PLC+ CHALLENGE 2017 FAQ – Frequently asked questions english
- Meaning of disturbance variables are:
N_LKva (Disturbance_0) – Number of light vehicles on E portal,
N_HKva (Disturbance_1) – Number of heavy vehicles on E portal,
N_LIst (Disturbance_2) – Number of light vehicles on W portal,
N_HIst (Disturbance_3) – Number of heavy vehicles on W portal,
v_Kva (Disturbance_4) – Vehicle speed in E portal,
v_Ist (Disturbance_5) – Vehicle speed in W portal,
p_Kva (Disturbance_6) – Pressure in E portal,
p_Ist (Disturbance_7) – Pressure in W portal.
- In the task pressures on each portals are mentioned in measured values (controller input) and also in disturbances. Furthermore, under the conditions that apply to the controller, it is said that they are inputs of the controller, but are located in DB4 Disturbances – Disturbance_7. Are the pressures on portal measured or disturbing variables. In TIA Portal the output of function block “Tunnel” just CO measurements and air follow speed, while in the task is mentioned that the pressure on portals is measured. Which one of these two are correct?
Pressures are measured disturbing variables, their current and former values can be used in control algorithm the same way as measured output variables are used (air flow speed and CO concentration in tunnel). Number of vehicles and average vehicle speed in both directions (as disturbing variables) can’t be measured and they can’t be used in control algorithm.
- Disturbance variables are mentioned as number, speed and a type of the vehicles that go into a tunnel. During the project simulation, are the variables estimated or are they input variables? If the disturbance variables are written in DB4, why are they called Distrubance_1..7 and not by their names?
Immeasurable variables can be, but don’t have to be estimated in controller, it all depends on chosen control algorithm that will be implemented.
- What are actually disturbing variables and in which way are they accessed?
They are described in the task (Disturbance variables are..), the part of the code which access there variables in order to determine them for specific sampling moment doesn’t have to be changed, they are all available to the process simulation block, while the mentioned pressures can be used also as a signal for the controller because it is considered that these signals are measureable.
- Are the wind direction change and the change of rotational direction of ventilators considered as turning the ventilators on and off?
No. It is assumed that upon change of the air direction ventilators automatically change rotational direction without requirement from the controller to turn them off and on. For example if 10 ventilators are active and air direction is changed, all 10 ventilators will change rotational direction automatically and follow the natural air direction.
- In the task, it is written that the wall friction force is linearly dependent on v_a, but in the lectures it is said it depends quadratically on the v_a? Which one of these statements is true? Thank you for your comment.
You are right, there is an error in the task – the friction force depends on square of v_aest. However, correct equation (with square dependency) is implemented in the model realized on PLC.
- Regarding piston forces: should we limit them to never go below zero (or above if we’re talking about west piston forces)? For example, let’s take F_pistHE (piston force created by heavy vehicles coming into the east portal). If their speed is, for example, v_hE = 3 m/s and air speed is v_a = 7 m/s, using equation (2) we’ll get a huge negative result (N_hE*CdH*Ah*rOA*0.5 is always greater or equal to zero). This means that those vehicles are creating force in the opposite direction of their heading direction, but both the air and the vehicles are moving from east to west, in the “positive” direction so I’m a little confused by this. Thank you in advance.
The piston forces are part of the model. As a challenger you have no authorization to change any part of the model. So, you cannot limit the forces. Furthermore, the piston forces shouldn’t be limited – they can take positive and negative values (that is exactly how this part of the model is realized on PLC). In the case that you described vehicles actually represent ‘obstacles’ (since v_a is greater than v_hE) and as such their piston force can be treated as some sort of a friction force.
- What values are we supposed to multiply by 10^6 in equation (8) to get CO in ppm units. I’m confused about this because k_CO also depends on the CO concentration and if I multiply entire right side of equation by 10^6 the expressed units don’t match? (because Q_co is expressed in [m^3/s] and the value it is subtracted from is expressed in [ppm/s]) Also, are we supposed to subtract the piston effect forces F_pist_E from F_pist_W, or do we just take the speed values from v_E as a positive number and v_W as a negative number and just add the piston effect forces.
CO value should be multiplied by 10^6 in order to get CO measured value that is used as i) a controller input, and ii) as a variable that is displayed on HMI. Multiplied value shouldn’t be used in mathematical expressions. Velocities v_E and v_W are always positive numbers. va_est can take both, positive and negative, values. Hence, depending on velocities’ signs, piston force can support or oppose air flow in the tunnel.
- Where are we supposed to find the input of the reference (border) allowed value of CO concentration in the tunnel (we haven’t found it in the assignment or in the program). This data would provide a great stepping stone for the optimization of the program. Are we supposed to use values of some kind of regulation?
Reference or maximum value can’t be discerned from the assignment text or from simulation model. In our instance of the assignment (tunnel Učka) the value is 15 ppm, like we discussed with colleagues from Zagreb. I mentioned an “issue” about the solution from Učka at the presentation, since the new method surpasses the “border” values. The suggested criteria function “J” is formulated in a way to allow for surpassing the border values, but it still penalises it with appropriate criteria. I suggest the use of 15 ppm value, but make sure that you will be able to change it fi the need arises (don’t hardcode it).
- Where can I find tab protection when installing PLCSIM Advanced?
Right click on project name -> Properties
- Will it be considered as an input to the controller if we use the number of vehicles as a condition?
No. Professor suggested to use a constant traffic profile at the presentation, which you can save into the regulator.
12. Can we change the start value of the ventilators (Constants.n_F)?
YES, this is the controller’s output.
13. In the task it is said that number of vehicles per hour in the tunnel is kept below 500, but time intervals are in 24 minutes span. How are those disturbances calculated, in 48 minutes or in 1 hour and 12 minutes?
The traffic profile that will be given in DB4 for evaluation, will not exceed 500 vehicles per hour. However, it’s value can be changed every 24 minutes (60 intervals per 24 hours). Disturbances (including traffic, speed, air pressure…) are given in DB4 (Q&A no. 1 on http://www.eestec-lj.org/plcplus2017faq/).