Analysis of the principle of the temperature and humidity monitor and the distance measuring instrument
Laser rangefinders generally use two methods to measure distance: pulse method and phase method. The process of pulse ranging is as follows: the laser emitted by the rangefinder is reflected by the measured object and then received by the rangefinder, which simultaneously records the time of the laser traveling back and forth. Half of the product of the speed of light and round-trip time is the distance between the rangefinder and the object being measured. The accuracy of pulse method for measuring distance is generally around+/-1 meter. In addition, the measurement blind spot of this type of rangefinder is generally around 15 meters.
Laser ranging is a distance measurement method in optical wave ranging. If the time required for light to travel back and forth between points A and B at a speed c in the air is t, then the distance D between points A and B can be represented as follows.
D=ct/2
In the formula:
D - Measure the distance between two points at stations A and B;
C - The speed of light propagation in the atmosphere;
T - The time required for light to travel back and forth between A and B.
As can be seen from the above equation, to measure the distances A and B, it is actually necessary to measure the time t of light propagation. According to different measurement methods, laser rangefinders can usually be divided into two measurement forms: pulse type and phase type.
Phase type laser rangefinder
A phase laser rangefinder is a device that uses the frequency of the radio band to modulate the amplitude of the laser beam and measure the phase delay generated by the modulated light traveling back and forth to the measuring line. Based on the wavelength of the modulated light, the distance represented by this phase delay is then converted. Measure the time required for light to travel back and forth through the measuring line using indirect methods.
Phase laser rangefinders are generally used in precision ranging. Due to its high accuracy, usually in the millimeter range, in order to effectively reflect signals and limit the measured target to a specific point that is proportional to the instrument's accuracy, this type of rangefinder is equipped with a reflector called a cooperative target.
If the modulation angle frequency is ω, The phase delay generated by a round trip on the distance D to be measured is φ, The corresponding time t can be expressed as:
T= φ/ω
Substituting this relationship into equation (3-6), the distance D can be expressed as
D=1/2 ct=1/2 c· φ/ω= C/(4 π f) (N π+ Δφ)
=C/4f (N+ Δ N) =U (N+)
In the formula:
φ-- The total phase delay generated by a signal traveling back and forth to the measuring line.
ω-- The angular frequency of the modulated signal, ω= 2 π f.
U - Unit length, numerical value equal to 1/4 modulation wavelength
N - The number of modulated half wavelengths included in the measurement line.
Δφ-- The signal generates a phase delay of less than π when traveling back and forth to the measuring line.
Δ N - The decimal part of the modulation wave contained in the measurement line that is less than half the wavelength.
Δ N= φ/ω
Under given modulation and standard atmospheric conditions, the frequency c/(4 π f) is a constant, and the measurement of distance becomes the measurement of the number of half wavelengths contained in the measuring line and the measurement of the fractional part less than half wavelength, i.e. measuring N or φ, Due to the development of modern precision machining technology and wireless phase measurement technology φ The measurement has achieved high accuracy.
To measure the phase angle of less than π φ, Measurement can be carried out through different methods, with the most commonly used being delayed phase measurement and digital phase measurement. Currently, short-range laser rangefinders all use the principle of digital phase measurement to obtain φ.
As mentioned above, in general, phase laser rangefinders use a continuous laser beam with modulated signals. In order to achieve high ranging accuracy, a cooperative target needs to be configured. The currently launched handheld laser rangefinders are another new type of pulse laser rangefinders. They are not only small in size and light in weight, but also use digital phase measurement pulse broadening and subdivision technology, which can achieve millimeter level accuracy without the need for cooperative targets. The measurement range has exceeded 100m and can quickly and accurately display the distance directly. It is the latest type of length measurement standard instrument in short-range precision engineering measurement and building area measurement.
