To implement field-oriented control (FOC) for a brushless DC (BLDC) motor, measure phase current in sync with the inverter’s PWM, obtain the rotor’s electrical angle, transform the currents into the rotor-aligned d/q frame, regulate those currents, and convert the resulting voltage commands back into PWM outputs. The work is not just the transform equations: motor and inverter limits, current-sensing topology, rotor-angle feedback, timing, startup, and fault handling all shape a viable implementation.
What FOC does in a BLDC drive
FOC, also called vector control, expresses three-phase stator currents in a coordinate frame that rotates with the rotor flux. The Clarke transform maps measured phase quantities into a stationary two-axis representation; the Park transform rotates that representation into direct-axis (d) and quadrature-axis (q) current components. The d/q components are regulated separately, and inverse transforms turn the commanded voltage vector back into phase commands for the inverter’s PWM.
In common permanent-magnet motor control, q-axis current is the principal torque-producing component. The d-axis reference is not universally zero: it depends on motor characteristics and operating range, and field weakening may command d-axis current at higher speeds. Microchip’s FOC materials cover BLDC and PMSM applications; its AN1292 sensorless PMSM example includes field weakening.
The resulting control path is: current and rotor-angle acquisition, phase-current reconstruction if needed, coordinate transforms, d/q current regulators, voltage limiting and modulation, inverse transforms, and PWM update. A speed regulator can sit outside the current loop and request torque through q-axis current; a position regulator can sit outside the speed loop when the application needs position control.
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Decisions to make before coding
FOC examples are implementation references, not universal recipes for sizing a motor drive. Establish the motor, power stage, sensing, and feedback requirements first.
- Motor and application: record phase connection, pole-pair count, rated and peak current, bus voltage, speed range, winding parameters if available, and whether the application needs torque, speed, or position control.
- Power stage and protection: select the inverter, MCU, sensing range, and protections to fit the motor and bus limits. Confirm board and device voltage and current ratings rather than assuming a development board is suitable for the target motor.
- Control timing: determine whether the MCU can synchronize ADC sampling to PWM, run the control calculations within the available cycle, and update the PWM at the intended point in the switching cycle.
- Rotor angle: choose a physical sensor or a sensorless estimator, and account for electrical-angle offset and startup behavior.
- Current feedback: decide how many shunts to use, where to place them, and whether the PWM scheme provides usable sampling windows for the selected topology.
Choose rotor-position feedback
The Park transform needs rotor electrical angle at the sampling instant. A position error rotates the measured current into the wrong d/q axes, so angle acquisition and its timing are part of the control loop rather than a separate convenience.
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| Approach | What it provides | Design considerations | Documented starting point |
|---|---|---|---|
| Hall sensors | Physical rotor-position states from Hall devices. | Requires sensors and wiring; use an appropriate method to derive the angle needed for FOC and validate the sensor-to-phase alignment. | Microchip AN4064 describes Hall-sensored FOC of a three-phase BLDC motor using dsPIC33CK. |
| Encoder or resolver | Physical rotor-position feedback. | Requires compatible sensor interfaces, installation, and angle alignment. The appropriate sensor and interface depend on the application. | TI TIDA-010250 documents a reference design with sensored Hall or quadrature-encoder modes. |
| Sensorless estimator | An estimated rotor angle from electrical measurements rather than a position sensor. | Estimator assumptions and tuning matter. At very low speed, weak back-EMF makes angle estimation particularly difficult, so startup alignment or another suitable startup method must be designed and validated for the motor. | Microchip AN1292 uses a PLL estimator; AN1078 uses a sliding-mode observer. They are distinct PMSM examples, not evidence that the algorithms have equivalent assumptions or performance. |
A sensored path is often the more straightforward choice when reliable low-speed position information is essential and the application can accommodate a sensor. Sensorless control can avoid a position sensor, but shifts complexity into estimation, startup, and validation. Choose based on the full operating range and reliability requirements, not on a general claim that one method is always better.
Choose current sensing and PWM sampling
Current feedback has to be both measurable and correctly timed. Shunt count and placement affect amplifier and ADC needs, the portions of each PWM cycle in which current can be sampled, reconstruction work, noise exposure, and hardware cost.
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| Topology | Implementation implication | Reference documentation |
|---|---|---|
| Three shunts | Measures each phase through a shunt-based path; implementation still depends on amplifier range, ADC timing, and switching noise. | TI TIDA-010250 supports configurations from one to three shunts. |
| Two shunts | Measures two phase currents and derives the remaining phase current according to the implementation’s current relationship and sampling validity. | TI TIDA-010250 supports configurations from one to three shunts. |
| Single shunt | Requires reconstructing phase-current information from samples taken in valid PWM measurement windows; modulation and sampling timing constrain the implementation. | Microchip’s single-shunt PMSM FOC documentation treats current reconstruction as a distinct design problem and points to AN1299. |
Whichever topology is selected, calibrate ADC offsets, scale readings into current units, check polarity, and ensure samples fall in valid measurement windows. Switching noise, amplifier saturation, and invalid windows can produce incorrect feedback even when the transform and regulator code are correct.
Implement the control loop in stages
- Describe the operating limits. Gather the motor and application requirements, then choose a compatible inverter, controller, current measurement range, feedback method, and protection strategy. The cited vendor designs illustrate particular platforms; they do not establish a general motor-sizing method.
- Bring up synchronized measurements. Configure PWM and ADC timing so current is sampled in the chosen topology’s valid window. Calibrate offsets, apply the appropriate scaling, and check that the measured sign and phase mapping match the hardware.
- Acquire or estimate electrical angle. Implement the sensor interface or estimator, account for pole pairs when relating mechanical and electrical angle, and establish the angle offset relative to the motor phases. Validate startup behavior, especially for a sensorless drive.
- Close the current loop first. Reconstruct any unmeasured phase current, apply Clarke and Park transforms, compare d/q feedback with their references, run the d/q regulators, limit the voltage vector to available inverter and bus constraints, inverse-transform, and update PWM. Align acquisition, computation, and PWM update timing.
- Add outer loops only after current control is stable. A speed loop can generate the torque or q-current request. Add a position loop only when needed. Apply appropriate command ramps and current and voltage limits rather than allowing outer-loop requests to exceed the drive’s operating range.
- Implement operating states and faults. Define startup, run, stop, and fault behavior, including how the drive responds to invalid feedback or electrical limits. Exact thresholds and regulator gains depend on the motor and platform; the cited references do not supply universally safe values.
The loop structure is common, but regulator gains, sampling placement, modulation constraints, and execution timing are hardware- and motor-dependent. Treat them as design and validation tasks, not constants to copy blindly from an unrelated example.
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Commission with controlled limits
Commissioning should proceed with a current-limited supply and appropriate electrical safety practices. Do not begin by applying full operating limits to an unverified phase mapping or angle estimate.
- With the power stage disabled as appropriate, check ADC offset, current scaling and polarity, sensor signals, and phase wiring.
- Use a conservative, application-appropriate current limit to establish rotor alignment and verify the electrical-angle offset.
- Command a cautious rotation and confirm phase order, feedback direction, and current waveforms before increasing operating limits.
- Increase limits incrementally while monitoring faults and temperature. Stop and diagnose unexpected current, angle, or protection behavior rather than masking it with higher limits.
Primary implementation references
- Microchip AN4064: Hall-effect-sensored FOC of a three-phase BLDC motor using dsPIC33CK. The associated DM330031 is a dsPIC33CK low-voltage motor-control development board for that implementation path, not a general-purpose drive for every motor.
- Microchip AN1292: sensorless PMSM FOC using a PLL estimator and field weakening. The manufacturer page lists source packages and board/device variants, with entries updated as late as 2025; check the current package and target-hardware fit before use.
- Microchip AN1078: sensorless PMSM FOC using a sliding-mode observer; the manufacturer page also lists a tuning guide.
- Microchip single-shunt PMSM FOC documentation and AN1299: useful for the specific current-reconstruction problem in single-shunt implementations.
- TI TIDA-010250: a reference inverter design rated at 1 kW by TI, with sensorless FOC and sensored Hall or quadrature-encoder modes, and support for one to three shunts. The 1-kW label is the design’s stated rating, not a comparative performance result or a suitability guarantee for another application.
- Microchip AN1208: covers integration of power-factor correction and sensorless PMSM FOC using a dsPIC DSC. It is relevant when the input-power architecture includes PFC, not a required element of every motor drive.
Before adopting a reference design, confirm the latest application-note revision, firmware package, device errata, development-board voltage and current limits, and applicable electrical-safety requirements for the intended build.
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