Modern sensors vary greatly in terms of their principles and structures. The challenge lies in selecting the most appropriate sensor based on the specific measurement objectives, measurement targets, and environmental conditions. Choosing the right sensor is the first problem that needs to be solved when measuring a particular quantity.
Once the sensor is selected, the corresponding measurement methods and equipment can be determined. The success or failure of the measurement largely depends on whether the sensor selection is appropriate.
1. Determining the Type of Sensor Based on the Measurement Target and Environment
The first step in any specific measurement task is to decide what type of sensor to use. This decision requires a careful analysis of several factors.
Even when measuring the same physical quantity, multiple sensor principles may be available for selection. To choose the most appropriate principle, the characteristics of the measured quantity and the sensor’s operational conditions should be carefully considered. Some of the key factors to evaluate include:
- Range: The measurement range required for the task.
- Size requirements: The physical dimensions of the sensor based on where it will be placed.
- Contact or non-contact measurement: Whether the sensor needs to make direct contact with the measured object or can be non-contact.
- Signal output method: Whether the sensor will use wired or wireless signal transmission.
- Source of the sensor: Whether to opt for a domestic or imported sensor, and whether the price fits within budget or if a custom-built solution is required.
After evaluating these factors, you can determine the type of sensor needed, and from there, you can consider the specific performance parameters required for the chosen sensor.
2. Sensitivity Selection
In general, a sensor’s sensitivity should be as high as possible within its linear range. This is because a higher sensitivity results in larger output signals corresponding to changes in the measured quantity, which is beneficial for signal processing. However, it’s important to note that higher sensitivity can also amplify unwanted noise from external sources, which can interfere with measurement accuracy.
To mitigate this, the sensor should have a high signal-to-noise ratio (SNR), reducing interference from external signals. Additionally, the sensor’s sensitivity may vary with direction. If the measured quantity is directional and requires high precision, a sensor with low sensitivity in other directions should be chosen. In multi-dimensional measurements, the sensor’s cross-sensitivity should be as low as possible.
3. Frequency Response Characteristics
The frequency response characteristics of a sensor determine the range of frequencies that the sensor can measure. To avoid distortion, the sensor should operate within the permissible frequency range. In practice, sensors always exhibit some delay, and it is ideal to minimize this delay time.
A sensor with a high-frequency response can measure a broader range of signal frequencies. However, mechanical systems generally have larger inertia, meaning that low-frequency sensors can measure lower frequency signals more effectively. When conducting dynamic measurements, it’s important to select a sensor that matches the characteristics of the signal (steady-state, transient, or random) to avoid introducing significant errors.
4. Linear Range
The linear range of a sensor is the range in which its output is directly proportional to the input. In theory, within this range, the sensor maintains a constant sensitivity. The wider the linear range, the greater the measurement range and the better the measurement accuracy. When choosing a sensor, the first step after determining the sensor type is to verify if its measurement range meets the requirements.
However, in practice, no sensor can guarantee perfect linearity. The linearity of a sensor is relative. In cases where high measurement precision is not critical, sensors with minor non-linear errors within a certain range can often be approximated as linear, which simplifies the measurement process.
5. Stability
Stability refers to a sensor’s ability to maintain consistent performance over time. Several factors affect the long-term stability of a sensor, the most significant being the sensor’s operating environment. To ensure good stability, a sensor should have strong environmental adaptability.
Before selecting a sensor, it is crucial to assess the operating environment and choose a sensor that can withstand these conditions or take steps to minimize environmental influences. Sensors also have quantitative stability indicators, which should be checked periodically, especially after extended use. In applications where sensors cannot be easily replaced or recalibrated, strict stability requirements must be met to ensure long-term performance.
6. Precision
Precision is a critical performance metric for sensors, as it directly impacts the overall accuracy of the measurement system. Generally, the higher the precision of the sensor, the higher the cost. Therefore, it is important to select a sensor with sufficient precision to meet the measurement requirements, without overspending on unnecessarily high precision.
If the measurement task is qualitative, a sensor with high repeatability is often more important than one with high absolute precision. For quantitative analysis, however, the sensor’s accuracy must meet the required precision standards.
In cases where a suitable sensor is unavailable, custom-designed sensors may be necessary. These sensors should be designed to meet the specific performance requirements of the task.