Control and Quality Systems

10 min readJan 4, 2021

Operation modes:

Off: The controller does not manipulate the control valve, although the appropriate information is still tracked.
Automatic: The set point should be given by the operator to the controller which manipulates the output according to the logic defined by tuning parameters.
Manual: manipulates the controller output manually.
Cascade: The main controller reacts to the fluctuations in the process variable, and send signals to the slave controller.
Indicator: Allows you to simulate the controller without controlling the process
Set point ramping: Ramping protects a process from rapid changes in the setpoint and the resulting thermal shock as the controller tries to force the process variable to follow. If the set point is reached too slowly via the automatic way, the controller should not have been tuned very well. A solution could be a manual ramp through adding the set point in small steps, 0.001, or the ramp should be improved.

Ramp means the variable divided by the time. A ramp of mass flowrate has a unit of kg/h/time. If it is said, that 150 kg should be dosed within one hour. It means that the suface area would be 150 kg and the ramp with an infinite slope would be 150 kg/h. If the slope would be like a triangle, the ramp should be 300 kg/h/h.

Regelung means contolling with a control valve, while Steuerung is done, when the valve is controlled manually with a defined position.

Die Durchflussregelung wird gebrückt: It means that the flow rate controller is not working.

Single Control Loop

2. Cascade Control Loop: having two or more controllers,

It should be used if you have a process with realtively slow dynamics (level, temperature, composition and humidity) and inner loop (liq. and gas flowrate) is at least 3 times faster than the outer loop

Advantages:

Speeding up the response to a disturbance
Smooth out nonlinearities
Isolating the slower outer loop from valve problems and disturbances

Disadvantages:

A second measurement device
A second controller
Complicated process

Control Loop System:

Open-Loop: heating boiler by a timer
Closed-Loop (Feedback): dependent on the desired and actual process variable

I. On-Off Control

II. Split Range Control: keep one controlled variable at the set point using two manipulated variables

III. Linear Control

Proportional:

- correction to the controlled variable in the outlet of valve which is proportional to the difference between the desired value and measured value.

- more complex than on-off

- cannot eliminate the residual sp-pv error, as it requires an error to generate a proportional output

PID Control (Proportional Integration Derivation): integrates the error to get the sum of deviation which will be added to the process variable over a long period of time.
If there are a lot of vibrations, the Kp value should be smaller.
If there is always a huge difference between set point and process variable, the integral term should be imporved-

Error function: E(t)=SP(t)-PV(t)/ SP: Set point/PV: Process variable

Process gain(K): (change of output/derivation of output)/(change of input/derivation of inout)

Proportional control: The quickest control system which has an offset. : OP(t)=KE(t)+bias/OP: Output variable

Increasing Kp: reaching the set point and steady state more quickly, increasing the oscillation (overshoot)

Proportional + integral control: which eliminates offset, as long as there is an error with a relatively quick response. Integral control:

OP(t)=OP(t0)+1/Ti(Integral of E(t) over the time)/Proportional + integral control: OP(t)=K(E(t)+1/Ti(Integral of E(t) over the time))

Increasing KI (Reducing Ti): reaching the set point exactly,

Proportional+integral+derivative control: to elimante the future deviation. It is not recommended for flow measurements, since it has a lot of noises. OP(t)=Kp(E(t)+1/Ti(integral of E(t) over the time)+Td dEt/dt)

Increasing KD: reducing the oscillation

Response Time: P or P+I+D>P+I>I

Tuning of PID Controller

The vast tuning algorithms for PID controller are designed based on the First Order plus Dead Time Model (FOPDT) .

Ziegler & Nichols method:

Open Loop: The Process reaction curve tuning technique which is generated in response to a disturbance. Three variables, namely, process gain , dead time , and time constant (how fast , are calculated from the open-loop time-domain plot.
Closed Loop:
start with small kp and Ti/Td=0
increase kp to achieve neutral stability, which oscillates …
look up kp, Ti and Td values
If there are lot of vibrations, the Kp-value of the controller should be modified.

Control System:

DCS (Distributed Control System): handling many process, continuous control, for large process, slower than PLC
PLC (Programmable Logic Controller): handling single process, discrete control, for small batch process, high speed control

Control loop components:

Sammelstörung (Collective disturbance): coming from different alarms, which may be the high temperature of the control room. In the control room, there is a cooling medium for keeping the control room cold. If the temperature of the room and the boxes are exceeded, some alarms can triggered.
Sensor: converts an environmental phenomena, light, heat, motion, moisture, pressure to a human readable display
Transmitter: transmit the data. The measuring range doesn’t begin with 0 for defective cases.
A diode: is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. A fuse is an automatic means of removing power from a faulty system; often abbreviated to ADS (Automatic Disconnection of Supply).
A potentiometer: is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider.

Scanner: a laser will be sent to the bar code. If there is only black or white, the black attracts the intensity completely and the white doesn’t attract. It means that there is whether 1 or zero, which will be converted to the number.(11001)2=(19)10
In the electronic, the current is like volumetric flow rate, the pressure is the same as voltage. The current is proportional to the voltage difference. The power is the current times voltage.

I. Pneumatic 3–15 psig (1 psi=0,06 bar)

II. Electrical 4–20 mA

If there is a flow meter with a signal span 4–20 mA for a measurement between 0–400 m3/h. It means that if you have a signal of 12 mA, there is a flow of 200 m3/h.

III. Optical

3. Transducer: to convert a signal in one form of energy to a signal in another , while sensor has always a non electrical input and an electrical output. All sensors are a kind of transducers, while all transducers are not sensors.

4. Interlock: makes the states of two functions dependent

5. Fieldbus: to replace the point-to-point wiring that connects each sensor and actuator to the controller.

6. Proximity sensor (Näherungsschalter):is a sensor able to detect the presence of nearby objects without any physical contact.

Level Measurement:

All level measurements should have a function test to check the alarm test and level measurement for a specific medium.

Point Level:

Capacitance: is the ability of a body to store electrical charge. C=Q (charge on the capacitor)/ V (voltage across the capacitor)
Optical
Conductivity: It is good for mediums having alot of ions
Float switch and tuning fork (die Schwinggabel): working by changing the frequency (vibration)

2. Continuous Level:

Radar: measuring the time of flight of a traveling wave (radio wave), with having the frequence of radar and the time, the level can be measured.

Delay time: the signal has a delay of action. It depends on the dimensions of vessel. The bigger the vessel is, the higher the delay is.

Viscous medium with changing densities: Shwinggabel with flansch (Gewinde) or screw is highly recommended.

Medium with unchangeable viscosity: the level can be measured through pressure.

Level trip: level trip can issue alarm to prevent high level in the vessel or low level in the vessel to protect the pump from dry running.

It is better to have two level measurements, where the level swings. One is used for the safety and the other for the controlling.

Trouble Shooting by Level Measurement:

Auftriebskörper/Schwimmkörper: ist hohl und schwimmt auf der Flüssigkeit. Er ist mit einem Arm drehbar an einem Messumformer befestigt. Der MU wandelt den gemessenen Winkel des Arms in den Level um.
Anlenkungen: Gestänge zur Übertragung eines Winkels auf einen Sensor
Check, whether there is dirt in the Auftriebskörper or Beirohr
Beirohr can be leer to check the 0 level and the electronic
If there is a pressure measurement, we may add another pressure measurement on another nozzle to check the level.
If the mechanic (Auftriebskörper…) can stand a higher pressure, we may check the level: PV=PV…

Flow Measurement:

Orifice Plate (DP Transmitter): The decrease in pressure as the fluid passes thruough the orifice is a result of the increased velocity of the gas passing thru the reduced area of the orifice. You may write a bernoulli equation for two points in the vera contracta and before it.
Venturi flow meter: The principle behind the operation of the Venturi flowmeter is the Bernoulli effect, which has a pressure loss of 20% of the pressure loss in the orifice plate, because it has a long way to reduce the pressure, while the orifice plate has a quick hole.
Oval gear flow meter (Ovalradzähler): The medium fills the defined space between the oval gears and turns the gears depending on the volume throughput. Permanent magnet or stainless steel pins are embedded in the gears themselves; the on-site electronics detect an impulse for each intermediate gear volume transported. The volume is proportional to the detected frequency.

5. Ringkolbenzähler: That’s the same to oval gear flow meter

6. Turbinenradzähler: similar to oval gear flow meter

2. Fluid dynamic (vortex shedding): vortex: a region, in which the flow revolves around an axis line. By having a medium, there is a pressure difference on the axis line, which is the basis of the flow measurement. The type of measurement cannot measure the density.

3. Ultrasonic: The sound signals are sent. Regarding the time and sound velocity, the flow rate can be calculated.

4. Mass flowmeter (Coriolis force) such as micro motion having two small pipelines. As the fluid divides into two tube lines, a corriolis force will be issued between two pipelines. A time difference would be between two pipelines, which is to be used for measuring the mass flow rate.

Micro motion can be also used for measuring the density based on the frequence and the calculated volume rate.

5. Thermal energy with a heating element

6. Pitottube (Staurohr) can be used for the flow measurement

7. Rotameter: through the energy balance between drag (acting opposite to the relative motion of any object moving), buoyoncy (caused by the differences in pressure acting on opposite sides of an object immersed in a static fluid) and weight force.

Pressure Measurement:

The pressure of a vessel can be controlled through the gas. But it can be also controlled through a heater on the bottom of the column and condensor on the top of the column.

Diaphragm seal pressure transmitter:

. to prevent pipe directly into the transmitter in the medium pressure sensor assembly using fluid by capillary (1,6–2 mm)

. no need for heat tracing of transmitter due to use of another medium

2. Gas pressure manometer: fluid raises or falls until its weight is in equilibrium.

To check the flooding or pulver formation or any problem, the pressure difference will be measured using height and adensity.

Temperature Measurement:

To avoid failure, high velocity, corrosion, turbulence, thermowell is highly recommended. The sensor’s response to process temp. is slowed by the addition of the wall. But if the sensor fails, there is enough time to change it. The electrical temperature measurement reacts well to process variations.