Electronics Guide

Basic Control Objectives

Motor control systems typically aim to regulate one or more of the following parameters:

• Speed control - Maintaining a desired rotational velocity regardless of load variations, commonly measured in revolutions per minute (RPM) or radians per second
• Position control - Moving to and holding a specific angular or linear position with defined accuracy and repeatability
• Torque control - Producing a specified output torque, essential for applications requiring force or tension regulation
• Current control - Regulating motor current for thermal protection, efficiency optimization, or as an inner loop for higher-level control

Many applications require coordinated control of multiple parameters. For example, a CNC machine must control position accurately while respecting velocity and acceleration limits and monitoring torque to detect tool breakage.

Open-Loop versus Closed-Loop Control

Motor control systems fall into two fundamental categories based on their use of feedback:

Open-loop control applies predetermined commands without verifying the actual motor response. This approach is simpler and less expensive but cannot compensate for disturbances or variations in motor characteristics. Open-loop control is suitable for applications where precise motion is not critical or where the load is well-characterized and consistent, such as simple fans or pumps.

Closed-loop control continuously measures the motor's actual state and adjusts commands to minimize the error between desired and actual behavior. This feedback-based approach provides superior accuracy, disturbance rejection, and the ability to adapt to changing conditions. Most precision motion applications require closed-loop control, though it demands additional sensors and more sophisticated control algorithms.

Power Electronics Fundamentals

Motor control systems rely on power electronic switches to modulate the energy delivered to motor windings. The key components include:

• MOSFETs - Field-effect transistors offering fast switching and low on-resistance, ideal for low to medium voltage applications
• IGBTs - Insulated-gate bipolar transistors combining MOSFET gate characteristics with bipolar output stages, preferred for high-voltage, high-power applications
• Gate drivers - Interface circuits that translate low-voltage control signals to the levels required to switch power devices reliably
• H-bridges - Four-switch topologies enabling bidirectional current flow for reversible motor operation
• Three-phase inverters - Six-switch configurations used to generate the rotating magnetic fields required by AC and brushless DC motors

PWM Fundamentals

The key parameters of a PWM signal include:

• Frequency - The switching rate, typically ranging from a few kilohertz for brushed DC motors to tens of kilohertz for brushless motors. Higher frequencies reduce acoustic noise and current ripple but increase switching losses
• Duty cycle - The percentage of each period during which the switch is on, directly controlling the average voltage applied to the motor
• Dead time - In bridge configurations, the brief interval when both switches are off to prevent shoot-through currents that could destroy the power stage

Modern microcontrollers include dedicated PWM peripherals with hardware support for generating precise, synchronized switching signals. These peripherals typically offer center-aligned or edge-aligned modes, programmable dead-time insertion, and fault detection inputs for rapid shutdown.

Advanced PWM Techniques

Beyond basic duty cycle modulation, several advanced PWM techniques optimize motor control performance:

• Space Vector Modulation (SVM) - A technique for three-phase inverters that synthesizes desired voltage vectors by appropriately combining the available switch states. SVM provides better DC bus utilization than sinusoidal PWM and reduces harmonic content
• Complementary PWM - Generates paired signals for upper and lower switches in a half-bridge, with automatic dead-time insertion to prevent shoot-through
• Phase-shifted PWM - Interleaves switching events across multiple phases or parallel converters to reduce input and output ripple
• Random or spread-spectrum PWM - Varies the switching frequency slightly to spread electromagnetic emissions across a wider band, reducing peak EMI levels

Current Sensing for PWM Control

Accurate current measurement is essential for current-loop control, overcurrent protection, and efficiency optimization. Common current sensing approaches include:

• Shunt resistors - Low-value precision resistors in the current path, with voltage drop measured by amplifiers or ADCs. Placement can be low-side (simpler amplifier design), high-side (protects against ground shorts), or in-line with phase windings
• Hall-effect sensors - Isolated current measurement using the magnetic field generated by current flow. Provides galvanic isolation but with bandwidth and accuracy limitations
• Current transformers - For AC current measurement, providing isolation and high bandwidth but requiring AC signal content

Current sampling must be synchronized with PWM switching to obtain accurate measurements during the active portion of the PWM cycle, avoiding transients associated with switching events.

DC Motor Characteristics

The fundamental relationships governing DC motor behavior include:

• Back-EMF - The voltage generated by motor rotation that opposes the applied voltage. Back-EMF is proportional to speed and limits maximum velocity for a given supply voltage
• Torque-current relationship - Motor torque is directly proportional to armature current, making current control equivalent to torque control
• Speed-voltage relationship - For a given load, speed is approximately proportional to applied voltage, enabling straightforward speed control through voltage modulation

Key motor parameters including the torque constant, back-EMF constant, winding resistance, and inductance determine control loop design and achievable performance.

H-Bridge Drive Circuits

The H-bridge is the fundamental circuit for bidirectional DC motor control. Four switches arranged in an H configuration allow current to flow through the motor in either direction:

• Forward operation - Diagonal switches conduct, driving current from supply through motor to ground
• Reverse operation - Opposite diagonal switches conduct, reversing current direction and motor rotation
• Braking modes - Both low-side or both high-side switches conduct, creating a short circuit that provides dynamic braking
• Coast mode - All switches off, allowing motor to spin freely with minimal electrical braking

Integrated H-bridge driver ICs simplify implementation by combining power switches, gate drivers, protection circuits, and control logic in a single package. These devices typically include thermal shutdown, overcurrent protection, and undervoltage lockout.

Speed and Position Control

Closed-loop DC motor control typically employs cascaded control loops:

• Inner current loop - The fastest loop, regulating motor current to provide precise torque control and overcurrent protection. Bandwidth typically ranges from hundreds of hertz to several kilohertz
• Velocity loop - Controls motor speed based on encoder or tachometer feedback. Bandwidth typically tens to hundreds of hertz
• Position loop - The outermost loop for positioning applications, commanding velocity to achieve desired position. Bandwidth typically a few hertz to tens of hertz

Each loop must have progressively lower bandwidth than the loop it contains to ensure stability. PID controllers are commonly used, though more advanced techniques such as model predictive control offer improved performance in demanding applications.

Stepper Motor Types and Operation

The primary stepper motor types each offer distinct characteristics:

• Permanent magnet steppers - Simple construction with permanent magnet rotor, offering good torque at low speeds but limited step resolution
• Variable reluctance steppers - Soft iron rotor with salient poles, providing high step rates but lower torque and no detent torque
• Hybrid steppers - Combine permanent magnet and variable reluctance principles for high torque, high resolution, and excellent positioning accuracy. The most common type for precision applications

Stepper motors typically have step angles of 1.8 degrees (200 steps per revolution) or 0.9 degrees (400 steps per revolution), though other resolutions exist for specialized applications.

Drive Modes and Microstepping

Stepper motor drive techniques trade off between simplicity, torque, and resolution:

• Full-step drive - Energizes one or two phases at a time, producing full step angles with maximum holding torque but rougher motion
• Half-step drive - Alternates between one-phase and two-phase states, doubling effective resolution with moderate smoothness improvement
• Microstepping - Precisely controls current in each phase to position the rotor between full step positions. Common resolutions include 8, 16, 32, or 256 microsteps per full step, dramatically improving motion smoothness and reducing resonance effects

Microstepping requires sinusoidal current control in each phase, typically implemented using PWM current regulation with lookup tables or real-time computation of the desired current profiles.

Current Control and Chopper Drives

Efficient stepper motor operation requires current-mode control rather than voltage-mode drive. Chopper drives rapidly switch the motor voltage while monitoring current to maintain the desired level:

• L/R drive limitation - Motor inductance limits current rise time, restricting high-speed operation if driven from a voltage equal to the motor rating
• Chopper operation - Higher supply voltages enable faster current rise, with PWM chopping maintaining rated current. Supply voltages of 5 to 10 times motor rating are common
• Current decay modes - Fast decay quickly reduces current but increases ripple, while slow decay provides smoother current but limited dynamic response. Mixed decay combines both for optimal performance

Integrated stepper driver ICs handle current regulation, microstepping sequencing, and protection functions, requiring only step and direction inputs from the host controller.

Motion Planning for Steppers

Proper motion planning prevents stalling and ensures reliable stepper operation:

• Acceleration profiles - Ramping step frequency prevents the motor from exceeding its torque-speed curve, which drops significantly at high speeds
• Resonance avoidance - Steppers exhibit mechanical resonances that cause vibration and potential stalling. Microstepping and careful speed selection help avoid resonant frequencies
• Load inertia matching - The ratio of load inertia to rotor inertia affects dynamic performance and should typically be kept below 10:1 for responsive motion

BLDC Motor Fundamentals

BLDC motors use permanent magnets on the rotor and three-phase windings on the stator. Key characteristics include:

• Trapezoidal back-EMF - BLDC motors are designed with trapezoidal back-EMF waveforms, enabling simple six-step commutation
• Three-phase topology - Windings are typically connected in wye or delta configuration, with a three-phase inverter providing the drive voltage
• Rotor position sensing - Unlike brushed motors where brushes handle commutation, electronic commutation requires knowledge of rotor position

Commutation Methods

Electronic commutation in BLDC motors can be achieved through several methods:

• Hall sensor commutation - Three Hall-effect sensors positioned 120 electrical degrees apart detect rotor magnets and indicate commutation timing. Simple and robust but requires sensor installation and wiring
• Back-EMF sensing - Sensorless operation detects zero-crossings of the back-EMF in the undriven phase to determine rotor position. Eliminates sensor cost and reliability concerns but cannot operate reliably at very low speeds or standstill
• Encoder-based commutation - High-resolution encoders provide precise rotor position for advanced control techniques. Enables field-oriented control and precise motion applications

Six-step commutation energizes two of three phases at a time, with commutation events occurring every 60 electrical degrees. This approach is simple to implement but produces torque ripple as current transitions between phases.

Sensorless Control Techniques

Sensorless operation reduces cost and improves reliability by eliminating position sensors:

• Back-EMF zero-crossing detection - Monitors the voltage on the floating phase during six-step commutation. Zero-crossing occurs 30 electrical degrees before the next commutation event
• Starting strategies - At standstill with no back-EMF, open-loop strategies such as alignment and ramp-up sequences bring the motor to speeds where back-EMF detection becomes reliable
• High-frequency injection - For advanced sensorless field-oriented control, injecting high-frequency signals enables position estimation at zero and low speeds by detecting position-dependent saliency

Field-Oriented Control

Field-Oriented Control (FOC), also known as vector control, provides superior BLDC and PMSM motor performance by controlling current in a rotating reference frame:

• Park and Clarke transforms - Mathematical transformations convert three-phase currents to a two-axis rotating reference frame aligned with the rotor flux
• d-q current control - Independent control of flux-producing (d-axis) and torque-producing (q-axis) current components enables optimal efficiency and dynamic response
• Space vector PWM - Implements the commanded voltage vector with optimal switching patterns for reduced harmonic content and better DC bus utilization

FOC requires precise rotor position information and significant computational resources but provides the best possible motor performance in terms of efficiency, torque ripple, and dynamic response.

Induction Motor Principles

Induction motors operate on the principle of electromagnetic induction:

• Rotating magnetic field - Three-phase currents in the stator create a magnetic field that rotates at synchronous speed, determined by supply frequency and pole count
• Rotor currents - The rotating field induces currents in the rotor conductors (either squirrel cage bars or wound windings), which interact with the stator field to produce torque
• Slip - The rotor must rotate slower than the synchronous speed for relative motion between rotor and field, enabling current induction. Slip typically ranges from 1% at no load to 3-5% at full load

Variable Frequency Drives

Variable Frequency Drives (VFDs) control induction motor speed by varying the frequency and voltage of the power supplied to the motor:

• V/Hz control - Maintains constant flux by keeping the voltage-to-frequency ratio constant. Simple but limited dynamic performance
• Vector control - Applies field-oriented control principles to induction motors by estimating rotor flux position. Provides excellent dynamic response and precise torque control
• Direct torque control - An alternative to vector control that directly selects inverter states based on estimated torque and flux errors, providing very fast torque response

VFDs incorporate power electronics including rectifiers, DC link capacitors, and three-phase inverters, along with sophisticated control algorithms running on dedicated motor control processors.

Motor Parameters and Modeling

Advanced induction motor control requires accurate knowledge of motor parameters:

• Stator resistance and inductance - Affect current dynamics and must be compensated in control algorithms
• Rotor resistance and inductance - Determine slip-torque characteristics and flux dynamics
• Magnetizing inductance - Relates to core flux and affects efficiency and power factor

Parameter identification procedures, either offline commissioning tests or online adaptive algorithms, enable the drive to tune its control for specific motors and applications.

Encoders

Rotary encoders provide digital position feedback through various technologies:

• Incremental encoders - Generate pulses as the shaft rotates, with resolution specified in counts per revolution. Quadrature outputs enable direction detection and 4x resolution through edge counting. Require homing at startup to establish absolute position
• Absolute encoders - Provide unique position codes at all angles, eliminating the need for homing. Single-turn types cover one revolution while multi-turn types track rotations through mechanical or battery-backed counters
• Optical encoders - Use light transmission through patterned disks for high resolution and accuracy. Susceptible to contamination in harsh environments
• Magnetic encoders - Detect magnetic patterns on rotating elements, providing robust operation in dirty or wet environments with moderate resolution

Resolvers

Resolvers are rotary transformers providing absolute position feedback with excellent durability:

• Operating principle - An AC excitation signal induces outputs in two stator windings that vary sinusoidally with rotor angle
• Resolver-to-digital conversion - Dedicated ICs or algorithms process the sine and cosine signals to extract angle with resolutions to 16 bits or higher
• Advantages - No electronics at the sensing element, inherent absolute position, and excellent temperature and vibration tolerance make resolvers preferred for harsh environments

Hall Effect Sensors

Hall sensors detect magnetic fields and are widely used for rotor position sensing in BLDC motors:

• Commutation sensing - Three Hall sensors positioned at 120-degree intervals provide six-state position information for electronic commutation
• Linear Hall sensors - Analog output proportional to field strength enables higher resolution position sensing when used with appropriate magnetic arrangements
• Integration considerations - Proper sensor alignment during motor assembly is critical for optimal commutation timing and smooth operation

Current Sensors

Current feedback enables torque control and protection functions:

• Shunt-based sensing - Precision low-value resistors with amplification provide direct current measurement with good bandwidth and accuracy
• Isolated sensing - Hall-effect current sensors or isolated amplifiers provide galvanic isolation between power and control circuits
• Integrated sensing - Many motor driver ICs include current sensing using internal shunts or by monitoring the voltage drop across power switches

PID Control

The Proportional-Integral-Derivative controller remains the workhorse of motor control:

• Proportional term - Produces output proportional to the current error, providing immediate response but unable to eliminate steady-state error alone
• Integral term - Accumulates error over time to eliminate steady-state offset, but can cause overshoot and windup issues if not properly managed
• Derivative term - Responds to rate of error change, providing damping and improving transient response. Sensitive to noise and often filtered or omitted

PID tuning methods range from manual trial-and-error to automated procedures. The Ziegler-Nichols method provides initial settings based on system response characteristics, while more advanced techniques optimize performance for specific applications.

Advanced Control Techniques

Beyond PID control, advanced algorithms address specific motor control challenges:

• Feedforward control - Adds commands based on known system dynamics to reduce the burden on feedback control. Particularly effective for acceleration and velocity feedforward in motion systems
• State-space control - Models the complete system state for optimal control design. Enables pole placement and observer-based techniques
• Model predictive control - Optimizes control actions over a prediction horizon, naturally handling constraints and multi-variable systems
• Adaptive control - Adjusts control parameters in real-time based on observed system behavior, compensating for parameter variations and unknown disturbances

Motion Profile Generation

Motion profiles define how a motor moves from one position to another while respecting physical constraints:

• Trapezoidal profiles - Constant acceleration to maximum velocity, constant velocity cruise, and constant deceleration to target. Simple to implement but produces discontinuous acceleration
• S-curve profiles - Smoothly varying acceleration with limited jerk produces gentler motion, reducing mechanical stress and settling time
• Polynomial profiles - Higher-order polynomials can match multiple boundary conditions for position, velocity, and acceleration at trajectory endpoints

Motion profile generators must respect motor and mechanical system limits including maximum velocity, acceleration, jerk, and available torque.

Coordinated Motion

Multi-axis systems require coordinated motion control:

• Interpolation - Generating intermediate points along a path to maintain smooth motion. Linear and circular interpolation are fundamental for CNC and robotic applications
• Electronic gearing - Synchronizing multiple axes in fixed or variable ratios, replacing mechanical gear trains with electronic coordination
• Camming - Implementing complex position-to-position relationships for applications like packaging machinery and printing presses

Integrated Driver ICs

Motor driver ICs combine power stage and control functions in compact packages:

• DC motor drivers - H-bridge ICs for small motors integrate all power switches, gate drivers, and protection circuits. Examples include DRV8xxx series, L298, and TB6612
• Stepper drivers - Combine current regulation, microstepping sequencing, and protection. The A4988, DRV8825, and TMC series offer various current ratings and microstepping resolutions
• BLDC drivers - Gate drivers, current sensing, and control logic for three-phase inverters. Some include integrated MOSFETs for lower power applications

Motor Control Microcontrollers

Dedicated motor control microcontrollers provide specialized peripherals:

• Advanced PWM units - Hardware for center-aligned PWM, dead-time insertion, fault handling, and synchronized ADC triggering
• High-speed ADCs - Fast analog-to-digital converters with sample-and-hold for current sensing during PWM cycles
• Encoder interfaces - Quadrature decoder peripherals that count encoder pulses without CPU intervention
• Math accelerators - Hardware for trigonometric functions, Park/Clarke transforms, and PID calculations

Popular motor control MCU families include ST STM32G4 and F3, TI C2000, Microchip dsPIC, and Infineon XMC series.

Protection and Safety

Robust motor control systems incorporate multiple protection mechanisms:

• Overcurrent protection - Hardware comparators and software limits prevent damage from shorts or overloads
• Overtemperature protection - Thermal sensors on power devices and motors enable temperature limiting and shutdown
• Overvoltage protection - Clamps and regenerative braking controls prevent DC bus voltage from exceeding safe levels
• Watchdog supervision - Independent timers detect control software failures and force safe shutdown
• Functional safety - Applications requiring SIL or PL ratings implement redundant sensing, diverse processing, and safe torque off functions

EMC and Noise

Motor drives generate significant electromagnetic interference:

• Conducted emissions - PWM switching creates high-frequency currents that propagate on power and signal cables. Input filters and proper grounding reduce emissions
• Radiated emissions - Motor cables act as antennas for switching noise. Shielded cables, ferrite chokes, and short cable lengths help
• Susceptibility - Sensitive analog circuits require isolation from noisy power stages through physical separation, shielding, and filtered supplies

Thermal Management

Power dissipation in motor drives requires careful thermal design:

• Switching losses - Increase with frequency and can dominate at high PWM rates. Slower switching reduces losses but increases EMI
• Conduction losses - Power dissipated in switches during the on-state, proportional to on-resistance and current squared
• Heat sinking - Proper thermal paths from semiconductor junctions to ambient air through heatsinks, thermal compounds, and forced airflow

Development and Debugging

Motor control development benefits from specialized tools:

• Motor control development kits - Reference designs with MCU, power stage, and motor enable rapid prototyping and algorithm development
• Real-time debugging - Tools for streaming internal variables during motor operation without affecting real-time performance
• Simulation - Motor and control system models enable algorithm development and testing before hardware is available