Basic Structured Text Programming

nicomp 2 years ago

Very cool. Brings back memories of long hours doing start-ups in warehouses and DCs.

K.I.S.S. 2 years ago

What's up with all this one-shot-trigger-rising-edge stuff?

IF s_s THEN

IF o_o THEN o_o:= TRUE;

ELSE o_o:=FALSE;

ENDIF;

ENDIF;

If that ain't all there is to it, what am I missing?

K.I.S.S. 2 years ago

Or rather:

IF s_s THEN

IF o_o THEN o_o:= FALSE;

ELSE o_o:=TRUE;

ENDIF;

ENDIF;

You know what I meant. :)

TDOT 2 years ago

Hey K.I.S.S -

The reason for the "rising edge trigger stuff" is that, in your example, if s_s is true for more than one scan (which it would be if tied to a pushbutton, lets say) then your logic would flip flop as long as s_s is true. The oneshot logic triggers the output once (and seals in) for the first s_s true scan.... and turns off the output on the next s_s true scan. Get it?

Mequonite 2 years ago

The chatter resulting from K.I.S.S. logic is a classic mistake. TDOT clearly understands how automation systems can easily be broken with goofy code.

K.I.S.S. 24 months ago

I think I've got it. I totally missed the real time aspect of PLC programming. Just to make sure I did get it and to maybe help those like me I renamed the variables in the structured text. Some programmers have come into PLC programming without ever seeing an oscilloscope, a coil or a contact. I know the veterans are shrieking in horror, but you know we are out there.

ONS_Trig -> ButtonPushed (abstracted logical button function)

Start_Stop -> Button (1=currently being physically pushed, input)

Rising_ONS -> AlreadyPushed (previous scan state)

ON_OFF -> On (0=off, output)

ButtonPushed := Button AND NOT AlreadyPushed;

On:= (ButtonPushed AND NOT On) OR (NOT ButtonPushed AND On);

AlreadyPushed := Button;

It almost reads like prose, doesn't it?

Thanks for helping me with this. Even if I didn't get it quite right, your responses have helped me think about it.

Post more stuff. How about something on debounce or alarm ack handshaking between PLC and HMI or anything on how PLC and HMI programs split up the work and keep in sync.

Mequonite 22 months ago

ST Programming allows for style in tag naming as well as logical technique. K.I.S.S. recoded the example with a commone VB coding style using leading character capitalization with merged words. This convention works well! Thanks for the update (AlreadyPushed :)

judi 21 months ago

Hi Mequonite,

It seems that you know alot about the PLCs and ST. I want to write a program in st to control an end-effector. It is very difficult for me to start with st. I have 2 amplifiers from Beckhoff. I could give a simple movement to my motors through a simple program from beckhoff but I even have problems to reproduce that program myself. I am really confused.

I really appreciate if you can help me. thanks in advance

Regards

Mequonite 21 months ago

There are many techniques for motor axis motion with ST. I prefer to create a simple case statement that reflects the status of the motor. The idea is to trigger the Beckhoff motion commands at the start of each status state. Here is an example I use with Siemens controllers:

FUNCTION_BLOCK FB100 // Tech CPU Basic Axis Motion Block

VAR_INPUT

Axis_Num : INT; // Axis Number from Technology Controller

Estop_Okay : BOOL; // Signal to Enable Axis or Remove Power

Jog_FWD : BOOL; // Run or Jog Forward request bit

Jog_REV : BOOL; // Run or Jog Reverse request bit

Move_ABS : BOOL; // Absolute Positioning Move

Move_REL : BOOL; // Relative Positioning Move

Stop_Now : BOOL; // Set Stop Now to Stop Axis at any time

Home_Axis : BOOL; // Reference Axis to Encoder Homing Position

Tech_Error : BOOL; // Technology Fault bit from Axis TO

Clear_Fault : BOOL; // Reset Axis Faults

Params : BasicDynamics; // Structure of Dynamic Parametes for Velocity & Position

END_VAR

VAR_OUTPUT

AxisState : INT;

Enabled : BOOL;

Faulted : BOOL;

Warning : BOOL;

Moving : BOOL;

Target_Reached : BOOL;

END_VAR

// MC_Power --- Axis Enable ---

AxisPower(Axis := Axis_Num // IN: INT

// MC_Reset --- Acknowledge Axis errors ---

AxisReset(Axis := Axis_Num // IN: INT

IF NOT Estop_Okay THEN

AxisState := 0;

END_IF;

IF Faulted THEN

AxisState := 20;

END_IF;

IF AxisState AxisStateMem THEN

AxisStateMem := AxisState;

StateUpdate := TRUE;

ELSE

StateUpdate := FALSE;

END_IF;

// Axis State Model of Operation

CASE AxisState OF

0 : // Axis at ESTOP or Disabled

IF Enabled THEN

AxisState := 1;

END_IF;

1 : // Axis is Ready to accept command

// Request to Jog or Run Axis with MC_Velocity

2..3 : // Axis is in a Jog or Run State

IF StateUpdate OR (AxisVelocity.Busy AND JogVelocity Params.Velocity) THEN

JogTrig := TRUE;

JogVelocity := Params.Velocity;

ELSE

JogTrig := FALSE;

END_IF;

// MC_MoveVelocity --- Jog or Run Axis at Velocity Setpoint ---

AxisVelocity(Axis := Axis_Num // IN: INT

4 : // Axis is Commanded to an Absolute Move

// MC_MoveAbsolute --- Absolute Axis Move ---

ABSMove(Axis := Axis_Num // IN: INT

IF Stop_Now THEN

AxisState := 6;

END_IF;

5 : // Axis is Commanded to a Relative Move

// MC_MoveRelative --- Absolute Axis Move ---

RELMove(Axis := Axis_Num // IN: INT

6 : // Axis is Commanded to a STOP

// MC_Stop --- Reference Axis position ---

AxisStop(Axis := Axis_Num // IN: INT

IF AxisStop.Done THEN

AxisState := 1;

END_IF;

7..10 : // Axis is Commanded to a Automatic Machine State

;

19 : // Axis is Commanded to Reference the Encoder

// MC_Home --- Reference Axis position ---

HomingActive(IN := NOT StateUpdate// IN: BOOL

IF HomingActive.Q (* AND AxisHome.Done *)THEN

AxisState := 1;

END_IF;

20 : // Axis is Reported to be Faulted

IF Enabled OR NOT ESTOP_Okay THEN

AxisState := 0;

END_IF;

ELSE:

// Statements_ELSE

;

END_CASE;

// One Shot Control Bits

Jog_ONS := Jog_FWD OR Jog_REV;

Move_ABS_ONS := Move_ABS;

Move_REL_ONS := Move_REL;

Stop_Now_ONS := Stop_Now;

Home_Axis_ONS := Home_Axis;

FaultTimed(IN := (Estop_Okay AND NOT Enabled) OR Tech_Error// IN: BOOL

,PT := T#500ms// IN: TIME

);

IF FaultTimed.Q THEN

Faulted := TRUE;

ELSE

Faulted := FALSE;

END_IF;

END_FUNCTION_BLOCK

judijudi 21 months ago

Thanks Mequonite,

I hope I can handle it although I think it is too far. Thank you very much for your useful information.

Can you tell me how can I couple two or three motors with each other?

If I had more questions can I ask you?

Regards

Oinky 18 months ago

I'm wondering how can we program complex Ladder sequences (Interlocking/Priorities) in terms of Structured Text.

I've been programming PLC in Ladder ever since and just started learning Structured Text. I'm having trouble interpreting Ladder sequence into Structured Text. Can anyone provide a link of a helpful ST programming manual?

Thanks very much,

Andrei

Philippines

Shekhar 8 months ago

Hi Mequonite,

Currently existing PLC is S5 & Hw is 101U.

Can you explain me couple of instructions.

what is mean by

L IW 0

L KM 11000000 00000011

AW

T FW 34

L FW 34

L FW 36

>