground faults, locked-rotor events, repeated start–stop cycles and regenerative currents that push DC-link hardware beyond its safe envelope. Reliable current and temperature signals are the first line of defence for these scenarios.

Thermal derating uses monitored temperature to modulate the allowed motor current. As winding and housing temperatures rise, the permissible continuous current shrinks and the axis transitions from nominal operation to reduced torque and, if needed, a controlled shutdown. The same monitoring data defines safe S1, S2 and S6 duty profiles and ensures that momentary peaks do not accumulate into excessive thermal stress.

Health monitoring treats current and temperature as indicators of mechanical and insulation ageing. RMS and peak phase currents, the frequency and duration of overloads, and the occurrence of thermal excursions all contribute to a lifetime model. By tracking winding, housing and connector temperatures alongside operating current, the system can estimate the remaining margin before insulation breakdown, connector fatigue or bearing damage becomes likely.

For these reasons, the monitoring chain normally measures RMS and peak line currents, winding and housing temperatures, connector hot spots and ambient conditions, rather than only a single shunt or thermostat contact.

Current Monitoring Topologies for Motor Phases

Phase current monitoring in an industrial robot axis can be implemented at the DC-link, on low-side phase legs or directly in the phase legs themselves. Each shunt placement changes what can be seen in the current waveform, how much blind zone exists around freewheel intervals and how difficult it is to achieve robust EMC performance on a crowded multi-axis servo board.

DC-link shunts minimise cost and board area by using a single sense element for the whole inverter. They provide a good view of overall power, short-circuit events, stalls and regenerative stress, but carry less information about individual phase loading and phase imbalance. Low-side phase shunts, implemented as a single shared shunt or three individual shunts, move the sensing point closer to each phase and improve diagnostics at the expense of extra components and tighter layout constraints.

Phase-leg or high-side shunts provide the most complete view of phase currents across operating quadrants, but sit in high-voltage, high dv/dt regions and therefore require isolated amplifiers or ΣΔ modulators with high CMTI. Ground-referenced current-sense amplifiers are well suited to DC-link and low-side shunts, whereas isolated amplifiers and ΣΔ modulators are preferred for high-side and phase-leg schemes in 400 V or 690 V drives.

Hall-based current sensors add another option where galvanic isolation, creepage and clearance dominate over shunt cost, for example on larger axes or where the current path must remain mechanically robust. For the motor temp and current monitoring role, the choice between DC-link, low-side and phase-leg shunt placement, and between ground-referenced vs isolated AFEs, determines whether the same current path can serve both FOC control and slower monitoring, or whether a partially independent monitoring channel is justified.

The topology decision therefore balances information content, EMC difficulty, isolation requirements and the cost of duplicating or reusing phase current paths between servo control and monitoring.

NTC / Motor Temperature Sensing Strategies

Motor temperature monitoring in a robot axis typically combines several sensing points: embedded NTC or PTC elements in the windings, sensors on the housing or endcaps around the bearings, and additional sensing near cables and terminal boxes. Each point sees a different part of the thermal path, so combining them gives a more realistic view of insulation stress, bearing health and connector safety than any single thermostat.

Winding NTC or PTC elements are the primary reference for insulation life and allowable current. Housing and endcap sensors respond more slowly but correlate better with surface temperature limits and bearing margins. Cable and terminal-box sensing focuses on local hot spots caused by contact resistance, poor crimping, contamination or ageing insulators, and is especially relevant for robots that see frequent reconnections or harsh cabinet environments.

The excitation and sampling network must convert these sensor resistances into clean, reproducible voltage signals. Simple divider networks with precision resistors cover most use cases, while constant-current excitation may be adopted when long harnesses, multiple series sensors or strong supply variations need to be tolerated. RC low-pass filtering at the ADC input is required to tame noise on long, high-impedance runs, but the cutoff frequency and capacitor placement must be chosen so that legitimate temperature changes still propagate within the time budget of derating and protection.

Linearisation and calibration turn raw ADC codes into temperatures that can drive derating curves and trip thresholds with known error margins. Steinhart–Hart equations, lookup tables with interpolation and piecewise linear approximations provide different balances between accuracy, memory and computation effort. End-of-line factory calibration is commonly used to absorb NTC tolerance, resistor error and wiring losses, while field calibration compensates for installation-dependent thermal paths and motor substitutions in long-lived installations.

A robust motor temperature strategy therefore defines which points are monitored, how the NTC network is excited and filtered, and how calibration data is maintained so that derating and protection thresholds remain credible over the life of the robot.