Lab Volt Process Control Trainer

The Lab Volt Process Control trainer model (3521) was purchased by the School of Engineering & Physics, University of the South Pacific for use in control engineering courses. The model came with a customised data acquisition device and proprietary software for control. One of its disadvantages was the lack of an interface with Matlab for labs and projects that required design using root locus and the implementation of difference equations. To overcome this problem, a National Instruments USB DAQ card was interfaced with the model and connected to a PC running Matlab. Fig. 1 below shows the complete system setup. Fig. 2 illustrates the DAQ card and connections. Fig. 3 shows step response data collected and displayed on a computer screen using Matlab.

Fig. 1 System set up for Matlab control

Fig. 2 National Instruments USB DAQ card

Fig. 3 Step response data plotted "on the fly" in Matlab

Matlab Code

% This m-file is designed for use with the lab-volt process control trainer

% model. A National Instruments NI DAQPad-6015 is used for data acquistion.

% Feedback temperature is input via analog input 0 (AI 0) while the

% controller output is sent via analog output 0 (AO 0).

% The difference equation of a digital controller can be programmed by the user.

% Sample Time T, execution time TEX, and reference input refInputP

% can be set by the user.

% Feedback temperature and controller output are output to the matlab workspace

% and also plotted in graphs.

% © Praneel Chand 05/04/2010

clear all

clc

% specify sample time in sec

T = 2; %two seconds

% specify execution time in minutes

TEX = 20; %twenty minutes

% specify reference input per unit value

refInputP = 0.8;

% DO NOT EDIT BELOW THIS LINE----------------------------------------

% convert reference input PU value to voltage

inputRange = 5;

minInput = 0;

refInput = minInput + refInputP*inputRange;

% create data acquisition device objects.

tempInputObj = analoginput('nidaq','Dev1');

addchannel(tempInputObj,0); % add channel 0 to analog input object

set(tempInputObj,'InputType','Differential');

set(tempInputObj,'SamplesPerTrigger',101); % read in 101 samples each time

set(tempInputObj,'TriggerType','Manual'); % manual trigger

set(tempInputObj,'TriggerRepeat',inf); % manual trigger

contOutputObj = analogoutput('nidaq','Dev1');

addchannel(contOutputObj,0); % add channel 0 to analog output object

% INITIALISE VARIABLES

% initialise loop counter

count = 1;

% loop counter limit

countMax = ceil(TEX*60/T) + 1;

% intialise temperature input data

tempData = 0;

% initialise controller output data

contData = 0;

% initialise arrays for temperature input and controller output storage

tempDataArray(1:countMax) = 0;

contDataArray(1:countMax) = 0;

% initialise error

error = 0;

error1 = 0;

% intialise timer variables

t1 = 0;

t2 = 0;

% start daq objects

start(tempInputObj)

%start(contOutputObj)

figure(1)

title('Sensed Heater Output');

xlabel('Sample Number');

ylabel('Sensed Temp. (V)');

hold on

tic

while count <= countMax

t1 = clock;

trigger(tempInputObj)

tempData = median(sort((getdata(tempInputObj))); % read 101 samples of temperature input data and compute median (acts as a filter)

% check if input data is within 0-5 V

if tempData < 0

tempData = 0;

elseif contData > 5

tempData = 5;

end;

tempDataArray(count) = tempData; % store temperature input data

plot(count,tempData,'*');

drawnow;

%DO NOT EDIT ABOVE THIS LINE-----------------------------------------

% insert your digital controller difference equation here...

error = refInput - tempData; % calculate currrent error

% continue with your difference equation programming...

contData = refInput; % e.g. open loop system, step input applied to plant directly OR

contData = (2.802*error-2.758*error1+contData); % e.g. feedback system difference equation

% DO NOT EDIT BELOW THIS LINE----------------------------------------

%contData = 2.2

% check if output data is within 0-5 V

if contData < 0

contData = 0;

elseif contData > 5

contData = 5;

end;

contDataArray(count) = contData; % store controller output data

putsample(contOutputObj,contData)% set controller output data

count = count + 1;

%t2 = clock

timep = T-(etime(clock,t1));

pause(timep); % insert pause to meet sample time requirement

end;

% set output to zero to disable heater

putsample(contOutputObj,0)% set controller output data

% stop objects

stop(tempInputObj)

stop(contOutputObj)

% delete objects

delete(tempInputObj)

clear tempInputObj

delete(contOutputObj)

clear contOutputObj

% plot output data

for i = 1:length(tempDataArray)

timeArray(i) = (i-1)*T/60;

end;

figure(2)

plot(timeArray,tempDataArray,'o:')

title('Sensed Heater Output')

xlabel('Time (min)')

ylabel('Volts (V)')

figure(3)

plot(timeArray,contDataArray,'o:')

title('Controller Output')

xlabel('Time (min)')

ylabel('Controller Output (V)')