11.3.1 Potentiometers

11.3.2 Encoders

11.3.3 Resolvers

11.3.4 Problems

Answer 11.1 360°/64steps, 360°/512seps, 360°/4096 steps

11.4 Linear Position

• A variable resistor is used to convert a displacement to resistance/voltage.

• These give absolute position readings.

• The basic principle of operation is that a moving wiper (sensor input) moves a contact along a resistor. The ends of the resistor are connected to reference voltages. As the wiper moves the potentiometer acts as a voltage divider and produces a voltage proportional to position.

 

• This is effectively a transformer with a moving inner core. The moving core changes the inductance, and this can be converted to a position.

• In these devices there are three sets of windings. The first set of windings is in the center, and is powered by an AC source. The other two coils are at the opposite sides of the main coils. As the core is moved forward/back the magnetic coupling will change.

 

• If the core shown is in the center, the output will be 0Vac, as it shifts to the left there will be more coupling between the center and left coil, and the signal magnitude will be larger than the right hand coil. This difference can be converted to a calculated difference.

• This sensor is used for linear position sensing to high accuracies such as,

load cells

Bourdon tubes

dimensional measurement for SPC

bellows/diaphragm

• The signals from the LVDT can be conditioned with the circuit below.

 

• Near the center of the movement range the voltage is linearly proportional to core movement.

• By overlapping two fine lines patterns we can measure displacement.

 

 

• These are used in high precision applications, and are not available as off the shelf sensors.

• Uses the fact that when light is 180 degrees out of phase it cancels out.

• By finding the phase between outgoing and returning light (from lasers typically) the distance can be found.

11.5 Velocity

• This can be measured with dedicated sensors.

• We may also differential the output of a position sensor to find a velocity

• Output voltage is proportional to velocity (V/(cm/s))

• These devices have low natural frequencies, and are used for signals with higher frequencies.

• well suited to measuring severe vibrations, but it may be affected by noise from AC sources.

• because signals are velocity, some form of integration must be done, making these devices bulky, and somewhat inaccurate

• There are two common methods for mounting velocity pickups,

Magnetic mounts allow fast and easy mounting, but the magnetic mount acts as a slight spring mass isolator, limiting the frequency range.

Stud mounted transducers have a thin layer of silicone grease to improve contact

 

• These devices measure the angular velocity of a rotating shaft.

• One way is to connect a DC generator (motor). The faster the shaft turns, the higher the voltage.

• Another technique uses a magnet with a pickup coil. As the magnet passes the coil a pulse is generated. The pulse magnitude and frequency are proportional to speed.

11.6 Acceleration

• Like velocity we may also find acceleration by differentiating velocity, or by differentiating position twice.

 

Vibration Source: hammers can be used to generate impulse/step function responses. Load cells/vibrators can be used to excite frequency responses (Bode plots and phase shift plots)

Sensors: Velocity Pickups/Accelerometers: lightweight devices that are mounted on structures. They produce small voltages (approx. 10mV). Velocity meters are not as accurate as accelerometers. Accelerometers are very common, and are used for vibrations above 1KHz. Many other sensors are possible.

Preamplifiers: Can power sensors, filter and amplify output.

Signal Processor: Many types used, from software packages, to older pen based plotters, or tape recorders

• Compared to velocity pickups

smaller

more sensitive

wider frequency range

• electronic integrators can provide velocity and position

• The accelerometer is mounted with electrically isolated studs and washers, so that the sensor may be grounded at the amplifier to reduce electrical noise.

 

• Cables are fixed to the surface of the object close to the accelerometer, and are fixed to the surface as often as possible to prevent noise from the cable striking the surface.

• Background vibrations in factories are measured by attaching control electrodes to ‘non-vibrating’ surfaces. (The control vibrations should be less than 1/3 of the signal for the error to be less than 12%)

• Piezoelectric accelerometers typically have parameters such as,

-100to250°C operating range

1mV/g to 30V/g

operate well below one forth of the natural frequency

• Accelerometer designs vary, so the manufacturers specifications should be followed during application.

• There is often a trade-off between wide frequency range and device sensitivity (high sensitivity requires greater mass)

• Two type of accelerometers are compression and shear types.

• Mass of the accelerometers should be less than a tenth of the measurement mass.

• Accelerometers can be linear up to 50,000 to 100,000 m/s**2 or up to 1,000,000 m/s**2 for high shock designs.

• Typically used for 10-10,000 Hz, but can be used up to 10KHz

• Temperature variations can reduce the accuracy of the sensors.

• typical parameters are,

 

• These devices can be calibrated with shakers, for example a 1g shaker will hit a peak velocity of 9.81 m/s**2

11.7 Forces and Moments

• These values cannot currently be measured directly, and count on indirect measurements based on deflections or strain.

• These devices are attached to surfaces. As the surfaces experience stress/strain the devices are stretched and the resistance changes.

• The basic theory is based on a stretched wire.

 

 

• Changes in strain gauge values are typically small (large values would require strains that would cause the gages to plastically deform). As a result the resistance values are also small, so we can use resistor bridges (eg whetstone bridge) to amplify the effect. In this circuit the variable resistor R2 should be turned until the circuit is balanced for no strain.

 

• If the strain gauge is placed in the direction of the strain it will read the full strain. If the gauge is perpendicular, the reading will be zero.

uniaxial: the direction is critical

rosette: two gages at 90deg. to each other will measure strain components in two direction (and can measure shear).

 

• In some machines (etc.) a strain gauge is often mounted on a narrowed member to measure force. This is typically known as a load cell.

 

• Strain gages are normally made on thin films that are attached (mounted) to surfaces through a process that involves surface preparation and attachment with adhesives.

• These are ceramic and crystal materials that will generate a small amount of charge when deformed (the capacitance also changes).

• If the deformation is linear,

 

11.8 Flow Rate

• Fluid flow rate is important for a number of processes such as cooling and chemical.

• A Venturi valve uses a narrow section of a pipe to generate a pressure differential from the normal pipe diameter. The pressure differential will increase with the velocity of the flow. By measuring the pressure we can determine the flow rate.

 

11.9 Temperature

• Temperature is of significant concern in most disciplines.

fine temperature measurements 0-100 deg C: e.g. science/laboratory work

high temperature measurements <3000 deg F: e.g. metal refining/processing

low temperature measurement 0--60 deg C: e.g. freezers

very low temperatures <-60 deg C: e.g. superconductors in MRI units

very high temperatures > 2000 deg C: e.g. fusion research

• These devices use the effects of temperature change on conductivity.

• Uses resistive wires made of materials such as,

platinum

nickel

copper

nickel-iron

• Typical material properties are, [Bryan]

 

• Wires are wound about an insulator, for support, and covered for protection.

• These devices typically have positive temperature coefficients that cause resistance to increase linearly with temperature.

• Typical changes are doubling of initial resistance value over range of use.

• Advantages,

stable

accurate

more linear than thermocouples

• Disadvantages,

expensive

requires a power supply

small absolute resistance and resistance change

self heating

• Uses a junction of dissimilar metals to generate a voltage proportional to temperature.

• The bimetalic junction will generate an electrical potential between the metals as the temperature increases. The change in voltage is linearly proportional to the change in temperature.

• When using a thermocouple for precision measurement, a second thermocouple can be kept at a known temperature for reference.

• A series of thermocouples connected together in series produces a higher voltage and is called a thermopile.

• Basic thermocouple types are J, K, etc. These are designed for different temperature ranges.

• The basic calculations for thermocouples are given below,

 

• Advantages,

self powered

simple, rugged

inexpensive and commonly available

wide temperature ranges

• Disadvantages,

nonlinear

low voltage

reference devices needed

least stable and sensitive

• Non-linear devices that change conductivity with temperature. These are usually semiconductors.

• The resistance drops as the temperature rises. (Note: this is because the extra heat reduces electron mobility in the semiconductor.) This effect can change the resistance by more than 1000 times.

• The basic calculation for thermistors is given below,

 

• The circuit below can be used to amplify the output of these devices.

 

• These devices are normally small and come as beads or metalized surfaces.

• The advantages of these devices include,

fast response

small

higher resistance reduces the effect of lead impedance

inexpensive

• Disadvantages include,

smaller temperature range

signal is not linear

very wide resistance range

self heating

11.10 Sound

• Convert sound pressure to electrical signals

 

• A microphone has a finite area to measure the sound with. As the diameter ‘d’ approaches the wavelength of sound, the sound becomes distorted.

 

• When wind blows across a microphone, it generates a low frequency turbulence noise. This is often corrected by using dBA measurements, and foam balls.

• Corrections must also be made for changes in temperature and altitude

 

11.11 Light Intensity

• These devices change from high resistance (>Mohms) in the dark to low resistance (<Kohms) in bright light.

• The change in resistance is non-linear, and occurs relatively slowly also.

• A simple comparator circuit is shown below.

 

• This sensor is relatively easy to design with.

11.12 Pressure

• Converts pressure to a linear translation. Basically a twisted section of flexible tube will try to straighten out as pressure is applied.

• One end of the tube is fixed, and the other end moves freely as pressure is applied. An LVDT can be used to measure the position.

11.13 Problems

Problem 11.2 Suggest a couple of methods for collecting data on the factory floor

Answer 11.2 data bucket, smart machines, PLCs with analog inputs and network connections

Problem 11.3 If a thermocouple generates a voltage of 30mV at 800F and 40mV at 100F, what voltage will be generated at 1200F?

Problem 11.4 A certain potentiometer is to be used as the feedback device to indicate position of the output link of a rotational robot joint. The excitation voltage of the potentiometer equals 5.0 V, and the total wiper travel of the potentiometer is 300 degrees. The wiper arm is directly connected to the rotational joint so that a given rotation of the joint corresponds to an equal rotation of the wiper arm.

a) Determine the voltage constant of the potentiometer Kp,

b) The robot joint is actuated to a certain angle, causing the wiper position to be 38 degrees. Determine the resulting output voltage of the potentiometer.

c) In another actuation of the joint, the resulting output voltage of the potentiometer is 3.75V. Determine the corresponding angular position of the output link.

Problem 11.5 How is the efficiency of a motor defined?

Problem 11.6 What is the relationship between Real Power, Reactive Power and Apparent Power? (A sketch is acceptable)

Problem 11.7 What is the definition of the power factor of a motor?

Problem 11.8 Name two ways to improve motor efficiency.

Problem 11.9 Name two types of inputs that would be analog input values (versus a digital value).

Problem 11.10 Describe the following as either Transient or Steady State problems with respect to Power Quality:

Spike _______________________

Electrical Noise _________________________

Tingle Voltage _______________________

11.14 References

11.1 Bryan, L.A. and Bryan, E.A., Programmable Controllers; Theory and Implementation, Industrial Text Co., 1988.

11.2 Swainston, F., A Systems Approach to Programmable Controllers, Delmar Publishers Inc., 1992.