Kurfess T.R. Robotics and Automation Handbook

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-8 Robotics and Automation Handbook

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.

Ascomparedwithoptical time-of-ﬂight sensors, ultrasonic sensorsareless sensitivetolight 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 Halleffectreferstothevoltagegenerated when acurrentcarryingconductoror 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 in Figure 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

, multiplied by the

strain

R

= G

where R/R is the relative resistance change, G

is the gage factor along the sensor length, and ε

is the

strain along the sensor length. The gage factor for the strain gage is provided by the manufacturer.

Sensors and Actuators 12

strain gage

structure

metallic grid

FIGURE 12.10 Strain gage sensor applied to structure.

A Wheatstone bridge circuit is used to measure the resistance change of the sensor. Strain gages are

available with nominal resistance values between 30 and 3000 . Temperature variation is the primary

environmental concern for strain gages. Strain gage manufacturers have developed grid materials which

can compensate for the thermal expansion of the test structure. Temperature compensation may also be

achieved through the use of an additional strain gage applied in a direction normal to the measuring gage.

This additional gage is then used as one of the legs in the Wheatstone bridge circuit for comparison.

12.2.3 Force and Torque

Force and torque sensors typically employ an elastic structural element with deﬂection proportional to the

applied force or torque. Either strain gages or piezoelectric materials are used to measure the deﬂection

of the elastic structure. Designs based on strain gages are useful for measuring static and low frequency

forces and torques.

Piezoelectric materials develop an electrostatic charge when a mechanical load is applied. This charge is

collected by electrodes and is converted to an output voltage signal through the use of a charge ampliﬁer.

Piezoelectric sensors are useful for dynamic measurements, but due to charge leakage across the electrodes,

these sensors cannot measure static deﬂection.

Force sensors are frequently termed “load cells.” Different load cell designs are capable of measuring

compressive, tensile, or shear forces. With both force and torque sensors, the natural frequency of the

elastic structure used in the sensor must be taken into consideration for the measurement.

12.2.4 Acceleration

Acceleration sensors are based on the measurement of the deﬂection of a proof mass suspended in the

sensor housing. There are many types of accelerometers available for use in robotics. The various designs

include piezoelectric, strain gage, piezoresistive, capacitive, and inductive technologies.

Figure 12.11 shows two simpliﬁed examples of two accelerometer designs. The ﬁrst is that of a com-

pressive piezoelectric design. The proof mass is suspended within the sensor housing using piezoelectric

material. The output charge from the piezoelectric material, most often a PZT crystal, is proportional to

the inertial force acting on the proof mass.

proof mass

strain gage

PZT crystal

FIGURE 12.11 Piezoelectric and strain gage accelerometer designs.

-10 Robotics and Automation Handbook

The second designshown in Figure12.11 isthe basis for a straingage accelerometerdesign. 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. Digitalsensor technology includes the transductiontechnologies discussed in the previous

section on analog sensors:

SPDT DPST

FIGURE 12.12 Single pole double throw (SPDT) and double pole single throw (DPST) switches.

Sensors and Actuators 12

Excitation

Common

Sensor

Circuit

Output

FIGURE 12.13 Digital sensor with NPN open collector output.

Inductive

Capacitive

Optical

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

-12 Robotics and Automation Handbook

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.

Forwardisdeﬁned 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 camerasincludeembedded processingand machine vision softwareintheir design. Smartcameras

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 12.14 and 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

Sensors and Actuators 12

frame

coil

plunger

air gap

FIGURE 12.14 Linear solenoid concept.

initial air gap. This force is inversely proportional to the length of the air gap so that the solenoid exerts

greater force as the air gap is closed. In addition, the magnetic force is increased by increasing the applied

current and the number of turns of wire within the coil.

The magnetic ﬁeld around the coil will only cause motion towards the center of the coil. When no

current ﬂows in the coil, the plunger is free to move to its initial position due to a restoring force caused

by the load and, possibly, a return spring.

The diagram shown in the ﬁgure is an example of a pull type solenoid. All solenoids act to pull in the

plunger. However, push type solenoids are designed such that the plunger extends out the rear of the frame

and is then connected to the load. This causes a pushing action on the load when the solenoid is energized.

Solenoids are also speciﬁed based on whether they will be used continuously or only for intermittent

duty. Continuous duty assumes that the solenoid will have a 100% duty cycle. Because the coil has limited

heat dissipation, a 100% duty cycle means that the applied current will have to be reduced to prevent

damage to the coil. This, in turn, results in a decreased applied force. Intermittent duty means that the

duty cycle is less than 100%. Intermittent duty allows higher forces than continuous duty because more

current can be applied.

Rotary solenoids are also available and are similar in design to linear solenoids. However, the linear

motion is converted to rotary motion through the use of ball bearings moving down spiral grooves, called

“ball races,” to close the air gap.

12.5.1.2 Stepper Motors

Stepper motors convert input electrical pulses into discrete mechanical motion. They are available in

either rotational or linear designs, but rotational is far more common. For rotary stepper motors, each

step corresponds to a set number of degrees of rotation. The number of electrical pulses determines the

number of steps moved while the timing of the pulses controls the speed of the stepper motor. Because

no feedback sensors are required for their operation, they can be used open-loop and do not require any

tuning. They are well-suited for use in positioning applications and are easy to use.

Permanent magnet stepper motors are common and include a stator (stationary part) with windings

and a rotor (rotating part) made with permanent magnets. A simple example of a permanent magnet

stepper motor is shown in Figure 12.15. The stator has four windings or poles and the rotor has one

complete magnet or two poles. The stator itself is magnetically conductive. The windings on the stator are

wired in pairs such that there are two phases: phase A and phase B. Power is applied to only one phase at

a time and the power sequencing is known as “electronic commutation.” When this occurs, one of the

windings in the phase pair becomes a north pole and the other winding becomes a south pole. Magnetic

attraction therefore causes the permanent magnet rotor to be aligned with the energized winding pair.

The alignment of the rotor with a winding pair corresponds with a “full step.”

-14 Robotics and Automation Handbook

Phase A

Phase B

Phase A

Phase B

Phase A

Phase B

Phase A

Phase B

SN NS

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

Sensors and Actuators 12

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 variablereluctanceand 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

rotor

–Vc +Vc

FIGURE 12.16 Permanent magnet DC brush motor with two poles.

-16 Robotics and Automation Handbook

stator

rotor

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 lowmaintenancein comparison with DC brush motors. They are capableof 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 rotormust be known so that the polarityof 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.

Sensors and Actuators 12

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 incompressible 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 12.18 is 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. Thesetwo 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.