Tag Archives: Process Control

Monitoring Sound & Vibration for Process Control

In a production environment, one can always find two common themes for the successful application of acoustical or vibrational monitoring. Usually, workers judge the noise or vibration event as being the start or end of a particular process. Initiated by such an event, an automated control system can easily minimize any loss of production.

On the production floor, control of manufacturing processes have used continuous monitoring of sound and vibration for the past several years. For instance Brüel & Kjær had used their 2505 Multipurpose Monitor in the early 1980s to automatically monitor vibration signals. One could connect an accelerometer, a microphone, or other piezoelectric device to this monitor, and set limits for alerting the user whenever the levels exceeded them. They had filters to limit the signal bands, and detectors to average signals that fluctuated highly. On the output side, relays interfaced with the process control systems or other instrumentation. No other expensive analysis systems were necessary if the process control technician used this device to monitor acoustic or vibration levels automatically. People used these monitors also in the machine condition monitoring field as basic overall vibration detectors to switch off the machine if vibration levels exceeded the set limits.

Discrete analog circuit boards enclosed in weather proof enclosures made up these early monitors. The user had to select the circuit cards necessary for their specific application. Usually, a circuit card was capable of performing a specific function, such as RMS detector, amplifier or attenuator, high and/or low pass filter, and signal conditioner. The circuit cards worked together with the relays, alarm indicators, and the meter module. With very little dynamic range, users had to be very careful in selecting a circuit card for each application. One had to be knowledgeable about the transducer they employed and the particular measurement they were making. If conditions changed, they had to order additional circuit cards.

The above disadvantages of the analog system made Brüel & Kjær develop their digital signal processors replacing the monitors with modern electronics. They now had software controlling the functions of RMS detection, gain/attenuation, and filtering. End users found the application of the new monitors much simpler, as a monitor could be field-programmed for meeting the demands of the present task. The supplied software and its use in setting up and control of the unit allowed users to save time they earlier spent on analyzing the required settings before purchasing the monitor.

The new monitors use a PC interface for setting up and to display the results of their measurements. Users can store programmed data within the unit, so the monitor can operate even without the presence of the PC and retain measurements if the power fails. Digital signal processing within the unit allows the user to set up many low and high pass filters, true RMS, and peak-to-peak measurements. Users can set other built-in voltage references and test functions for set-ups related to new tests, including relays and indicators for system failure. In addition, the presence of electrical outputs for unconditioned and conditioned AC signals makes these new monitors ideal for real-time detection and control of acoustic and vibration events.

Differential Pressure with a Tiny Sensor

Process control requires system operators to monitor and control the condition and movement of liquids and gases. Several instruments are available for this, allowing measurement and monitoring of variables, and these fall under the categories of pressure, temperature, level, and flow. Among the pressure-gage category, differential-pressure gages receive the widest recognition for being the largest specialty type – useful in filtration, flow, and level measurements.

While standard pressure gages measure pressure at a single point in a system, differential pressure gages measure pressures at two points and display the difference on a single dial. This makes it easy for the operator to know at a glance, which of the two points is at a higher pressure, and by how much. Use of differential pressure gages greatly reduces operator error, protecting expensive equipment. They reduce operator training and maintenance time, thereby improving process efficiency.

For instance, differential pressure gages are popularly applied in filtration. In this process, a filter separates unwanted contaminants or particles from a gas or liquid system. However, with the progress of the process, the filter becomes increasingly clogged, leading to a drop in efficiency and pressure at the outlet.

It would seem enough to use a single standard pressure gage at the outlet to monitor the health of the filter and assess the time for its inspection and replacement. However, the situation is complicated, as most processes do not maintain a steady working pressure. Several factors are responsible for this, such as compressor or pump on-off cycles or valve open-close cycles, causing wide pressure fluctuations in most processes. For many systems, operators expect such fluctuations of pressure as normal, within limits.

Using two standard pressure gages, one at the input and the other at the output, introduces two additional problems for the operator. First, this compounds the accuracy errors resulting from the two gages as against error from one gage. Second, the operator needs training in reading the two gages, then subtracting the readings, and finally, interpreting the result. History shows many operators do not truly understand the importance of the calculation.

Installing one differential pressure gage using the same taps at the filter inlet and outlet solves all the problems listed above. The accuracy goes up as the rate of error drops. Additionally, the operator does not have to rely on mathematics to understand and interpret the reading – most differential pressure gage dials feature a red arc to indicate the clogging of the filter.

The SDP3x differential pressure sensor from Sensirion is a tiny device. Its dimensions are only 5x8x5 mm, making it one of the smallest of its kind, but with countless new possibilities of applications. It is well suited for use in portable medical devices as well as in consumer electronics.

Users can choose between an analog signal output and a digital one from two versions of the fully calibrated and temperature-compensated differential pressure sensor. The digital sensor, the SDP31, comes with an I2C interface, while the analog sensor, the SDP36, offers an analog output signal. The sensors have a sampling rate of 2 KHz with a resolution of 16-bits, and a measurement range of +/-500 Pa with a span accuracy of 3% of the reading.