Lab #7: Analog I/O and Closed Loop Programming - WVU ...

Lab #7: Analog I/O and Closed Loop
Programming
Objectives:
• Introduction to analog output
• Familiarization with basic output and functions
• Familiarization with scaling
• Introduction to closed loop control with analog sensors
Introduction:
During the course of mechanical engineering, an engineer may have a need to be
able to turn a motor or other apparatus at a variable speed by feeding it specific voltages.
An engineer may also have a need for a program the runs continuously to monitor or
record an input, or continuously change an output with respect to this input. These tasks
often require the use of analog inputs and outputs.
Analog Input
Analog input reads the state of an analog apparatus. Unlike digital input, however,
analog input can give us more specific information. An analog input returns a number
related to the voltage present on its input terminals. The voltage usually comes from a
sensor and is proportional to a measured quantity like pressure, temperature, speed, etc.
Alternately, the input can represent the desired temperature, pressure, etc. and in these
cases usually comes from a variable resistor called a potentiometer. The volume control
knob on your stereo is usually connected to a potentiometer.
Either way, the voltage represents an actual or desired level of something. The
voltage must be scaled to properly represent engineering units. Scaling is a simple
mathematical function—usually linear—more on that later. The analog voltage is
converted to a number by the Analog to Digital Converter (ADC or A/D Converter).
These will be discussed in detail in class. In your interface boards, the range can be
selected by software, but the system is set up to provide an input range of -10V - +10V as
the default. Your converters have 12 bits of resolution so what you read from the analog
input channels will be accurate to +/-0.00488 V.
Experiment 1: Reading a Potentiometer
• Hook the potentiometer to an analog input using the schematic below. The left
terminal should be attached to the 5V power supply with a 100Ω resistor in series, the
right terminal should be grounded both to the power supply and to the ground
terminal on the analog input and to the ground terminal on the power supply. The
center terminal should be connected to the positive terminal on the analog input.
• Turn the power supply on and use an infinite loop to read the potentiometer’s voltage
continuously and display it on the screen. Make sure that the values changes
continuously from approximately zero to 5V. Make note of the min and max values
observed here.

Fig. 6-3—Potentiometer diagram
Fig. 5-2—Analog inputs
Analog output
Analog output is similar to the digital output that has already been covered,
however, instead of just being on or off, you may output a variable voltage. The DAQ
Board is capable of outputting a voltage between positive 10V and negative 10V;
however this experiment will only use ±5V. If this output is hooked to a motor, as it is
here, it means that the speed of the motor can be controlled by changing the output
voltage.
The chip attached to the DAQ Board’s analog output pins and shown in the figure
serves an important purpose. Although the DAQ board can output a variable voltage, it is
only capable of supplying 5mA of current. Such a small current is insufficient to turn a
motor or other device. For this reason, the motor control chip is present. Using an
operational amplifier, it is possible to magnify the current using an external source, while
keeping the voltage output by the DAQ board the same. This allows the user to run a
small motor at variable speeds directly from the DAQ.

Fig. 6-1—Motor control chip
Using the DAQ board’s analog output function
The analog_out function in the mechlab toolbox requires two parameters, channel
and voltage. The DAQ board has two analog output channels, and can accept voltages
ranging from -10V to +10V, and is accurate to approximately 0.001V. This give much
flexibility to specify exactly what the user needs.
Aside from the analog_out function, a secondary function called motor_out has
been written. The purpose of this function is to easily scale -5V to +5V to -100% to
+100%. The reason for this is that this is a simpler way to command the motor when
using an encoder, as Matlab’s reaction time is too slow to accurately read the encoder if it
is running much faster than 5V. It is important to understand that these commands are
essentially the same, however, it is encouraged that you make use of the motor_out
command as often as possible for simplicity and avoidance of erroneous encoder
readings. The motor_out command, like the analog_out, requires two parameters: channel
(1 or 2) and speed (-100 to +100).
Scaling
Many sensors do not begin reading at 0 units of what they are measuring. For
instance, the temperature sensors you will use in this lab will output a voltage of
0.01V/°F. However, the sensor has a base temperature reading of 70°F, meaning if you
would get a reading of 0.01°F, the temperature is 71°F rather than 1°F. For this reason,
the sensor reading must be scaled using the line equation from basic Algebra:
y=mx+b (1)
Using this equation, the programmer can write a function to directly display the
meaning of the output, rather than a voltage that still needs to be interpreted.
Closed loop programming
A closed loop program is one that executes indefinitely until manually stopped. In
this lab, you will build a thermostat and a variable speed motor. Using the thermostat as

an example, think of how a heating system works. A desired temperature is given, which
is compared to the room temperature. If the room temperature is lower than the desired
temperature, the heat turns on, but if it is higher, the heat stays off. A person would not
want to manually turn the heat on or off constantly to keep the room at the desired
temperature, nor would the person want to have to continuously tell the thermostat to
check the temperature. In essence, this is closed loop control. An infinite loop is created,
so that the program will run on its own, over and over, without any external intervention,
aside from changing the desired setpoint. “Set it and forget it,” as they say.
Experiments:
Experiment 1: Using analog speed control
Connect the motor to one of the motor outputs and turn it on using the syntax
motor_out(ch,speed). Change the speed several times and notice how the motor speed
changes.
Fig. 6-2—Motor connected to analog output
Experiment 2: Variable speed control
• Using the potentiometer as a setpoint, write a program to continuously vary the speed
of the motor with respect to the position of the potentiometer. The motor should spin
in reverse from -100 to 0 while the potentiometer is between 0 and 2V. The motor
should remain off while the potentiometer reads between 2V and 3V. The motor
should spin forward from 0 to +100 while the potentiometer reads between 3V and
5V. You will need to using scaling for this part.
Project: Thermostat

• Attach the temperature sensor to the analog input. Scale its output to print the
temperature in degrees. (See previous information on how this sensor reads.)
• Scale your potentiometer’s reading of 0 to 5V to be 70°F to 140°F
• Connect a 100Ω resistor directly to the digital output, so that when turned on it will
heat up. CAUTION: THIS GETS HOT! Do not leave this on long or the resistor will
burn.
• Using the temperature sensor to detect the temperature of the resistor, write a program
so that the potentiometer can be used to give a setpoint, this setpoint will be compared
to the temperature of the resistor, and the resistor will turned on or off according to
the temperature of the resistor compared to the setpoint temperature. Demonstrate this
program to a TA.
Fig. 6-5—Thermostat

Questions:
1. (5 pts) Describe how the analog_out and the motor_out are related, and explain
how the motor_out works, in relation to the analog_out command.
2. (5 pts) Show the calculations needed to scale the motor input versus the
potentiometer output in the closed loop motor control.
3. (5 pts) Tell why the input is not simply scaled, but a “space” called a deadband
is left in the center of the potentiometer input.
4. (10 pts) Include your commented and formatted code for the closed loop motor
speed control program.
5. (Project)
a. (15 pts) Include the code for temperature control project.
b. (10 pts) Show the calculations for scaling the temperature input.