Electronics Guide

DC Tachogenerators

DC tachogenerators are small permanent-magnet DC generators that produce a voltage directly proportional to rotational speed. The output polarity indicates direction of rotation, making them ideal for bidirectional speed control applications.

Key characteristics of DC tachogenerators include:

• Linearity - Output voltage varies linearly with speed, typically within 0.1% to 1% of full scale
• Voltage gradient - Expressed in volts per 1000 RPM, common values range from 3 to 100 V/kRPM
• Ripple - Commutation causes AC ripple superimposed on the DC output, typically 1-5% of output voltage
• Temperature coefficient - Output varies with temperature due to magnet and winding resistance changes
• Brush wear - Mechanical brushes require periodic maintenance and limit maximum speed

DC tachogenerators are commonly used in analog servo systems, industrial drives, and applications requiring simple, reliable speed feedback without digital processing.

AC Tachogenerators

AC tachogenerators produce an alternating voltage with frequency proportional to speed. They eliminate brush wear issues but require additional signal processing to extract speed information.

Two main types exist:

• Permanent magnet AC tachometers - Generate sinusoidal output with both frequency and amplitude proportional to speed
• Drag cup tachometers - Use a rotating magnetic field and conducting cup to generate speed-proportional voltage with direction sensitivity

Digital Tachometers

Digital tachometers generate pulse trains with frequency proportional to speed. They interface directly with microcontrollers and digital control systems, offering noise immunity and easy signal transmission over long distances.

Common implementations include:

• Slotted disc with optical sensor - Interrupts light beam to generate pulses
• Magnetic pickup with toothed wheel - Variable reluctance generates voltage pulses
• Hall effect sensors with magnetic targets - Detects passing magnets or ferrous teeth

Incremental Encoders

Incremental encoders generate pulses as the shaft rotates, with each pulse representing a fixed angular increment. The controller counts pulses to track relative position changes and calculates speed from pulse frequency.

Standard incremental encoder outputs include:

• Channel A - Square wave pulse train, one cycle per encoder line
• Channel B - Square wave in quadrature (90 degrees phase offset) with Channel A for direction detection
• Index (Z) - Single pulse per revolution for absolute position reference

Quadrature decoding provides four times the resolution of the encoder line count by detecting all rising and falling edges of both channels. A 1000-line encoder thus provides 4000 counts per revolution when using full quadrature decoding.

Key specifications include:

• Lines per revolution (PPR) - Ranges from 100 to over 10,000 for optical encoders
• Maximum speed - Limited by output circuit response time, typically 3,000 to 10,000 RPM
• Output type - TTL, open collector, differential line driver (RS-422), or push-pull
• Accuracy - Angular error between actual and indicated position, typically specified in arc-minutes or arc-seconds

Absolute Encoders

Absolute encoders provide a unique digital code for each shaft position, eliminating the need for homing routines after power-up. The position is known immediately without any shaft movement.

Encoding methods include:

• Gray code - Only one bit changes between adjacent positions, preventing ambiguous readings during transitions
• Binary code - Standard binary representation, requires careful timing to avoid glitches
• BCD (Binary Coded Decimal) - Simplifies display but uses more bits than pure binary

Absolute encoders are available as:

• Single-turn - Provides unique code within one revolution, typically 10 to 16 bits (1024 to 65536 positions)
• Multi-turn - Tracks multiple revolutions using gear-driven secondary encoders or battery-backed counting, offering 12 to 16 bits of turn counting in addition to single-turn resolution

Communication interfaces for absolute encoders include parallel outputs, SSI (Synchronous Serial Interface), BiSS, EnDat, and various industrial fieldbus protocols.

Optical Encoder Technology

Optical encoders use light-based sensing to achieve high resolution and accuracy. Key components include:

• Code disc - Glass or plastic disc with precisely etched opaque and transparent patterns
• Light source - LED with collimating optics provides parallel light beam
• Photodetectors - Phototransistors or photodiodes detect transmitted or reflected light
• Signal conditioning - Comparators convert analog signals to digital outputs

Transmissive encoders pass light through the disc, while reflective encoders detect light reflected from the disc surface. Transmissive designs generally offer better performance but require more precise alignment.

Magnetic Encoders

Magnetic encoders use magnetized targets and magnetic field sensors to detect position. They offer superior resistance to contamination, temperature extremes, and shock compared to optical encoders.

Technologies include:

• Magnetoresistive sensors - Detect field direction from multipole magnetic ring
• Hall effect sensors - Respond to field strength from magnetic targets
• Inductive sensors - Detect eddy current variations in conductive targets

While magnetic encoders traditionally offered lower resolution than optical types, modern designs achieve resolutions exceeding 16 bits per revolution.

Hall Sensor Fundamentals

The Hall effect produces a voltage across a current-carrying conductor when exposed to a perpendicular magnetic field. In motor applications, Hall sensors detect the position of permanent magnets on the rotor.

Hall sensors are available as:

• Linear Hall sensors - Output voltage proportional to field strength
• Hall switches - Digital output changes state at defined field thresholds
• Hall latches - Toggle between states at opposite polarity thresholds

Three-Phase Hall Configurations

BLDC motors typically use three Hall sensors spaced 120 electrical degrees apart. For a motor with p pole pairs, the mechanical spacing is 120/p degrees. The three sensor outputs create six unique states per electrical cycle, defining the rotor position within 60 electrical degrees.

Hall sensor placement considerations include:

• Electrical vs mechanical degrees - A 4-pole motor has 2 electrical cycles per mechanical revolution
• Sensor alignment - Sensors must be precisely positioned relative to stator windings
• Temperature compensation - Hall sensitivity varies with temperature, affecting switching thresholds

The six-step (trapezoidal) commutation pattern derived from Hall sensors is simple to implement but produces torque ripple. Higher-performance systems use encoder feedback for sinusoidal commutation.

Integrated Hall Sensor Solutions

Modern motor designs often integrate Hall sensors directly into the motor housing or stator assembly. This simplifies installation but limits field replacement options. Some integrated circuits combine multiple Hall sensors with signal conditioning and interface logic in a single package.

Back-EMF Fundamentals

When a motor rotates, its windings cut through magnetic flux lines, inducing a voltage proportional to speed. In BLDC motors, the back-EMF waveform contains information about rotor position that can be used for commutation timing.

The back-EMF voltage is described by:

• Magnitude - Proportional to speed and motor voltage constant (Ke)
• Frequency - Equals electrical frequency (speed times pole pairs)
• Phase - Related to rotor position, enabling commutation timing

Zero-Crossing Detection

The most common back-EMF sensing method detects when the back-EMF voltage crosses zero in the undriven motor phase. During six-step commutation, one phase is always inactive and can be monitored.

Implementation challenges include:

• PWM noise - Switching transients obscure the back-EMF signal, requiring filtering or synchronous sampling
• Low-speed operation - Back-EMF is too small at low speeds for reliable detection
• Starting - No back-EMF exists at standstill, requiring open-loop starting sequences
• Voltage dividers - Motor voltages must be scaled for controller input ranges

Advanced Sensorless Techniques

Beyond zero-crossing detection, sophisticated algorithms extract position information across the entire speed range:

• Observer-based methods - State estimators predict rotor position from measured currents and voltages
• High-frequency injection - Injects test signals to detect rotor position through saliency effects, enabling operation at zero speed
• Direct back-EMF integration - Integrates back-EMF to calculate flux linkage and position
• Model reference adaptive systems (MRAS) - Compare motor model with measurements to estimate speed and position

These advanced techniques enable sensorless operation of PMSM and induction motors with performance approaching that of encoder-based systems.

Resolver Operating Principle

A resolver is essentially a rotating transformer with one primary winding (rotor) and two secondary windings (stator) positioned 90 mechanical degrees apart. The rotor is excited with a sinusoidal reference signal, and the stator windings output amplitude-modulated signals whose amplitudes vary as sine and cosine functions of rotor angle.

The output signals are:

• Sine output - Vs = V * sin(theta) * sin(wt), where theta is rotor angle and wt is excitation frequency
• Cosine output - Vc = V * cos(theta) * sin(wt)

The rotor angle is calculated as theta = arctan(Vs/Vc), providing absolute position within one revolution.

Resolver Types

Different resolver configurations serve various applications:

• Brushless resolvers - Use rotary transformers to couple excitation to rotor, eliminating wear items
• Variable reluctance resolvers - Rotor has shaped laminations rather than windings, improving reliability
• Multi-speed resolvers - Provide multiple electrical cycles per revolution for increased resolution

Resolver-to-Digital Converters

Resolver-to-digital converters (RDCs) process resolver signals to produce digital position and velocity outputs. Key functions include:

• Excitation generation - Provides sinusoidal reference signal, typically 2-20 kHz
• Signal demodulation - Extracts position-dependent amplitudes from modulated signals
• Angle calculation - Computes arctangent using tracking converter or direct computation
• Velocity derivation - Calculates speed from position changes or internal tracking loop

Modern RDCs achieve 12 to 16-bit resolution with tracking rates exceeding 100,000 RPM. Integrated RDC chips simplify resolver interface design significantly.

Resolver Advantages

Resolvers offer significant benefits for demanding applications:

• Temperature range - Operate reliably from -55C to +155C or higher
• Contamination immunity - Sealed construction resists oil, dirt, and moisture
• Shock and vibration - No fragile components, withstand severe mechanical stress
• EMI immunity - High-level analog signals resist electromagnetic interference
• Absolute position - Position known immediately after power-up
• Long cable runs - Differential signals allow cable lengths exceeding 100 meters

Thermistors

Thermistors are temperature-sensitive resistors commonly embedded in motor windings. Two types are used:

• PTC (Positive Temperature Coefficient) - Resistance increases sharply at a defined temperature, providing simple overtemperature detection. Common trip points are 130C, 140C, and 150C
• NTC (Negative Temperature Coefficient) - Resistance decreases continuously with temperature, enabling analog temperature measurement across a range

PTC thermistors are typically used with relay circuits that disconnect motor power when resistance exceeds a threshold. NTC thermistors interface with analog-to-digital converters for continuous temperature monitoring and thermal modeling.

Resistance Temperature Detectors

RTDs use the predictable resistance change of metals with temperature. Platinum RTDs (Pt100, Pt1000) offer excellent accuracy and stability for precision temperature measurement.

RTD characteristics include:

• Linearity - More linear than thermistors over wide temperature ranges
• Accuracy - Class A Pt100 sensors accurate to 0.15C at 0C
• Stability - Maintain calibration over years of service
• Self-heating - Measurement current causes slight temperature errors

RTDs require more sophisticated interface circuits than thermistors, typically using three-wire or four-wire connections to compensate for lead resistance.

Thermocouples

Thermocouples generate voltage proportional to temperature difference between the sensing junction and reference junction. While less common in motors than thermistors or RTDs, thermocouples offer:

• Wide temperature range - Type K covers -200C to +1250C
• Self-powered - No excitation current required
• Small size - Junction can be very compact

Thermocouple signals require cold junction compensation and amplification, adding complexity to the interface circuit.

Thermal Modeling

Beyond direct measurement, advanced motor controllers implement thermal models that estimate internal temperatures from current, speed, and ambient temperature. These models protect against thermal runaway in applications where direct sensing is impractical or during transient overloads.

Accelerometers

Accelerometers measure vibration acceleration and are the most common sensors for motor condition monitoring. Technologies include:

• Piezoelectric accelerometers - Piezoelectric crystals generate voltage proportional to acceleration. They offer wide frequency range (1 Hz to 30 kHz), high sensitivity, and excellent durability for permanent installation
• MEMS accelerometers - Micromachined silicon sensors provide lower cost for integrated monitoring systems. They typically cover DC to several kHz with adequate sensitivity for most applications
• Capacitive accelerometers - Measure capacitance change between proof mass and fixed electrodes. They excel at low-frequency measurements including DC response

Key accelerometer specifications include:

• Sensitivity - Typically 10-100 mV/g for industrial sensors
• Frequency range - Must capture bearing defect frequencies, typically requiring response to at least 10 kHz
• Mounting - Stud mounting provides best frequency response; adhesive and magnetic mounts trade convenience for reduced bandwidth

Velocity Sensors

Velocity sensors measure vibration velocity, which correlates well with vibration severity in rotating machinery. Moving coil velocity sensors use a coil suspended in a magnetic field, generating voltage proportional to velocity.

Velocity sensors offer:

• Direct velocity output - No integration required for velocity-based analysis
• Self-generating - No power supply needed
• Limited frequency range - Typically 10-1000 Hz, adequate for general machinery monitoring

Proximity Probes

Eddy current proximity probes measure shaft displacement directly, detecting shaft runout, bearing wear, and rotor eccentricity. They are essential for monitoring large machines with fluid film bearings.

Proximity probe systems include:

• Probe - Coil that induces eddy currents in the shaft
• Driver electronics - Provides excitation and signal conditioning
• Installation gap - Probes require precise positioning relative to shaft

Vibration Analysis Techniques

Raw vibration signals are processed to extract diagnostic information:

• Overall level - RMS or peak vibration indicates general machine condition
• Spectrum analysis - FFT reveals frequency components related to specific faults (imbalance at 1x, misalignment at 2x, bearing defects at characteristic frequencies)
• Envelope analysis - Detects modulation caused by bearing and gear defects
• Time waveform - Reveals impacting and looseness not apparent in spectra

Strain Gauge Torque Sensors

Strain gauge sensors measure the twist of a shaft under torque. Strain gauges bonded to the shaft deform with the shaft surface, changing their electrical resistance proportionally to applied torque.

Configurations include:

• Full bridge - Four active gauges provide temperature compensation and maximum sensitivity
• Dual bridge - Two bridges at 45 degrees cancel bending and axial forces
• Reaction torque sensors - Measure torque through stationary housing, simplifying signal connection
• Rotating torque sensors - Inline sensors require slip rings or wireless telemetry for signal transmission

Rotary Torque Transducers

Purpose-built rotating torque transducers incorporate strain gauges, signal conditioning, and data transmission in an integrated assembly. Signal transmission methods include:

• Slip rings - Sliding contacts transfer power and signals but add friction and require maintenance
• Rotary transformers - Contactless inductive coupling for power and analog signals
• Wireless telemetry - Digital transmission eliminates all mechanical coupling, modern sensors achieve high bandwidth and accuracy

Magnetoelastic Torque Sensors

Magnetoelastic sensors detect torque through changes in magnetic properties of ferromagnetic materials under stress. A magnetic field is applied to the shaft, and torque-induced stress changes the field pattern, which is detected by sensors surrounding the shaft.

Advantages include:

• Non-contact operation - No physical connection to rotating shaft
• High overload capacity - No delicate strain gauges to damage
• Potentially lower cost - Shaft can serve as the sensing element

Optical Torque Measurement

Optical methods measure the angular twist between two points on a shaft under torque. A light beam or encoder tracks the relative rotation between shaft sections, with the angle proportional to applied torque.

This approach enables:

• High bandwidth - No mechanical elements limit dynamic response
• Non-contact sensing - No loading of the shaft
• Integration with encoders - Same shaft can provide position and torque

Torque Estimation from Motor Current

Motor current is proportional to torque in many motor types, enabling torque estimation without dedicated sensors. For DC motors, torque is directly proportional to armature current. For AC motors, the torque-producing current component can be calculated using vector control techniques.

Current-based torque estimation offers:

• Zero additional hardware - Uses existing current sensors
• Lower accuracy - Affected by motor parameters and temperature
• Practical for many applications - Adequate where high precision is not required

Performance Requirements

• Resolution - Determines minimum detectable position change and velocity measurement at low speeds
• Accuracy - Affects positioning precision and velocity control quality
• Bandwidth - Must exceed control loop requirements for stable operation
• Repeatability - Critical for applications requiring return to defined positions

Environmental Factors

• Temperature range - Optical encoders typically limited to -40C to +100C; resolvers extend to +155C or beyond
• Contamination - Sealed or magnetic sensors for dirty environments
• Shock and vibration - Resolvers and magnetic encoders outperform optical types
• EMI exposure - Shielding and differential signals for high-interference environments

System Integration

• Interface compatibility - Match sensor output to controller input requirements
• Cable length - Consider signal degradation over distance; differential outputs for long runs
• Mounting constraints - Available space, shaft access, and mechanical attachment method
• Power requirements - Supply voltage and current consumption

Cost Considerations

• Sensor cost - Ranges from a few dollars for Hall sensors to thousands for precision resolvers
• Interface electronics - Some sensors require significant signal conditioning
• Installation cost - Sensorless techniques eliminate sensor installation entirely
• Maintenance - Brushed tachometers and slip ring sensors require periodic service

Encoder Issues

• Intermittent counts - Check connections, inspect disc for contamination, verify signal integrity with oscilloscope
• Position drift - Indicates missed counts from noise, speed exceeding capability, or failing sensor
• Noisy signals - Verify proper shielding and grounding, check for damaged cables
• Channel imbalance - Quadrature signals should be symmetric; asymmetry indicates alignment or contamination issues

Hall Sensor Problems

• Motor runs rough - One or more Hall sensors may have failed; check all six state combinations
• Motor fails to start - Verify Hall signals present and correct timing relationship
• Temperature-related failures - Hall sensors may drift or fail at temperature extremes

Resolver Diagnostics

• Position jitter - Check excitation signal quality and amplitude
• Offset errors - Verify resolver alignment and RDC calibration
• Loss of tracking - Speed may exceed RDC tracking rate capability