Where do so many magical details come from in ordinary analog signals? Today, let's take the Sanjing 8000B general-purpose frequency converter as an example and delve into its intricacies.
Analog quantities and sensors
Analog quantities are various continuous measurement objects, such as pressure and temperature.
Analog sensors are specialized devices that convert measurement results into signals. When these signals are output to a frequency converter , they can be used for signal conversion and control.
Frequency converters typically have two analog input ports, as shown in the figure below:
Application Tips: First, determine the optimal measurement principle based on the process requirements, and then select the sensor, as shown in the example table below:
Measurement category examples, process characteristics, optional sensor types
Pressure measurement of conventional cryogenic gases and liquids using piezoresistive force sensors
Ceramic pressure sensor
High-pressure, high-temperature, high-precision measurement diffused silicon pressure sensor
Sapphire pressure sensor
Liquid level contact pressure sensor
Capacitive liquid level sensor
Non-contact radar level sensor
Ultrasonic liquid level sensor
Output signal type
Sensor output signals commonly come in four types: 0-5V, 0-10V, 0-20mA, and 4-20mA.
Signal Selection Tips
1. Select 0/4-20mA as the current-type signal, which has stronger anti-interference capability than the 0-5V/10V voltage-type signal.
2. If the system needs to detect disconnection, it is better to use a 4-20mA signal - because the signal zero point is 4mA, not 0mA, so the disconnection and zero point can be distinguished.
Sensor wiring configuration
Sensor output channels are divided into two categories: active and passive.
1. Passive output sensors typically use a three-wire or four-wire wiring system, and their output signal path is separate from their power supply circuit, as shown in the following example:
2. Active output sensors are generally two-wire systems, meaning their signal circuit is also the sensor's power supply circuit. These sensors typically integrate a small probe and transmitter, and their output is usually a 0-20mA signal. Wiring is simpler than with three-wire or four-wire sensors.
Note: The load-carrying capacity of the sensor output is an important indicator, and the input impedance of the frequency converter must be matched accordingly.
For voltage-type sensors, the inverter input impedance cannot be too small, as shown in the figure where the input impedance of RV is 20kΩ. For current-type sensors, the inverter impedance cannot be too large, as shown in the figure where the input impedance of Rc is 500Ω. Generally, these requirements are met; otherwise, the sensor signal will lose linearity, resulting in significant distortion of the output signal.
Common Problem Case Analysis
The air compressor controller and frequency converter shared a single current-type pressure sensor (4-20mA), resulting in a significantly lower pressure reading. Investigation revealed that the pressure sensor's allowable load resistance should not exceed 750Ω, but the actual total internal resistance of the circuit exceeded 750Ω, causing severe output signal distortion. Solution: Connect a 500Ω resistor in parallel between the frequency converter's ACI and GND lines, and adjust the frequency converter's ACI lower and upper limits. Afterward, the pressure display and system operation returned to normal.
Anti-interference measures
Sensors generally convert the measured physical quantity into an electrical signal. Most measurements rely on indirect physical quantities, such as resistive pressure sensors which convert the deformation of a sheet silicon semiconductor into a change in resistance, and then detect the change in the bridge voltage signal. These signals are weak electrical signals and are easily distorted by deviations in physical quantities or electromagnetic interference.
Common anti-interference measures: specific practices and effects
Keeping signal lines away from power lines and not parallel to them reduces or minimizes the effects of strong electric fields, as well as reducing distributed capacitance and common-mode interference.
Install nearby/use twisted pair cables to reduce common-mode interference
Shielding uses shielded wires to eliminate interference caused by electromagnetic emissions from interference sources.
Most are single-ended grounded to prevent potential differences from affecting current.
Power isolation of isolated sensors reduces interference from sources conducted through the power supply.
Signal isolation (such as connecting a signal conditioning module) improves signal transmission quality.
Hardware filtering, built into the frequency converter to reduce harmonics and high-frequency interference.
Software filtering and inverter parameter settings neutralize interference deviations through sampling and averaging.
Signal verification/conversion accuracy
Linear and nonlinear processing: When the signal input is linear, the inverter's default analog input curve definition can be used.
Signal verification tips
1. For sensors with significant nonlinearity, try to select a measurement range that is close to linear.
2. When there is signal zero-point offset: the problem can be solved by setting the zero point of the inverter's analog channel to a non-zero value.
3. Select a frequency converter with high analog-to-digital conversion accuracy. Generally, it should have 12-bit accuracy or higher, which can better meet the control requirements of PID and other similar systems.
Analog quantities for general use
Properly design analog applications to achieve accurate control. Analog signal readings can be used in the following situations.
• The frequency setting of the inverter can be used for master-slave speed following, compensation superposition, etc.
• The PID feedback signal from the frequency converter can be used for closed-loop process control.
• User comparison signal: can be used for specific purposes, such as material breakage detection, etc.
• Conversion output: Used for analog or pulse output of frequency converters for multi-machine chain speed proportional synchronous control, or for display and other control.
Analog Special Purpose
• Cleverly use frequency converters as signal conversion devices:
The host computer can acquire data by reading the analog values from the frequency converter via serial communication. For example, some concrete mixing systems are designed to directly use the on-site frequency converter as a signal conversion device, reducing project investment and achieving good results.
• Make good use of analog inputs as digital signals:
When there are more digital inputs than digital input channels, simplified processing can be achieved. This typically requires the assistance of a frequency converter or host computer programming.
Optimization and Improvement Tips
Commonly used tips can improve application reliability and control effectiveness.
1. Sensor range: Usually 1.5 times the maximum measured value.
2. Thermocouples and resistance temperature detectors (RTDs) have different applications: Thermocouples are generally suitable for measurements above 500℃, while RTDs are suitable for applications at 500℃ and below.
3. Installation location and form: Example 1: The balancing pipeline of the pressure sensor prevents direct impact from liquid. Example 2: The static pressure sensor of a variable air volume (VAV) air conditioner should not be installed too close to the blower to prevent measurement distortion due to turbulence.
4. Current instead of voltage: For example, using a 4-20mA signal instead of a 0-10V signal makes it more resistant to interference.
5. Non-contact instead of contact: For example, using an angle sensor instead of a lever potentiometer improves reliability and lifespan.
6. Replace mechanical measurement with non-mechanical measurement: For example, use piezoresistive pressure sensors instead of pointer-type remote pressure gauges to improve reliability and lifespan.
7. Installation direction: For example, installing the tension sensor in the direction perpendicular to the tension is better than installing it in the direction inclined to the tension.
Analog input applications involve control system design, process matching, product selection, signal processing, interference prevention, and reliability optimization, making it a comprehensive technical practice!
