12.3.1 Switches as Digital Sensors A mechanical switch is the simplest and lowest cost type of digital sensor used in robotics.. 12.3.2 Noncontact Digital Sensors In order to reduce prob
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B
I
VH
FIGURE 12.9 Hall effect sensor.
12.2.1.7 Ultrasonic Sensors
Ultrasonic displacement sensors use the same approach as optical time-of-flight sensors with an analog output proportional to the distance to target Instead of a light pulse, however, an ultrasonic pulse is generated, reflected from a target, and then detected by a receiver The response is dependent upon the density of the target The frequency of the ultrasonic pulse is greater than 20 kHz and thus beyond the audible range for humans Piezoelectric and electrostatic transducers are typically used to generate the ultrasonic pulse
As compared with optical time-of-flight sensors, ultrasonic sensors are less sensitive to light disturbances However, ultrasonic sensor output can be affected by large temperature gradients because the speed of sound is affected by air temperature
12.2.1.8 Hall Effect Sensors
Hall effect sensors are noncontact sensors which output a signal proportional to input magnetic field strength The Hall effect refers to the voltage generated when a current carrying conductor or semiconductor
is exposed to magnetic flux in a direction perpendicular to the direction of the current A voltage, the Hall
voltage, is generated in a direction perpendicular to both the current, I, and the applied magnetic field, B,
as shown in Figure 12.9 In order to use a Hall effect sensor as a displacement sensor, the sensor is typically matched with a moving permanent magnet This magnet is applied to the target
Since the output of the Hall effect sensor is directly proportional to the applied magnetic field strength, the output will be nonlinearly related to the distance from the sensor to the permanent magnet This nonlinear relationship must be included in the post processing of the output signal The sensor is most often made of a semiconductor material and therefore is available in standard integrated circuit packages with integrated electronics
12.2.2 Strain
The most common strain sensor used in robotics is the strain gage The electrical resistance of this sensor changes as a function of the input strain The input strain may be either positive (tensile) or negative (compressive) An example of a bonded metallic strain gage is shown inFigure 12.10.The sensor consists
of an insulating carrier, a foil or wire grid bonded to the insulating carrier, and an additional insulating layer applied to the top of the grid The sensor is bonded a structure such that the grid legs are aligned with the desired direction of strain measurement The design of the grid makes it most sensitive to strain
along its length The change in resistance of the gage is a function of the gage factor, G L, multiplied by the strain
R
R = G L ε L
whereR/R is the relative resistance change, G L is the gage factor along the sensor length, andε L is the strain along the sensor length The gage factor for the strain gage is provided by the manufacturer
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The second design shown inFigure 12.11is the basis for a strain gage accelerometer design A cantilevered beam suspends the proof mass within the sensor housing Strain gages are applied to the cantilevered beam and measure the deflection of the beam MEMS accelerometers have found wide use in the automotive market and are suitable for many robotics applications The most common designs in this category are based on capacitive displacement sensing
The sensitivity, usable frequency range, and maximum input acceleration are limited by the mechanical dynamics of the accelerometer For example, if the suspension is designed to be very compliant, the sensitivity of the accelerometer will increase with respect to input acceleration However, the natural frequency of the suspension will be decreased and this will lead to a decreased frequency range for the measurement
12.3 Digital Sensors
A digital sensor will output either an “on” or an “off ” electrical state Apart from encoders, the majority
of digital sensors used in robotic applications are static digital sensors in that their value is solely based on
the digital state of the output as opposed to the frequency of an output pulse train Static digital sensors
do not require counter electronics for acquisition
12.3.1 Switches as Digital Sensors
A mechanical switch is the simplest and lowest cost type of digital sensor used in robotics Switches may be purchased with “normally open” (NO) or “normally closed” (NC) contacts The word “normally” refers
to the inactive state of the switch They also may be available with both options and can be wired in one configuration or the other When a switch is configured as normally open, it will have zero continuity (or infinite resistance) when it is not activated Activating the switch provides electrical continuity (zero resistance) between the contacts Normally closed switches have continuity when they are not activated and zero continuity when they are activated
Switches may also be designed to have multiple poles (P) and multiple throws (T) at each pole A pole
is a moving part of the switch and a throw is a potential contact position for the pole Figure 12.12 shows
an example of a single pole double throw switch (SPDT) and a double pole single throw (DPST) switch Because mechanical switches are contact sensors, they can exhibit failures due to cyclical wear In addition, the acquisition electronics used with switches must be able to filter out electrical noise at switch transitions due to contact bounce
12.3.2 Noncontact Digital Sensors
In order to reduce problems of contact wear and switch bounce, noncontact digital sensors are frequently used in robotics Digital sensor technology includes the transduction technologies discussed in the previous section on analog sensors:
FIGURE 12.12 Single pole double throw (SPDT) and double pole single throw (DPST) switches.
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Excitation
Common
Sensor Circuit
E B C
Output
FIGURE 12.13 Digital sensor with NPN open collector output.
r Inductive
r Capacitive
r Optical
r Hall effect
An analog sensor design can be converted to a digital sensor design through the use of additional electronic circuitry A comparator or Schmitt trigger circuit is frequently used to compare the output of the sensor amplifier with a predetermined reference The Schmitt trigger will activate when the amplifier output is greater than the reference If the amplifier output is less than the reference, then the Schmitt trigger will turn off
12.3.3 Solid State Output
Digital sensors frequently use transistors as the output driver technology These sensors are classified as having NPN or PNP outputs depending upon the type of transistor used for the output stage Figure 12.13 shows a sensor with NPN open collector output The base of the transistor is connected to the sensor circuit, the emitter to common, and the collector to the output Since the output is not yet connected to any other signal, the NPN sensor has an “open collector” output In the active state, the transistor will drive the output to common In the inactive state, the transistor will leave the output floating Typically, the output is connected to a data acquisition circuit with a pull-up resistor to a known voltage In this way, the inactive state of the sensor will register at the known voltage and the active state will register as common or ground
In the case of a PNP sensor, the base of the transistor is still connected to the sensor circuit, the collector
is tied to the DC excitation source for the sensor, and the emitter is used as the output In this case, the PNP sensor has an “open emitter” output When active, the sensor output switches from floating to a positive voltage value When inactive, the sensor output is left floating In the inactive case, a pull-down resistor in the acquisition circuitry will bring the output to common or ground
Any type of digital sensor can be classified as having a sinking or a sourcing output A sinking output sensor means the sensor can sink current from a load by providing a path to a supply common or ground This type of sensor must be wired to a sourcing input in a data acquisition system A sourcing output sensor means the sensor can source current to a load by providing a path to a supply source This type of sensor must be wired to a sinking input in a data acquisition system NPN sensors are examples of sinking output devices while PNP sensors are examples of sourcing output devices Switches and mechanical relays may be wired as either sinking or sourcing sensors
12.3.4 Common Uses for Digital Sensors
Digital sensors can be used in a wide variety of applications within robotics These include proximity sensors, limit sensors, and safety sensors such as light curtains
12.3.4.1 Proximity Sensors
Proximity sensors are similar to analog displacement sensors, but they offer a static digital output as opposed to an analog output Proximity sensors are used to determine the presence or absence of an
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object They may be used as limit sensors, counting devices, or discrete positioning sensors They are typically noncontact digital sensors and are based on inductive, capacitive, photoelectric, or Hall effect technology These technologies are discussed in the previous section on analog sensors Their design is frequently similar to that of analog position sensors but with threshold detecting electronics included so that their output is digital
12.3.4.2 Limit Switches and Sensors
Limit switches or limit sensors are digital inputs to a robot controller that signal the end of travel for motors, actuators, or other mechanisms The incorporation of limit sensors helps prevent mechanical failure caused by part of a mechanism hitting a hard stop in the system The limit sensor itself can be a physical switch with mechanical contacts or a digital proximity sensor as described above Limit sensors may be mounted to individual joints in the robot or to axes of motion in a robotic workcell When the limit sensor is encountered for a particular joint or axis, the robot controller will bring the motion to a safe stop Both a forward and a reverse limit sensor can be connected to each joint or axis in a robotic system Forward is defined as the direction of increasing position as measured by the encoder or analog feedback signal Limit sensors can be used with both rotational and linear axes A home switch or sensor can also
be built into each axis and used to indicate a center position reference for the axis
12.3.4.3 Light Curtains
Light curtains can automatically detect when an operator moves within the danger area for a robot or robotic operation This danger area will usually include the entire workspace of the robot Light curtains are typically based on photoelectric sensors which emit multiple beams of infrared light When any of the beams of light is broken, the control circuit for the light curtain is activated, and the robotic system is immediately shut down in a safe manner
12.4 Vision
Many robots use industrial cameras for part detection, inspection and, sometimes, guidance The camera output may be analog or digital and may be acquired by a computer through several means Often, a frame-grabber or image acquisition plug-in board is used and installed in a computer More recently, computer bus technologies such as IEEE 1394, Camera Link®, Gigabit Ethernet, and USB have been used to transfer data between the camera and the computer Machine vision software is installed on the computer and is used to examine the image so that image features can be determined and measurements can be made Smart cameras include embedded processing and machine vision software in their design Smart cameras may be one integrated unit or they may use a tethered design with an electronic link between the processor and the camera
12.5 Actuators
12.5.1 Electromagnetic Actuators
12.5.1.1 Solenoid
Solenoids are the most basic type of electromagnetic actuator The linear solenoid concept is shown in Figure 12.14and consists of a coil of wire, a fixed iron or steel frame, and a movable iron or steel plunger The part of the plunger that extends from the frame is attached to the load
When current flows in the coil, a magnetic field is generated around the coil The frame serves to concentrate the magnetic field such that the maximum magnetic force is exerted on the plunger The magnetic force causes the plunger to be attracted to the rear of the frame or the back stop and close the
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N S S
N Phase A
Phase A
N S Phase A
Phase A
Phase A
Phase A
S N N
S Phase A
Phase A
S N
0 degrees
90 degrees
180 degrees
270 degrees
FIGURE 12.15 Two-phase stepper motor power sequence.
As shown in the diagram at the top of the figure, phase A is powered (indicated by shading) causing the rotor to be aligned at the 0◦position Then, going clockwise in the figure, phase B is powered while phase
A is unpowered, causing the rotor to rotate to 90◦ Then, phase A is again powered but with current of opposite polarity compared with the 0◦position This causes the rotor to rotate to 180◦ The next step of
270◦is achieved when phase B is powered again with opposite polarity compared with the 90◦position Thus, the two poles in the rotor result in four discrete steps of 90◦ within one complete rotation The direction of rotation can be changed by reversing the pulse sequence applied to the phases
The torque required to move the rotor when there is no power applied to either phase is known as the “detent torque.” The detent torque causes the clicks that can be felt if an unpowered stepper motor
is moved manually The torque required to move the rotor when one of the phases is powered with DC current is known as the “holding torque.”
Stepper motors may have higher numbers of poles in the rotor and the stator These are created using
a magnet with multiple teeth on the rotor and additional windings on the stator This will increase the number of full steps per revolution Many stepper motor manufacturers make stepper motors with 200 and 400 steps per revolution This corresponds to step angles of 1.8◦and 0.9◦, respectively Microstepping
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is a common feature on stepper motor drives that adjusts the amount of current applied to the phases This allows each full step to be subdivided into smaller steps and results in smoother motion
Other types of stepper motors include variable reluctance and hybrid stepper motors Variable reluctance stepper motors have a stator which is similar to the permanent magnet stepper motor, but the rotor is composed of soft iron rather than a permanent magnet and also has multiple teeth Because there are no permanent magnets on the rotor, it is free to move with no detent torque if there is no power applied to either of the phases Hybrid stepper motors combine the principles of permanent magnet and variable reluctance motors The rotor has teeth like a variable reluctance motor but contains an axially magnetized cylindrical magnet which is concentric with the shaft
12.5.1.3 DC Brush Motor
DC brush motors convert applied current into output mechanical motion They are a type of servomotor, and the output torque is directly proportional to the applied current As compared with stepper motors,
DC brush motors require a feedback signal for stable operation They must be used closed-loop, and this can make the system more complex than one using stepper motors They also use conductive brushes for mechanical rather than electric commutation and thus can have higher maintenance costs due to wear However, DC brush motors have smooth motion and higher peak torque and can be used at higher speeds than stepper motors
DC brush motors incorporate a stator with either permanent magnets or windings and a rotor with windings Windings in the stator are known as field windings or field coils DC motors using permanent magnets in the stator are known as permanent magnet DC motors DC motors with windings in the stator are known as series-wound or shunt-wound motors depending upon the connectivity of the windings in relation to rotor windings Series-wound motors have the field windings in series with the rotor while shunt-wound motors have the field windings in parallel with the rotor
All DC brush motors include conductive brushes which make sliding contact with a commutator as the rotor turns The commutator is attached to the rotor shaft and the rotor windings are connected to the individual sections of the commutator The commutator has as many sections as there are poles in the rotor
A simple example of a permanent magnet DC motor is shown in Figure 12.16 This motor has two poles
in the rotor When voltage is applied to the brushes, current flows from the brushes to the commutator and, in turn, to the windings This creates a magnetic field in the rotor The interaction between this field
stator
commutator brush
S
N rotor
FIGURE 12.16 Permanent magnet DC brush motor with two poles.
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S N stator
rotor
Phase A
Phase A
Phase C
Phase B Phase C
Phase B
FIGURE 12.17 Three-phase DC brushless motor.
and that of the stator causes the rotor to move until it is aligned with the magnetic field of the stator Just as the rotor is aligned, the brushes switch to the next section or contact of the commutator and the magnetic field of the rotor is reversed This causes the rotor to continue moving until it is aligned with the stator again
DC brush motors typically include multiple poles in the rotor to smooth out the motion and increase torque
12.5.1.4 DC Brushless Motor
DC brushless motors are another type of servomotor in that feedback is required for stable operation A DC brushless motor is like a DC brush motor turned inside out because the rotor contains a permanent magnet and the stator contains windings The windings are electronically commutated so that the mechanical commutator and brushes are no longer required as compared with a DC brush motor Figure 12.17 shows
an example of a brushless motor with three phases (six poles connected in pairs)
DC brushless motors are commonly used in robotics applications because of their high speed capability, improved efficiency, and low maintenance in comparison with DC brush motors They are capable of higher speeds because of the elimination of the mechanical commutator They are more efficient because heat from the windings in the stator can be dissipated more quickly through the motor case Finally, they require less maintenance because they do not have brushes that require periodic replacement However, the total system cost for brushless motors is higher than that for DC brush motors due to the complexity
of electronic commutation
The position of the rotor must be known so that the polarity of current in the windings of the stator can be switched at the correct time Two types of commutation are used with brushless motors With trapezoidal commutation, the rotor position must only be known to within 60◦so that only three digital Hall effect sensors are typically used Sinusoidal commutation is employed instead of trapezoidal commutation when torque ripple must be reduced for the motor In this case, the rotor position must be determined more accurately so that a resolver is used or an encoder is used in addition to Hall effect sensors The Hall effect sensors are needed with the encoder to provide rotor shaft position at startup The resolver provides absolute position information of the rotor shaft, so that the Hall effect sensors are not required for startup
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12.5.2 Fluid Power Actuators
12.5.2.1 Hydraulic Actuators
Hydraulic actuators are frequently used as joint or leg actuators in robotics applications requiring high payload lifting capability Hydraulic actuators output mechanical motion through the control of incom-pressible fluid flow or pressure Because incomincom-pressible fluid is used, these actuators are well suited for force, position, and velocity control In addition, these actuators can be used to suspend a payload without significant power consumption Another useful option when using hydraulics is that mechanical damping can be incorporated into the system design
The primary components in a hydraulic actuation system include:
1 A pump — converts input electrical power to hydraulic pressure
2 Valves — to control fluid direction, flow, and pressure
3 An actuator — converts fluid power into output mechanical energy
4 Hoses or piping — used to transport fluids in the system
5 Incompressible fluid — transfers power within the system
6 Filters, accumulator, and reservoirs
7 Sensors and controls
Positive displacement pumps are used in hydraulic actuator systems and include gear, rotary vane, and piston pumps The valves that are used include directional valves (also called distributors), on-off or check valves, pressure regulator valves, flow regulator valves, and proportional or servovalves
Both linear and rotary hydraulic actuators have been developed to convert fluid power into output motion A linear actuator is based on a rod connected to a piston which slides inside of a cylinder The rod
is connected to the mechanical load in motion The cylinder may be single or double action A single action cylinder can apply force in only one direction and makes use of a spring or external load to return the piston to its nominal position A double action cylinder can be controlled to apply force in two directions
In this case, the hydraulic fluid is applied to both faces of the piston
Rotary hydraulic actuators are similar to hydraulic pumps Manufacturers offer gear, vane, and piston designs Another type of rotary actuator makes use of a rack and pinion design where a piston is used to drive the rack and the pinion is used for the output motion
Working pressures for hydraulic actuators vary between 150 and 300 bar When using these actuators, typical concerns include hydraulic fluid leaking and system maintenance However, these can be mitigated through intelligent engineering design
Hydraulic actuators have been used in many factory automation problems and have also been used
in mobile robotics.Figure 12.18is a picture of the TITAN 3 servo-hydraulic manipulator system from Schilling Robotics This is a remote manipulator that was originally developed for mobile underwater applications but is also being used in the nuclear industry
12.5.2.2 Pneumatic Actuators
Pneumatic actuators are similar to hydraulic actuators in that they are also fluid powered The difference
is that a compressible fluid, pressurized air, is used to generate output mechanical motion Pneumatic actuators have less load carrying capability than hydraulic actuators because they have lower working pressure However, pneumatic actuators have advantages in lower system weight and relative size They are also less complex in part because exhausted pressurized air in the actuator can be released to the environment through an outlet valve rather than sent through a return line
Because compressed air is used, the governing dynamic equations of pneumatic actuators are nonlinear
In addition, compressed air adds passive compliance to the actuator These two factors make these actuators more difficult to use for force, position, and velocity control However, pneumatic actuators are frequently used in industry for discrete devices such as grippers on robotic end effectors
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Motion Control Course Manual, National Instruments, Part Number 323296A-01, March 2002.
National Instruments, 11500 N Mopac Expwy, Austin, TX 78759, Measuring Strain With Strain Gages,
website:www.zone.ni.com
National Instruments, 11500 N Mopac Expwy, Austin, TX 78759, Choosing The Right Industrial Digital I/O Module for Your Digital Output Sensor, website: www.zone.ni.com.
Prosser, S.J., The evolution of proximity, displacement and position sensing, Sensors, 15(4), April 1998 Sorli, M and Pastorelli, S., Hydraulic and pneumatic actuation systems, in: The Mechatronics Handbook,
Bishop, R.H., ed., CRC Press, Boca Raton, FL, 2002
Welsby, S.D., Capacitive and inductive noncontact measurement, Sensors, 20(3), March 2003.
Trang 10Precision Positioning
of Rotary and Linear
Systems
Stephen Ludwick
Aerotech, Inc
Pittsburgh, Pennsylvania
13.1 Introduction 13.2 Precision Machine Design Fundamentals
Definitions of Precision • Determinism • Alignment Errors (AbbePrinciple) • Force and Metrology Loops • Constraint
• Thermal Management • Cable Management
• Environmental Considerations • Serviceability and Maintenance
13.3 Mechatronic Systems
Definition of Mechatronic Systems • Discrete-Time System Fundamentals • Precision Mechanics • Controller Implementation • Feedback Sensors • Control Algorithms
13.4 Conclusions
13.1 Introduction
Precision positioning systems have historically been a key part of successful industrial societies The need
to make something requires the ability to move something with a very high level of accuracy This has not
changed in the Information Age, but instead has become even more important as global competition forces manufacturers to hold ever tighter specifications, with increased throughput and reduced costs Automation is the equalizer that allows manufacturers in countries with high labor rates to compete
globally with developing countries The definition of a precision machine continues to evolve as technology
advances The components used to build machines become more precise, and so the machines themselves improve As loosely defined here, precision machines are those that repeatably and reliably position to within a tolerance zone smaller than is possible in commonly available machines Designing machines to best use the available components and manufacturing techniques requires specialized skills beyond those
of general machine designers It is interesting to note that many of the fundamental rules for designing precision machines have not changed for hundreds of years Evans [9] tracks the evolution of precision machines and provides a historical context to the present state-of-the-art
Modern precision-positioning systems are largely mechatronic in nature Digital computers interface
with electronic sensors and actuators to affect the motion of the system mechanics Auslander and Kempf [2] review the basic elements required to interface between system mechanics and control electronics, while A◦str¨om and Wittenmark [24] and Franklin et al [11] present the required discrete-time control theory and implementation Kiong et al [17] describe how these elements are specifically combined