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Sensing and Control
Digital or Analog
Digital or Analog
Digital Input: The interface device can detect a simple
change in an electrical voltage level. That is, it can detect if
a switch is on or off.
Digital Output: The interface device can change an
electrical signal to on or off and therefore turn on motors,
lights, etc.
Analog Input: The interface device can detect a continuously
changing electrical voltage coming from sensors which may
be responding to changing parameters such as light, sound,
liquid levels in tanks, etc. The changing voltage levels are
changed to digital numbers to be manipulated and stored in
the computer. ( done by ADC)
Analog Output: The interface device can take a series of
stored digital numbers and change them into a varying
electrical voltage. This voltage can be used to control an
external device. (done by DAC)
The Simplest Circuit
The simplest circuit consists of:
– A source of electrical current
– A conducting path
– A control element (Switch)
– Something that converts electrical energy to
some other form of energy (called a load
resistance)
– See Mims, pp 14,18-25
There are a number of electrical parameters
(measurements) that are of importance.
The 3 most basic are:
– Electric Potential Difference - measured in
Volts
– Electric Current - measured in Amperes
– Resistance - measured in Ohms
Electric Potential Difference can be very
loosely defined as the “electric pressure”
which forces electric current through a
conductor
Electric Current is the flow of electric
charge through a conductor in a unit of
time
Resistance is the opposition to the flow of
current through the conductor
The relationship is expressed as Ohm’s Law
V= I * R
Rt = R1 + R2…. (series)
1/Rt = 1/R1 + 1/R2…(parallel)
Pulse Width Modulation
DC motors are often run at a constant speed.
Other uses require variable speed.
One of the most efficient methods of DC
motor speed control is called Pulse Width
Modulation
• Pulses are simply on/off voltages.
• The length of time the voltage in on is the
pulse width.
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•
•
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Cycle: a single pulse
Period: Time to complete one cycle
Duty Cycle: time on/length of cycle
Frequency: number of cycles per second
PhidgetLVMotor Controller
• Two independent low-voltage terminal
blocks
• Uses PWM for motor control
• 65 pulse steps (Code allows -100 to 100)
• Frequency: 2.5KHz
• Lamp – Brightness
• DC Motor - Torque
Servo Motors
Servos do not have a continuous supply of voltage given to them. Instead, they are
supplied with electricity using Pulse Width Modulation. What does this mean?? There
are a series of pulses sent to the motor controller which judges how much you want the
motor to turn and this causes the shaft to turn that amount.
Servo Motor Control
The DC motor is connected to a set of
gears which reduces speed and increases
torque or the force of the turn.
There is a control circuit which controls
how much the motor turns.
The position sensor is a variable resistor
or potentiometer which is responsible for
sensing the position of the motor.
Servo Motor Control
The position of the motor is monitored by the potentiometer sensor. If the motor is not in
the position as sent by the external servo controller, the internal control circuit causes
the motor to turn until it is. When the motor is in the correct position, the sensor signals
the control circuit to turn the motor off.
The process of the sensor sending information to the control electronics which turns the
motor which turns the sensor is called feedback or a closed loop control circuit.
How does the potentiometer sensor know when the motor is in the correct position??
Servo Motor Control
Each position of the servo is referenced by
an internal control circuit pulse. 0 degrees
is 1ms, 90 degrees is 1.5 ms and 180
degrees is 2 ms. When the external pulse
is issued by the controller, it is issued in
milliseconds as well.
The internal circuit compares the external
pulse to the internal and starts turning the
motor. As the motor turns, the internal
pulse changes relative to the position.
When the difference in the two pulses is
zero, the motor will stop turning.
The difference in pulses is called the error
signal.
If the incoming pulse is greater than the
internal pulse, it turns in the clockwise. If it
is smaller, it turns counter-clockwise.
Referencing the Phidgets
• Install the Phidget Library on the computer
• Before you can use Phidget code in Visual
Basic the library must be referenced
Open the Project menu and select
References. Scroll down and select
Phidget Library by clicking the check box.
Declaring the Phidgets
In General...Declarations enter the following code:
Dim WithEvents MyPhidgetName As PhidgetDevice
Note: MyPhidgetName is called an instance of the
class of objects called PhidgetDevice You can
use any name as long as it is not a Visual Basic
reserved word.
Opening the Phidget
Code the Form_Load procedure:
Set MyPhidgetName = New PhidgetDevice
Call MyPhidgetName.Open(True, -1)
Call MyPhidgetName.Open
Servo Motor Control Phidget Code Example
Assuming the Phidget library is installed and referenced in this Visual Basic
application.
Declare a variable of PhidgetServo type:
Dim WithEvents Servo As PhidgetServo
We then have to instantiate that servo instance when we load our form:
Set Servo = New PhidgetServo
Call Servo.Open(True, xxxx)
The Servo instance now exists and has a set of member functions which can be
accessed using dot notation. One of these member functions is called
MotorPosition( n ) and is accessed by placing the dot between the instance
of Servo and the member function.
Servo.MotorPosition (n)
The (n) is an index which represents which of the motors we are controlling at any
given time. The servo controller can control up to four motors. The first is powered by
the +5 v supplied by the USB cable. The other three are powered by the power supply
attached to the device. Each device is independent of the other and can be assigned
a degree value between 0 and 180.