Thread Content
During operation of the flow sensor in Emerson’s mass flow meters, the elastic system of the measurement tube remains in a resonant state at all times. Without vibration of the measuring tube, no Coriolis force is generated, and the mass flow sensor stops functioning. During the vibration of the measuring tube, the flow of the fluid causes a change in the phase of the detection signals at its inlet and outlet sides. The two series of voltage sine signals measured by the left and right electromagnetic detectors L and R are respectively amplified by the isolation amplifiers A1 and A2 via inductive circuits. At the same time, the voltage sine signal output by the left-side signal detector L is amplified by the isolation amplifier A2, and after further amplification by the drive amplifier, it provides an excitation current to the drive coil, thereby causing the measurement tube to oscillate via the electromagnetic driver. This vibration signal is then picked up by the signal detector, thereby forming a positive feedback loop consisting of the signal detector, amplifier, and electromagnetic driver, which maintains the self-oscillation of the vibration system in the measurement tube. To maintain a stable amplitude of vibration in the measuring tube, a negative feedback AGC automatic gain control circuit is connected to the drive amplifier. In this AGC circuit, the signal u is compared with a reference level, and a control signal with a stable amplitude is generated and sent to the excitation coil, so that the electromagnetic driver can cause the measuring tube to vibrate at the excitation frequency. The temperature sensing element in the sensor of Emerson mass flow meters is generally a platinum resistor attached to the wall at the inlet end of the measurement tube. The platinum resistor is powered by a constant current source, and the terminal voltage of the platinum resistor is taken; this voltage is processed by a voltage conversion circuit to generate the voltage Ug. The microprocessor calculates the temperature value of the measuring tube based on the voltage Ug, and corrects the temperature-related errors in mass flow rate and density measurements caused by changes in the elastic modulus of the measuring tube material with temperature. The microprocessor directly drives the current loop and frequency loop components, which output standard current signals of 0/4–20mA and frequency signals respectively; both the quantities represented by these standard current and frequency signals as well as their ranges can be configured. When the detection signals from the electromagnetic detectors L and R on the left and right sides of the flow sensor are amplified by the isolation amplifiers A1 and A2, they enter the signal processing circuit. With the assistance of a microprocessor, this circuit determines the phase difference or time difference Δt between the left and right detection signals; this time difference is in a linear relationship with the mass flow rate. At the same time, the signal processing circuit generates the vibration frequency signal of the measuring tube based on its input signal u and U2. The vibration frequency of the measuring tube is a function of the fluid density, decreasing as the fluid density increases. The microprocessor calculates the mass flow rate and density of the fluid based on the time difference Δt of the detection signals and their frequency f.