Feedback sensors in motor control include position sensors that track rotor position, current sensors that measure motor phase current, voltage sensors that monitor DC bus voltage, and temperature sensors that track heat. Position sensors, such as resolvers, as well as current, voltage, and temperature sensors, output analog values that must be converted to digital values using an analog interface and an ADC converter.
Current sensor
Current measurements in control systems are typically performed using either resistors or human effect sensors, depending on the specific hardware implementation.
Hall effect sensors measure current without direct electrical contact with a conductor, providing electrical isolation between the measurement circuit and the current-carrying circuit. They can measure both AC and DC currents and can handle a wide range of currents, from very small to very large. There are two main types of Hall effect sensors: open-loop Hall effect current sensors, which directly measure the magnetic field without feedback, offering simplicity and cost-effectiveness; and closed-loop Hall effect current sensors—often called compensated or zero-fluid sensors—which utilize a feedback winding to cancel the main magnetic field, thus improving accuracy and linearity over a wide range. If your motor control application requires high accuracy, high bandwidth, and resistance to temperature fluctuations, a closed-loop Hall effect current sensor is often the preferred choice. However, the specific requirements and cost implications of your application should always be considered, as closed-loop sensors can be more expensive than their open-loop counterparts.
Resistors, which are typically cheaper than other current sensing methods, are particularly well-suited for measuring low currents, where the voltage drop across the shunt remains measurable, and in space-constrained applications. When deciding to use a shunt resistor in a motor control application, the specific requirements of the application should always be considered.
Examples of current sensing using Haar effect sensors and fully differential, low-power, low-noise amplifiers with rail-to-track outputs optimized for driving differential ADCs are demonstrated. The choice between using a differential or single-ended ADC depends on the application and requirements. Key advantages of using a differential ADC include noise immunity with tightly routed inputs, elimination of common-mode noise, doubled dynamic range near 0V, and improved performance for small voltage signals.
To handle harmonics and transients, the bandwidth of the output amplifier should be significantly higher than the pressure wave M frequency, preferably in the megahertz range. It should have low noise characteristics and a high common-mode rejection ratio (CMRR) to ensure that common-mode signals, such as those from the pump switch or other noise sources, are rejected and do not appear at the differential output. Suitable OPS-AMPS for motor control applications using differential ADCS include fully differential amplifiers with rail-to-track input and output capabilities, such as the LTC6363 and ADA4807 implemented via analog devices, and the THS452X family implemented by Texas Instruments.
12-bit ADCs are commonly used in many general-purpose applications, offering a reasonable trade-off between resolution, cost, and complexity. In precision motor control applications, finer current measurements are crucial for control accuracy, typically employing 16-bit ADCs. For most motor control applications, anything beyond 16 bits is generally excessive. The entire signal chain must always be considered, from the current sensor resistance or influencing sensor, through the signal conditioning circuitry, to the ADC itself. The weakest link in this chain determines real-world resolution and accuracy.
DC bus voltage sensing
In many applications, the DC bus voltage is relatively stable, while its current waveform changes rapidly. This means that fast sampling is not as critical because the values change relatively quickly. Given the relative stability of the DC voltage, any noise or fluctuations in DC bus voltage measurements can usually be handled by averaging or filtering. This means that a higher resolution ADC is not required to obtain a stable and accurate measurement. Considering that the storage capacitors of the DC bus power supply already provide a significant low-pass filtering effect, the ADC primarily captures the stable DC voltage and potential superimposed high-frequency noise. A median filter, adept at eliminating sporadic high-frequency peaks from the data, can be used in conjunction with a low-pass filter (LP). The cutoff frequency for the LP filter can be set relatively low, in the range of 10 Hz to 50 Hz. A simple first-order II (infinite impulse response) filter can serve as an LP filter.
Temperature sensing
Temperature monitoring at various locations is crucial for ensuring the safe operation, optimal performance, and extended lifespan of equipment.
The following are the commonly measured critical temperatures (and locations):
• Motor winding temperature is one of the most critical temperature measurements. Overheating of the motor windings will reduce insulation performance and lead to premature motor failure.
• NTC thermistors are the most commonly used temperature sensors in electric motors. However, some motors are equipped with resistance temperature detectors, such as semiconductor RTDs based on Pt100/Pt1000 platinum. Some motors only contain thermal switches or temperature controllers, which are bimetallic temperature switches placed inside the motor windings or on the motor housing. When a specific temperature is reached, these switches can open or close the circuit to activate protection mechanisms, such as stopping the motor or triggering an alarm.
• Connection temperature of power switching devices (such as GBts/MOSIFT). Some IGBT modules may have integrated temperature sensors to monitor connection temperature.
· temperature
• Ambient temperature
• Cooling temperature of electric motors with liquid cooling systems
• Connection temperature of control units such as microcontrollers, DSPs, or FPGAs
By monitoring these temperatures, operators and control systems can make informed decisions about the operation of the motor, potentially reducing power or shutting down the motor if the temperature exceeds safe limits.
IC temperature sensors, typically NTC thermistors or RDSs, are sometimes embedded in the windings or placed very close to them to provide measurements of the winding temperature. They provide digital or analog outputs corresponding to the temperature. IC sensors are known for their ease of integration, stability, and ability to communicate on bus systems like I2C or SPI.
The temperature of the motor and motor controller components changes relatively slowly. Depending on the dynamics and application requirements, the typical sampling rate for sensing motor temperature may range from once every few seconds to once every few minutes.
Position sensor
To achieve smooth speed control of the PMSM, accurate measurement of the motor shaft angle is essential. Inaccurate measurements can lead to motor instability, spasms, or increased current consumption, potentially damaging the PMSM. Therefore, the accuracy of position readings is crucial.
There are two main types of rotary position sensors used in motor control applications: encoders and resolvers.
Rotary encoder
Rotary encoders primarily employ optical or magnetic sensing mechanisms. Optical encoders offer high resolution, fast operating speeds, and reliable performance in a variety of environments. Conversely, magnetic encoders are favored for their more robust performance, offering commendable resolution, fast operation, and exhibiting exceptional resistance to environmental factors such as dust, humidity, temperature fluctuations, and mechanical shocks.
The unique advantage of an absolute encoder is its ability to maintain position memory, even in the absence of power. This feature ensures immediate position verification upon power restoration, eliminating the need for a reset to the home position. Essentially, the core benefit of an absolute encoder lies in its ability to bypass the need for reference operations and instantly recover its precise position. The resolution of an absolute encoder is determined by the number of bits in its output data.
Incremental encoders generate a series of pulses as they rotate, and the pulse count indicates the movement from a known position. Unlike absolute encoders, they do not track or remember the exact position when power is lost; instead, they require a reference point or "home" point for calibration each time they are started. The main output of an incremental encoder is a stream of pulses, the frequency of which indicates speed, and the pulse count indicating distance traveled or position changed.
Absolute rotary encoders provide high-resolution feedback on rotor position and are used in applications where accurate measurement is critical and initiating a "zeroing" step is impractical or undesirable.
The RS-422/RS-485 physical layer standard is favored by absolute rotary encoders due to its robust differential signaling, good noise immunity in industrial environments, and reliable long-distance data transmission capabilities. Besides RS-422/RS-485, several other physical layers are used to communicate with absolute rotary encoders. These include Ethernet, CAN, analog signals, and sometimes USB for configuration and diagnostics.
Synchronous, point-to-point, serial communication is a data transmission method in which devices directly and synchronously exchange data, one bit at a time. This protocol adheres to the RS-485 physical layer standard, ensuring robust and reliable communication, even in potentially disruptive industrial environments. Synchronous communication ensures that feedback and command data are consistent with the driver's control cycle.
Various protocols have been designed to facilitate communication between the encoder and the controller, providing short cycle times for seamless real-time interaction.
Decomposer
An electric motor resolver is a rotary transformer used to measure the angular position of an electric motor rotor. It consists of a rotor and one or more stator windings. As the rotor rotates, it generates a varying voltage in the stator windings, which can be decoded to determine the shaft's angular position. Resolvers are known for their durability, ability to withstand harsh environments, and are frequently found in applications where high reliability and robustness are essential, such as aerospace and industrial environments. Unlike encoders, which provide digital feedback, resolvers provide analog feedback, and their output needs to be converted using a digital control system.
A sinusoidal excitation signal is input to the rotor. As the rotor rotates, the phase and amplitude of this signal change relative to the stator windings due to the rotor's angular position. The outputs of these windings are sinusoidal signals, called sine and cosine outputs. The amplitudes of these signals are proportional to the sine and cosine of the rotor's angular position, respectively. To determine the rotor's angle, the controller or interface circuit typically uses the ratio of the UCOS output to the UCO output and calculates the arc to deduce the angle's position. Since the resolver provides analog feedback, an ADC is typically used to convert the analog information into digital data for further processing by the control unit. The resolver is robust, unaffected by certain types of noise and interference, and can operate reliably in harsh environments where digital sensors (such as encoders) may fail.
Key parameters applied to motor control systems
In control systems, especially in motor operation, measurement accuracy is crucial. Analog-to-digital converters (ADCs) play a vital role in converting analog signals into digital data for processing. When selecting an ADC, its resolution, sampling rate, accuracy, noise performance, input range, power consumption, interface type, and compatibility with the signal frequency and processing requirements of the intended application must be considered.
Some important parameters, such as TCD, signal-to-noise ratio, Enob, and throughput, are explained below.
Total harmonic distortion (THD) is a metric expressed in dB, representing the ratio between the root mean square sum of the initial five harmonic components and the RMS value of a complete input signal.
Signal-to-noise ratio (SNR) quantization refers to the degree to which a signal stands out from noise. SNR here is synonymous with dynamic range, which in signal processing and electronics refers to the ratio between the maximum and minimum values of a variable. An additional parameter closely related to the SNR of an ADC is signal-to-noise + distortion, often referred to as sino.
Introduce several key factors to reduce the signal-to-noise ratio in the application.
1. Quantization noise: Directly related to the resolution of the ADC. A higher bit depth in the ADC will provide a better signal-to-noise ratio because the step size is smaller, reducing quantization errors. This is very important for precise motor control tasks.
2. Thermal noise: This inherent noise, caused by the random motion of electrons, exists in all electronic components and can affect the accuracy of ADC readings.
3. Electromagnetic Interference: Electric motors, especially when they switch at high frequencies, can be a source of EMI. This interference can connect to ADCS and other sensitive electronic components, affecting their signal-to-noise ratio.
4. Power Supply Noise: Voltage fluctuations and noise from the power supply can affect ADC readings. Maintaining a stable power supply is crucial, especially when motors and control electronics share a power source.
5. Reference Voltage Noise: The ADC uses a reference voltage to determine the digital representation of the analog signal. If this reference voltage is noise, it will affect the signal-to-noise ratio.
6. Mismatch: In the context of motor control, aliasing may occur if the input signal (such as the back electromagnetic wave of the motor) contains frequencies exceeding half the ADC sampling rate. Sampling must be performed at an appropriate rate, and some form of filtering must be used to prevent this.
7. Jitter: Inaccurate timing, especially in the sampling clock of an ADC, can introduce errors. In motor control applications, time-based events such as pulsed M-signals frequently occur; therefore, minimizing jitter becomes crucial. For example, when the time intervals between position samples are inconsistent (due to jitter), the discriminant can produce jumps or spikes in the calculated speed. These jumps may be misinterpreted by the controller as rapid changes in speed, causing it to take unnecessary or excessive corrective actions.
8. External interference: In addition to the EMI of the motor itself, other nearby devices and systems can also introduce interference that affects the ADC input.
9. Circuit Noise: The design and layout of a PCB with ADC circuitry can introduce noise. Proper design measures, including decoupling, shielding, and grounding, can help minimize this noise.
10. Crosstalk: In a system using multiple ADC channels, one channel can receive interference from another channel.
11. Another important metric is the Effective Number of Bits (ENOB). ENOB measures resolution when the input is a sine wave. It is calculated by SINAD using the following formula: ENOB = (SINAD dB – 1.76) / 6.02 (Result in bits).
(Sampling rate) For an ADC, it describes the ADC's maximum sampling rate, or the speed at which it can convert an analog input signal into a digital output. It is usually expressed in "kilograms of samples per second".
Many microcontrollers for motion control are equipped with 12-bit, 14-bit, and even 16-bit automatic data processors to improve the resolution of current and voltage measurements. Popular microcontroller manufacturers, such as the STM32 series, Texas Instruments (C2000 series), NXP (Dynamics), and Microchip (DSPIC, PIC32), integrate high-sampling-rate ADCSs capable of simultaneously sampling multiple channels. Additionally, there are microcontrollers featuring differentials for precise current and voltage measurements, eliminating common-mode noise. Microcontroller ADCSs typically achieve oversampling by acquiring multiple consecutive samples and then calculating their average.
When selecting an ADC for controlling a motor, always consider the details of the application and the nature of the motor's operation. Ensure that the selected ADC meets the requirements for accuracy, speed, and other relevant parameters in the application-specific context.
Time and synchronization measurements in control systems
The motor controller is synchronized with the host system (usually a PLC, PC, or dedicated control system) to coordinate motion control tasks in real time. Synchronization ensures that commands, feedback, and other data exchanges between the host system and the motor controller are timely and consistent.
Some communication protocols allow for clock synchronization across devices. This ensures that the host and motor controllers operate based on the same time reference. One example is the Precision Time Protocol (PTP) used in ether-based systems.
Many fieldbus and industrial Ethernet protocols (Ethernet, PROFIBUS, ProFinnet, PowerLink, SECOS III, Ethernet/IP) support cyclic data exchange, in which data is read from or written to the motor controller at predetermined fixed intervals.
Some systems use dedicated input/output signals for synchronization. When an external host system provides a dedicated synchronization output signal as an input to the motor controller, the immediate priority is to synchronize the motor controller's internal processing cycle and all internal sampling clocks with this signal.
All measurement activities, major processing interruptions, and pump outputs should be coordinated with this system cycle. In this case, phase-locked loops (PLLs) are crucial because they ensure synchronization between the output and input signal phases. While many microcontrollers have built-in PLLs, for FPGA-based systems, it may be necessary to use the PLL as an IP address.
