Choose a robot’s motor, drive, sensing, mechanics and control software as one system—not as independent parts. Start with the required motion and load, then verify electrical and thermal limits, feedback accuracy, real-time control needs and safe behavior during faults before settling on hardware.
What a robot motor-control system includes
A motion-control system turns a task into controlled movement. Its signal path typically runs from application or trajectory software to a motion controller, then to a motor drive and power stage, motor and mechanical transmission. A feedback path may report position or speed; the drive may also measure voltage and current. The controller and power stage must work together with the motor and the mechanism they move.
That system view matters because a motor can meet a shaft-level speed or torque requirement yet still fail to deliver the robot’s needed motion at the load. Transmission behavior, available power, drive limits, feedback location and control computation all affect the outcome.
What to establish before choosing hardware
Describe the task in measurable terms before comparing motors or boards. Build a requirements sheet for each axis, including:
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- Load and required torque throughout the motion, including acceleration and other transient demands.
- Speed, acceleration, positioning accuracy and the motion profile the axis must follow.
- Duty cycle, operating environment and expected thermal conditions.
- Available power source, supply voltage and any battery-runtime constraints.
- What the system should do during a fault, loss of power or commanded stop.
- Protection, isolation, communications and functional-safety requirements for the machine and its deployment.
These requirements determine what ratings and behavior to verify. Without a specified robot, payload, supply, duty cycle, accuracy target and jurisdiction, there is no sound basis for prescribing a motor rating, loop frequency, drive or compliance design.
How to choose a motor and controller for a robot
Choose the motor family and drive together. The family influences the power stage, commutation or drive mode, feedback options and control work. The table is a starting point, not a universal ranking: actual selection depends on the axis requirements.
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| Motor or system family | What the cited vendor material establishes | Design implication |
|---|---|---|
| Brushed DC | Microchip describes on/off and variable-speed control, optional feedback, and unidirectional or bidirectional drive forms. | Choose the drive form and decide whether feedback is needed for the task; check current and voltage against the motor and application requirements. |
| BLDC/PMSM | ST discusses sensorless field-oriented control (FOC); TI’s humanoid-robotics brief presents PMSMs for higher-power needs and brushed DC motors for some low-power hand or finger applications. | Select the motor, commutation/control approach, drive and sensing as a coordinated set. The humanoid examples are context-specific, not a rule for every robot. |
| Stepper | Microchip describes unipolar or bipolar drives and wave, full-step, half-step or microstep modes, depending on phase configuration and application. | Verify that the chosen drive mode and motor configuration suit the required motion and positioning behavior. |
| Servo system | Kollmorgen’s selection guidance emphasizes matching motor and drive voltage, continuous current and peak current to the application. | Do not select from a nominal or continuous figure alone; transient demands such as acceleration also matter. |
For PMSM/BLDC systems, motor windings and the commutation or control strategy affect precision. TI’s humanoid brief emphasizes efficiency in the context of battery-powered robots, but that example should not be generalized into a recommendation that every robot use a brushless motor.
Match voltage and current across the motor and drive
Check the application supply, motor voltage and drive limits as a set. For brushless servo drives, include DC-bus compatibility in the check. Compare both continuous and peak current requirements with the motor and drive ratings; confirm that the drive can support the transients the motion profile calls for.
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Kollmorgen gives an illustrative selection example of a 240 Vac motor rated for 3 A continuous and 5 A peak, matched with a drive having suitable voltage, continuous rating and peak capability. Those figures explain the comparison; they are not a recommendation for a robot axis.
When does a robot need encoder feedback?
Use feedback when the control task needs measured position or speed rather than relying only on the commanded motion. Possible feedback sources include Hall-effect sensors, resolvers and optical encoders; a sensorless control scheme instead estimates rotor position. Which option is appropriate depends on the required precision and the conditions under which the robot must start and operate.
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Place sensing where the controlled quantity matters
A motor-mounted encoder reports shaft motion, not necessarily the position of a tool or end effector. Compliance, backlash and other mechanical effects can make the two differ. If the task’s accuracy requirement applies at the load, assess whether shaft feedback can establish load position adequately; consider sensing nearer the load when it cannot.
Weigh sensorless FOC against measured feedback
ST describes sensorless FOC as estimating rotor position from synchronized phase-current and voltage readings with real-time computation. Removing a sensor can reduce hardware and mechanical complexity, while increasing computation and programming demands. It is not an automatic, cost-free replacement for an encoder: assess estimation confidence across the required operating range, startup behavior, load conditions and precision.
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How to work from requirements to a validated design
- Define the motion and load. Write down the required torque, speed, acceleration, positioning accuracy, duty cycle and behavior at stops or faults for each axis.
- Set the electrical and operating boundaries. Identify the power source, motor and supply voltage constraints, continuous and peak current needs, environment and thermal limits.
- Choose a motor-and-drive family. Compare brushed DC, BLDC/PMSM, stepper and servo-system options against the axis task; establish the compatible drive form or control approach.
- Choose feedback and its location. Decide whether the task needs an encoder or other sensor, whether an estimator is appropriate, and whether motor-shaft measurement represents the load closely enough.
- Check real-time computation and power efficiency. Confirm that the intended algorithm and controller can do the required work. For battery-powered robots, account for efficiency and its effect on runtime.
- Validate on the actual mechanism. Test the completed motor, drive, transmission, feedback and control combination against the required motion and fault behavior; shaft-only checks cannot establish load-side accuracy where the mechanics intervene.
How to use evaluation kits and reference designs
Evaluation boards and reference designs can shorten the path to learning or prototyping. Renesas documents a low-voltage RA-family motor-control evaluation system for PMSM/BLDC control and a separate RZ/T1 motion-control solution kit. Their existence does not establish suitability for a particular robot or confirm present availability.
Before choosing a kit, verify that it supports the intended motor voltage and current, feedback sensors, MCU and software environment, and control algorithm. Check whether its protection and safety provisions fit the prototype’s actual use; a demonstration setup does not by itself establish that a complete machine is safe.
Real-time control claims and what they mean
Texas Instruments’ servo and stepper drive resource reports less than 1 µs of computation time for field-oriented or direct-torque control in its design-resource context. The page’s date is not stated, and the figure is a vendor claim about the described implementation—not a general guarantee for every controller, algorithm or robot.
Microchip’s 2015 AN532 application note describes a 2 kHz control-loop sample-time range in a PIC17C42 brushed-DC servo-control example. That is a historical result for that example, not a current general performance benchmark. Neither figure should substitute for confirming timing and behavior on the hardware and mechanism being designed.
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Plan electrical fault protection, isolation, braking, safe shutdown and any functional-safety measures at the system level. TI’s design resources discuss functional-safety-related designs and Safe Torque Off or safe-brake-control material, but those resources alone do not establish that an individual design complies with a standard or is safe for a particular machine. Qualified engineers must apply the requirements relevant to the deployment and verify the complete system.
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