OPC connection with siemens TIA portal

I have developed a model of motor speed control using variable frequency drive and established the server by defining OPC connection. The motor that I am using is asynchronous motor with parameters 5.5kw power and 1500 rpm. When I run the model and log the data through PLC , the value changes from full load condition to the fed value. For example , If I feed the rpm speed as 1000 rpm , the motor speed will first go to the full load condition i.e 1500 rpm and then go to 1000 rpm. What I want to do is the value should move from 0 to 1000 rpm(fed value). I am attaching the code that I am using for it.

function [x] = Read_OPC_Func(y)

% variables

persistent init_Server;

persistent init_Nodes;

persistent uaClient;

persistent Var_Node_In;

persistent Var_Node_Out;

persistent testVal;

% initialize variables

if (isempty(init_Server))

testVal = 0;

init_Server = 0;

init_Nodes = 0;

end

% OPC UA server (PLC) address and connecting client (Simulink) to the

% server

if init_Server == 0

init_Server = 1;

uaClient = opcua('172.16.4.20',4840);

connect(uaClient);

end

% define variable nodes in the server

if uaClient.isConnected == 1 && init_Nodes == 0

init_Nodes = 1;

% find DB "OpcInterface" of the server

%DB_Node = findNodeByName(uaClient.Namespace, 'OpcInterface', '-once');

% find variables "ref_speed" and "Motor_speed" in the DB "OpcInterface

Var_Node_In = opcuanode(3, '"OpcInterface"."ref_speed"',uaClient);

Var_Node_Out = opcuanode(3, '"OpcInterface"."Motor_speed"',uaClient);

end

% read and write variables of the server

if uaClient.isConnected == 1 && init_Nodes == 1

% read "ref_speed" value from server and store in "val"

[val, ~, ~] = readValue(uaClient, Var_Node_In);

% assign input y of the function to "Motor_speed" variable

writeValue(uaClient, Var_Node_Out, y);

% assign "val" to variable "testVal"

testVal = val;

end

% assign "Motor_Speed" ("testVal") value to the output x of the function

x = double(testVal);

end

Answer 1

[Joel Van Sickel]

Hello Omkar,

it is quite possible the problem isn't with this code itself. As this is just a function, the question is where is the motor getting full speed from? Does the actuall controller of the motor start it at full speed, and then the first command from this code actually works and sets it to 1000. Do you have something that is calling this code before your submit the value of 1000? I would look beyond this code for your issue. Is there a default value for your ref speed that is automatically setting it to 1500?

To debug, you could modify this code to create a function that only reads speed and also read reference speed and check to see if these values are what you expect them to be without commanding anything.

Regards,

Joel

[omkar tulankar]

Hi joel ,

I am using asynchronous motor machine block of simscape electrical to define the motor characteristics.

There I am defining the characteristics i.e. squirrel cage , 5.5 KW , 1500 rpm (15 number under slection list).

I guess from here it is taking the default value 1500.

But if I try to control the speed of motor without linking this function to model , i.e manually feedind the input through constant value , the motor speed goes normally from 0 to fed value speed , but when I link this function to the model and try to feed the value through PLC established via OPC connection , the speed goes from full load condition (1500) to the set value which is not desired. And also simulation time gets severely affected i.e it goes ver slow. If you recommend any changes in parameter or function code define , please help me out with it. Thanks

[Joel Van Sickel]

Hello Omkar,

are you controlling the speed of your machine with a VFD or are you controlling it with a simulink signal specifying it's speed? The only way for the motor to set itself to 1500 would be through initial condition. Otherwise an external signal or the VFD controls the speed, depending on how you have set things up, so that is where you should be looking. Is the PLC doing the actual closed loop control or just serving to provide set points? It sounds like you are having a PLC connect to a desktop simulation. I still recommend writing a function that only reads motor speed and motor reference and seeing what results you get there. Otherwise, you will need to provide plots and simulation models and diagrams of how you have connected the PLC to the system and what signals are coming from where for us to be able to provide other advice.

Here is how to debug: You will need to write additional functions to do this.

1: Connect the PLC without writing or reading with a function call

2: Read the speed reference and measured speed without writing (only use read command) with a function call

3: Write a speed command of 0 with a function call

Now you can see where the motor goes to 1500 in this process.

Regards,

Joel

[omkar tulankar]

Hi joel ,

Stability of the output values is very low when attached to the function. I also tried to do it by reducing the sample time of the solver configuration. But it just reduces the simulation time but adccersely affecting the output values.

As per your suggestion , changes in the code is hardly making any diffrence.

Is there anything else that can be done for stability of the model.

[Joel Van Sickel]

Hello Omkar,

the speed probably has to do with the OPC connection. if you want something faster you would probably need to use real-time hardware to directly connect to the PLC.

Regards,

Joel

[omkar tulankar]

Hi Joel ,

Is there any limitations or prerequisite changes required to be done while using simscape electrical blocks for

establishing communication via OPC. Beacause when I try to establish communication for pure physical models , the communocation is working fine , but then I am trying to the same with using simscape electrical blocks , it is creating problem as mentioned before.

I am also attaching the simulation model I am trying to interface with OPC.

Please provide solution to this problem.

[Joel Van Sickel]

Hello omkar,

it possible that your simulation is not converging, and maybe even proving inf/nan values. The discrete time step you have isn't fast enough to simluate a switching frequency of 7500. You would ideally set your time step to be 1/7500/100, which would also make your model significantly slower. I would recommend using the universal bridge set to average mode as an inverter to start with before attempting to capture switching events in this way.

Regards,

Joel

[omkar tulankar]

Thanks Joel , I tried changing the time step according to the frequency and it is working much better, though the exact values are not achieved , but variations are lowered down. Thanks for your help ansd handling my queries patiently.