Process Level Indicators - Papermachine Automation

Level Indicators

Load cells

A load cell is a transducer which emits an electrical signal when a load is applied. The electrical signal is proportional to the force being applied. Strain gauge load cells work in a similar way to the paddle consistency meters. The electrical signal can be calibrated to measure the weight/ fill capacity of a tank or hopper. Some starch mixing tanks, for example, use load cells to weigh the amount of starch for each batch.

The downside to load cells are, they have to be mounted in such a way that all of the weight/ load is on the load cell. The load cell can become deformed/ inaccurate if the transmitter is overloaded and as such fail-safes have to be fitted to prevent overloads. After a time the load cell loses its accuracy due to constant deformation of the transducer. Compared with the ultrasonic level transmitter the operating range is low.

Ultrasonic level transmitter

An ultrasonic transmitter uses ultra-sonic radio waves to determine the fill level in a tank. The transmitter sends out a signal. The signal hits the top of the liquid in the tank, reflects back to the transmitter where the time taken is calculated. The time is compared with the time calibrated when the tank was empty. A percentage can be calculated and this is the level of the tank. For example

If the tank, when empty took 5 mili seconds to send and receive the signal, at 50% fill volume the transmitter would record a 2.5 mili second delay.

Ultrasonic transmitters have a high rate of accuracy and tend to maintain their calibration over time. The transmitter can work on lower ranges than load cell transmitters. 

The disadvantage of ultrasonic transmitters can be the substance you're trying to measure, for example in a starch silo lots of dust is produced when filling. this dust can obstruct the ultrasonic transmitter giving a false or erratic reading

Guided wave radar

fundamentals of guided wave radar level measurement come directly from Time Domain Reflectometry (TDR), a technology that has been employed for decades to find breaks in underground cables and in-wall cable installations in large buildings. TDR instruments launch low amplitude, high-frequency pulses onto the transmission line, cable, or waveguide under test, and then sequentially sample the reflected signal amplitudes.

Guided wave Transmitters work on a similar principle as ultrasonic transmitters. the only difference is how the wave is emitted. the radar wave is sent through a "cable probe" that spans the height of the tank for example. the radar wave travels down the cable and reflects back. the transmitter uses time of flight principle to calculate a level. 
the pulses traveling through the probe are disturbed by the liquid or dry powder medium and reflects the signal back early giving a level reading.

Hydrostatic Transmitters

Hydrostatic transmitters are similar construction to pressure transmitters. the transmitter design is the same the application and calculations behind the transmitter vary. Placed at the 0 level of a tank or silo the hydrostatic transmitter will measure a hydrostatic head or "total head pressure" which is basically the pressure exerted by the water column in the tank. 

So, the filling height is calculated from the distance of the medium surface to the measuring point by the pressure measurement. The weight force of the liquid column, thus the hydrostatic pressure, however, is not only directly proportional to the filling height but also varies with the specific gravity of the medium and the force of gravity.

PID Control loop parameters - Papermachine Automation

Control loop tuning

PID Control

PID control refers to the control system that alters the process outputs to bring the measured valued closer to the setpoint value.

The computer uses an error value (the difference between the setpoint and the measured value) to base the calculations on. PID is comprised of 3 parts, the proportionality value, the integral action and the derivate action. The majority of control systems tend to use PI control because of the limitations of the D action.

The Proportional Action

The proportional term produces an output that is proportional to the error value. The proportional response to the error value can be multiplied by the proportional gain constant (Value P on the control system).

A high proportional gain constant (Pk) relates to a larger change in the output to correct the error value. If the proportional gain constant value is too high it can make the control loop unstable. Likewise, if the value is too low the control action can be too small and results in a small output when responding to a higher input. This can lead to a less sensitive controller.

The integral action

The contribution to the PID control from the integral term is proportional to both the magnitude (highest value) of the error and the duration of the error. Plainly speaking the Integral value affects the time it takes for the proportional value to meet the set point by reducing the error value to 0. The controller works by adjusting the repeats per minute value. The bigger the integral action the quicker the proportional value meets the set point. By increasing the repeats per minute value the faster/ bigger the integral action is.

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The derivative action

The derivative action looks at the slope of the error value and tried to predict the measured value. This allows you to have higher P and me values while also keeping the system stable. The value of the derivative action describes how far in the future it should look so 20 would mean it looks 20 seconds into the future.

The issue with using the derivative action is if there is noise on the measured value (small spikes) this confuses the algorithm used and increases the effect of the Derivative value leading to an unstable control system. Most control systems do away with the Derivative action for this reason. The derivative function looks at the “steepness” of the curve, having noise on the MV leads to high steepness but no actual chance this is why the derivative action breaks down.

The PID control display page can be accessed through the face plates on the DCs system but cannot be altered. Fine tuning of the PID controls can be difficult. Fine tuning of the controls should dampen the oscillations created by the output so that the MV matches that of the SP quickly and stabilizes.

For more information of the devices that can be used with PID controllers within the papermaking industry take a look at this; Process Level Control - Paper machine Automation