Kurfess T.R. Robotics and Automation Handbook

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Design of Robotic End Effectors 11

-17

Vacuum Lifter

Normal Gripper

FIGURE 11.19 Multi-tool end effector.

11.8 Sensors and Control Considerations

A variety of sensors is available for inclusion in end effectors. The most common sensors employed in end

effectors measure proximity, collision, and force.

11.8.1 Proximity Sensors

Through beam and reﬂective optical proximity sensors are widely used to verify the grasping of an object.

Typically the sensors are used as presence indicators. The weight of the sensor on the end effector should be

minimized because of payload considerations. Most optical proximity sensors are available with an optical

ﬁber end connection so that sensor electronics will not necessarily be located on the end effector. Place

as much sensor hardware as possible remote to the end effector. Grippers may be designed with built-in

limit switches for grasping ﬁxed dimensional objects. A common implementation of this is the use of Hall

effect or reed switches with air cylinder pistons.

Inductive and capacitive proximity sensors may be used with compatible object materials such as carbon

steels. Materials such as stainless steel, aluminum, brass, and copper may also be sensed but at a reduced

detection distance.

If a non-contact distance measurement is required rather than a presence veriﬁcation, laser diode-based

analog sensors or a machine visionsensor can be used. For extremely close-range and precise measurements

(such as part dimensional measurements) eddy current sensors may be used on metallic components.

11.8.2 Collision Sensors

Although compliance centers (discussed earlier) help align parts, ease assembly, and reduce potential

damage, damage may still occur because of design limits (angle and position tolerances, as speciﬁed by

manufacturers or design). Thus, a collision detection sensor can be a good end effector component to

mitigate damage to the robot, inserted part, or workpiece where the part is being attached. Collision

detection sensors detect the ﬁrst onset of a collision in all directions. After detecting the start of a collision,

the robot can be stopped or backed away before damage has occurred. (Note that while proximity sensors

can also be used as collision avoidance devices, most are unidirectional with a limited ﬁeld of view.)

Collision detection devices are usually attached at the end of arm before the end effector.

There are several types of collision detection sensors available for wrist mounting. These include force

sensors as an active control device. Force sensors are primarily strain gauge devices and require additional

programming for monitoring and control.

Speciﬁc collision detection sensors are readily available in the market place. These are passive devices.

Some utilize compressed air for variability of fault loading in three-dimensional space. Other collision

sensor designs are based on mechanical springs at preset loads. Both types indicate a collision by the

opening of a normally closed switch. This design can be hardwired into the E-stop circuit for safety or be

wired to a digital input for programmatic monitoring.

-18 Robotics and Automation Handbook

11.8.3 Tactile Feedback/Force Sensing

Force sensing during grasping is important for successful grasping of varying parts, compliant part, and

for reduction of part deformation. In the past strain gauge sensors were the primary device used. These,

however, are of limited use in the multiple-point force sensing. Piezoelectric force sensors are also available

but are better for measuring dynamic loads because of charge leakage.

Piezoresistor force sensors can be used for tactile force sensing using the principle that the resistance of

silicon-implanted piezoresistors will increase when the resistors ﬂex under any applied force. The sensor

concentrates the applied force through a stainless steel plunger directly onto the silicon sensing element.

The amount of resistance changes in proportion to the amount of force being applied. This change in

circuit resistance results in a corresponding mV output level.

A relatively new development is the commercial availability of force sensing resistors (FSR). The FSR

is a polymer thick ﬁlm (0.20 to 1.25 mm PTF) device which produces a decrease in resistance with any

increase in force applied to its surface. Although FSRs have properties similar to load cells, they are not

well suited for precision measurement equipment (variation of force from grasping can be on the order of

10%). Its force sensitivity is optimized for use in human touch control switches of electronic devices with

pressure sensitivity in the range of 0.5 to 150 psi (0.03 to 10 kg/cm

) [24].

In general, the FSR’s resistance vs. force curve approximately follows a power-law relationship (resistance

decreasing with force). At the low force end of the force-resistance characteristic, a step response is evident.

At this threshold force, the resistance drops substantially. At the high force end of the sensor range, the

resistancedeviatesfrom the power-law behavior and ultimately levels to an output voltage saturation point.

Between these extremes is a limited but usable region of a fairly linear response. The relatively low cost,

small thickness, and reasonable accuracy make these sensors attractive for multi-sensor tactile control.

11.8.4 Acceleration Control for Payload Limits

Another important control aspect is the force caused during acceleration. Thus, the accelerations of the

joints must be known and prescribed (or carefully estimated). In most instances the acceleration limits

on each joint are known. Typically accelerations are deﬁned by S-curves for velocity, with maximum

accelerations limits.

11.8.5 Tactile Force Control

Traditional position and velocity control for trajectories are obviously important for painting, welding,

and deburing robots. However, to create quality products, it is also necessary to have force control for

material removal operations such as grinding and deburing. In these situation the compliance of the robot,

end effector, and tool make up total compliance (ﬂexibility). When a tool is moved against an object, the

inverse of this compliance (stiffness) creates a force. To assure that dimensional accuracy is maintained,

an additional position measurement of the tool tip must be made or position must be taken into account

by the use of a compliance model. The stiffness of each joint is easily determined. However, each unique

position of the joints creates a unique stiffness.

Generally ﬁxed-gain controllers (such as PID-type controllers) are preferred for force applications

because of their simplicity in implementation. These controllers are easily tuned using standard

approaches.

Tuning the force control loop is best done using position control and the effective stiffness from the

end effector-workpiece interaction. Many industrial controllers provide utilities for automatic tuning of

position loops. Many are based on the Ziegler-Nichols PID tuning [25] or other heuristic techniques.

Although this technique is based on continuous systems for very fast sampling rates (greater than 20 times

the desire bandwidth), the results also apply well to discrete systems.

Design of Robotic End Effectors 11

-19

11.9 Conclusion

From the preceding text it is clear that the design, selection, control, and successful implementation of a

robotic system relies heavily on the end effector subsystem. End effector designs and technology continue

to evolve with new actuators, sensors, and devices.

References

[1] Ceglarek, D., Li, H.F., and Tang, Y., Modeling and optimization of end effector layout for handling

compliant sheet metal parts, J. Manuf. Sci. Eng., 123, 473, 2001.

[2] Ciblak, N., Analysis of Cartesian stiffness and compliance with applications, Ph.D. Thesis Defense,

Georgia Institute of Technology, 1998.

[3] Robotic Accessories, Alignment Device 1718 Speciﬁcations, 2003.

[4] Thermo CRS, A465 Six Axis Robot Speciﬁcation, 2003.

[5] Kane, T.R., Dynamics: Theory and Applications, McGraw-Hill, New York, 1985.

[6] Yang, K. and Gu, C.L., A novel robot hand with embedded shape memory alloy actuators, J. Mech.

Eng. Sci., 216, 737, 2002.

[7] Francois, C., Ikeuchi, K., and Herbert, M., A three ﬁnger gripper for manipulation in unstructured

Environments, IEEE Int. Conf. Robot. Autom., 3, 2261–2265, 1991.

[8] Foster, A., Akin, D., and Carignan, C., Development of a four-ﬁngered dexterous robot end-effector

for space operations, IEEE Int. Conf. Robot. Autom., 3, 2302–2308, 2002.

[9] Kaneko, M. et al., Grasp and manipulation for multiple objects, Adv. Robot., 13, 353, 1999.

[10] Kumazaki, K. et al., A study of the stable grasping by a redundant multi-ﬁngered robot hand, SICE,

631, 2002.

[11] Mason, M. and Salisbury, J., Robotic Hands and the Mechanics of Manipulation, MIT Press, Boston,

1985.

[12] Seguna, C.M., The design, construction and testing of a dexterous robotic end effector, IEEE SPC,

2001.

[13] Bicchi, A., Hands for dexterous manipulation and robust grasping: a difﬁcult road towardsimplicity,

IEEE Trans. Robot. Autom.,16, 652, 2000.

[14] Caldwell, D.G. and Tsagarakis, N., Soft grasping using a dexterous hand, Indust. Robot, 3, 194,

2000.

[15] Bicchi, A. and Kumar, V., Robotic grasping and contact: a review, IEEE Int. Conf. Robot. Autom.,

200, 348, 2000.

[16] Taylor, C.L. and Schwarz, R.J., The Anatomy and Mechanics of the Human Hand: Artiﬁcial Limbs,

vol. 2, 22–35, 1955.

[17] Zajac, T., Robotic gripper sizing: the science, technology and lore, ZAYTRAN, 2003.

[18] Baumeister, T. (ed.), Marks’ Standard Handbook for Mechanical Engineers, 8th ed., McGraw-Hill,

New York, 6–24, 1978.

[19] Causey, G.C. and Quinn, R.R., Gripper design guidelines for modular manufacturing, IEEE Int.

Conf. Robot. Autom., 1453, 1998.

[20] Walsh, S., Gripper design: guidelines for effective results, Manuf. Eng., 93, 53, 1984.

[21] Adept Technologies, AdeptSix 300 Speciﬁcations Data Sheet, 2003.

[22] Zhang, T. and Goldberg, K., Design of robot gripper jaws based on trapezoidal modules, IEEE Int.

Conf. Robot. Autom., 29, 354, 2001.

[23] Derby, S. and McFadden, J., A high precision robotic docking end effector: the dockbot(TM), Ind.

Robot, 29, 354, 2002.

[24] Interlink Electronics, FSR Data sheet, 2003.

[25] Franklin, G.F., Powell, J.D., and Workman, M.L., Digital Control of Dynamic Systems, Addison-

Wesley, Reading, MA, 1990.

Sensors and Actuators

Jeanne Sullivan Falcon

National Instruments

12.1 Encoders

Rotary and Linear Incremental Encoders

•

Tachometer

•

Quadrature Encoders

•

Absolute Encoders

12.2 Analog Sensors

Analog Displacement Sensors

•

Strain

•

Force and Torque

•

Acceleration

12.3 Digital Sensors

Switches as Digital Sensors

•

Noncontact Digital Sensors

•

Solid State Output

•

Common Uses for Digital Sensors

12.4 Vision

12.5 Actuators

Electromagnetic Actuators

•

Fluid Power Actuators

12.1 Encoders

12.1.1 Rotary and Linear Incremental Encoders

Incremental encoders are the most common feedback devices for robotic systems. They typically output

digital pulses at TTL levels. Rotary encoders are used to measure the angular position and direction of a

motor or mechanical drive shaft. Linear encoders measure linear position and direction. They are often

used in linear stages or in linear motors. In addition to position and direction of motion, velocity can also

be derived from either rotary or linear encoder signals.

In a rotary incremental encoder, a glass or metal disk is attached to a motor or mechanical drive shaft.

The disk has a pattern of opaque and transparent sectors known as a code track. A light source is placed

on one side of the disk and a photodetector is placed on the other side. As the disk rotates with the motor

shaft, the code track interrupts the light emitted onto the photodetector, generating a digital signal output

(Figure 12.1).

The number of opaque/transparent sector pairs, also known as line pairs, on the code track corresponds

to the number of cycles the encoder will output per revolution. The number of cycles per revolution (CPR)

deﬁnes the base resolution of the encoder.

12.1.2 Tachometer

An incremental encoder with a single photodetector is known as a tachometer encoder. The frequency

of the output pulses is used to indicate the rotational speed of the shaft. However, the output of the

single-channel encoder cannot give any indication of direction.

-2 Robotics and Automation Handbook

Light

Source

Disk

Photo Sensor

Pickup

FIGURE 12.1 Typical encoder design.

12.1.3 Quadrature Encoders

Quadrature encoders are another type of incremental encoder. A two-channel quadrature encoder uses

two photodetectors to sense both position and direction. The photodetectors are offset from each other by

◦

relative to one line pair on the code track. Since the two output signals, A and B, are 90

◦

out of phase,

one signal will lead the other as the disk rotates. If A leads B, the disk is rotating in a clockwise direction,

as shown in Figure 12.2. If B leads A, the disk is rotating in a counterclockwise direction, as shown in

Figure 12.3.

Figure 12.2 and Figure 12.3 illustrate four separate pulse edges occurring during each cycle. A separate

encoder count is generated with each rising and falling edge, which effectively quadruples the encoder

resolution such that a 500 CPR (cycles per revolution) encoder provides 2000 counts per revolution with

quadrature decoding.

The electrical complements of channels A and B may be included as differential signals to improve noise

immunity. This is especially important in applications featuring long cable lengths between the encoder

and the motion controller or electrical drive.

Channel A

Channel B

90°

FIGURE 12.2 Clockwise motion.

Channel A

Channel B

90°

FIGURE 12.3 Counterclockwise motion.

Sensors and Actuators 12

Some quadrature encoders include a third output channel, known as an index or zero pulse. This signal

supplies a single pulse per revolution and is used for referencing the position of the system. During power

up of the system, the motor can be rotated until the index pulse occurs. This speciﬁes the current position

of the motor in relation to the revolution.

Moving to the index position is not enough to determine the position of the motor or mechanical system

during system startup. The absolute position is usually determined through a homing routine where the

system is moved to limit switches or sensors. Once the robot is commanded to move to the limits, then

the encoder readings can be reset to zero or some other position value to deﬁne the absolute position.

Subsequent motion is measured by the encoders to be relative to this absolute position.

Encoders can be attached to components other than motors throughout a mechanical system for

measurement and control. For example, a rotary motor may be attached to a belt drive that is in turn

attached to a payload under test. A rotary encoder is attached to the motor to provide feedback for control,

but a second rotary encoder can also be attached to the payload to return additional feedback for improved

positioning. This technique is known as dual-loop feedback control and can reduce the effects of backlash

in the mechanical components of the motion system.

Some encoders output analog sine and cosine signals instead of digital pulses. These types of encodersare

typically used in very high precision applications that require positioning accuracy at the submicron level.

In this case, an interpolation module is necessary between the encoder output and the robot controller

or intelligent drive. This functionality may be included in the robot controller. Interpolation modules

increase the resolution of the encoder by an integer value and provide a digital quadrature output for

use by the robot controller. Some encoder manufacturers offer interpolation modules in a wide range of

multiplier values. Manufacturers of encoders include BEI, Renco, US Digital, Renishaw, Heidenhain, and

Micro-E.

12.1.4 Absolute Encoders

A position can be read from an absolute encoder if the application requires knowledge of the position of

the motors immediately upon system start-up. An absolute encoder is similar to an incremental encoder,

except that the disk used has multiple concentric code tracks and a separate photodetector is used with

each code track. The number of code tracks is equivalent to the binary resolution of the encoder, as shown

in Figure 12.4.

An 8-bit absolute encoder has eight code tracks. The 8-bit output is read to form an 8-bit word indicating

absolute position. While absolute encoders are available in a wide variety of resolutions, 8-, 10-, and 12-bit

binary are the most common. Due to their complexity, absolute encoders are typically more expensive

than quadrature encoders. Absolute encoders may output position in either parallel or serial format.

Because of the variety of output formats available for absolute encoders, it is important to ensure that

the robot controller or intelligent drive is compatible with the particular model of the absolute encoder.

111111 000000

010101

001110

FIGURE 12.4 Absolute encoder example.

-4 Robotics and Automation Handbook

12.2 Analog Sensors

Analog sensors commonly used in robotic applications include displacement, force, torque, acceleration,

and strain sensors. As in the case of encoders, these sensors may be used in either an open-loop or closed-

loop fashion within the robot system. For example, a force sensor may be used to measure the weight of

objects being assembled for quality control. Or, a force sensor may be added to the gripper in a robot end

effector as feedback to the gripper actuator. The gripper control system would allow objects to be held

with a constant force.

12.2.1 Analog Displacement Sensors

These sensors can include both angular and translation position measurement relative to a reference

position. They provide a continuously varying output signal which is proportional to the position of the

sensed object. The most common sensors and technologies used for displacement measurement include:

Potentiometer

LVDT — contact sensor

Resolvers

Inductive

Capacitive

Optical

Ultrasonic

Hall effect

12.2.1.1 Potentiometers

Potentiometers offer a low cost method of contact displacement measurement. Depending upon their

design, they may be used to measure either rotary or linear motion. In either case, a movable slide or

wiper is in contact with a resistive material or wire winding. The slide is attached to the target object in

motion. A DC or an AC voltage is applied to the resistive material. When the slide moves relative to the

material, the output voltage varies linearly with the total resistance included within the span of the slide. An

advantage of potentiometers is that they can be used in applications with a large travel requirement. Some

industrial potentiometers are offered in sealed conﬁgurationsthat can offer protection from environmental

contamination.

12.2.1.2 Linear Variable Differential Transformers (LVDT)

Linear variable differential transformers are used for contact displacement measurement. They include a

moving core which extends into the center of a hollow tube as shown in Figure 12.5. One primary and

two secondary coils are wrapped around the hollow tube with the secondary coils symmetrically placed

around the primary. The core is attached to the target object in motion.

An AC signal is applied to the primary coil and, because of coupling of magnetic ﬂux between the coils,

this induces AC voltages in the secondary coils. The magnitude and phase of the output signal of the LVDT

primary coil secondary coilsecondary coil

core

FIGURE 12.5 Linear variable differential transformer.

Sensors and Actuators 12

is determined by the position of the core within the tube. The magnitude of the output of the signal is a

function of the distance of the core from the primary coil, and the phase of the output signal is a function

of the direction of the core from the primary coil — towards one secondary coil or the other as shown in

the ﬁgure.

LVDTsensorscanbeused in applications with large travelrequirements.However,mechanicalalignment

along the direction of travel is important for this type of sensor.

12.2.1.3 Resolvers

A resolver is essentially a rotary transformer that can provide absolute position information over one revo-

lution. The resolver consists of a primary winding located on the rotor shaft and a two secondary windings

located on the stator. The secondary windings are oriented 90

◦

relative to each other. Energy is applied to

the primary winding on the rotor. As the rotor moves, the output energy of the secondary windings varies

sinusoidally. Resolvers are an alternative to encoders for joint feedback in robotic applications.

12.2.1.4 Inductive (Eddy Current)

Inductive sensors are noncontact sensors and can sense the displacement of metallic (both ferrous and

nonferrous) targets. The most common type of inductive sensor used in robotics is the eddy current sensor.

The sensor typically consists of two coils of wire: an active coil and a balance coil, with both driven with a

high frequency alternating current. Whena metallic target is placed near the activesensor coil, the magnetic

ﬁeld from the active coil induces eddy currents in the target material. The eddy currents are closed loops

of current and thus create their own magnetic ﬁeld. This ﬁeld causes the impedance of the active coil to

change. The active coil and balance coil are both included in a bridge circuit. The impedance change of

the active coil can be detected by measuring the imbalance in the bridge circuit. Thus, the output of the

sensor is dependent upon the displacement of the target relative to the face of the sensor coil.

The effective depth of the eddy currents in the target material, δ, is given by

δ =



π f µσ

where f is the excitation frequency of the coil, µ is the magnetic permeability of the target material, and

σ is the conductivity of the target material. In order to ensure adequate measurement, the target material

must be three times as thick as the effective depth of the eddy currents.

In general, the linear measurement range for inductive sensors is approximately 30% of the sensor

active coil diameter. The target area must be at least as large as the surface area of the sensor probe. It is

possible to use curved targets, but their diameter should be three to four times the diameter of the sensor

probe. In addition, the sensor signal is weaker for ferrous target materials compared with nonferrous target

materials. This can lead to a reduced measurement range and so this should be investigated with the sensor

manufacturer.

Inductive sensors can sense through nonmetallic objects or nonmetallic contamination. However, if

measurement of a nonmetallic target displacement is required, then a segment of metallic material must

be attached to the target.

12.2.1.5 Capacitive

Capacitive displacement sensors are another type of noncontact sensor and are capable of directly sensing

both metallic and nonmetallic targets. These sensors are designed using parallel plate capacitors. The

capacitance is given by

C =

whereε

is the relativepermittivity(dielectricconstant) of the material between the plates, ε

is the absolute

permittivity of free space, A is the overlapping area between the plates, and d is the distance between the

plates.

-6 Robotics and Automation Handbook

sensor plate sensor plate

targettarget

FIGURE 12.6 Distance and area variation in capacitive sensor measurement.

In displacement sensor designs, a capacitive sensor typically incorporates one of the capacitor plates

within its housing and the target forms the other plate of the capacitor. The sensor then operates on the

principle that the measured capacitance is affected by variation in the distance d or the overlapping area A

between the plates. The capacitance is measured by detecting changes in an oscillatory circuit that includes

the capacitor. Figure 12.6 shows the distance and area variation methods for capacitive displacement

measurement.

Fordetection of nonmetallic targets, a stationarymetallic referenceis used as the external capacitor plate.

The presence of the nonmetallic target in the gap between the sensor and the stationary metallic reference

will change the permittivity and thus affect the measured capacitance. The capacitance will be determined

by the thickness and location of the nonmetallic target. Figure 12.7 shows the dielectric variation approach

for capacitive displacement measurement.

In general, the linear measurement range for capacitive sensors is approximately 25% of the sensor

diameter. The target should be 30% larger than the sensor diameter for optimum performance. In addition,

environmental contamination can change the dielectric constant between the sensor and the target, thus

reducing measurement accuracy.

12.2.1.6 Optical Sensors

Optical sensors provide another means of noncontact displacement measurement. There are several types

which are commonly used in robotics: optical triangulation, optical time-of-ﬂight, and photoelectric.

12.2.1.6.1 Optical Triangulation

Optical triangulation sensors use a light emitter, either a laser or an LED, in combination with a light

receiver to sense the position of objects. Both the emitter and receiver are contained in the same housing

as shown in Figure 12.8. The emitter directs light waves toward a target. These are reﬂected off the target,

through a lens, to the receiver. The location of the incident light on the receiver is used to determine

the position of the target in relation to the sensor face. The type of receiver used may be a position

sensitive detector (PSD) or a pixelized array device such as a charge coupled device (CCD). The PSD

receiver generates a single analog output and has a faster response time than the output pixelized array

device because less post-processing is required. It is also typically smaller so that the overall sensor size

will be smaller. Pixelized array devices, however, are useful when the surface of the target is irregular or

transparent.

non-metallic target

metallic reference

sensor plate

FIGURE 12.7 Dielectric variation in capacitive sensor measurement.

Sensors and Actuators 12

emitter

receiver

target

range

lens

FIGURE 12.8 Optical triangulation displacement sensor.

Important speciﬁcations for this type of sensor are the working range and the standoff distance. The

standoff distance is the distance from the sensor face to the center of the working range.

Both diffuse and specular designs are available for this type of sensor. A diffuse design is useful for

targets with surfaces which scatter light such as anodized aluminum. A specular design is useful for targets

with surfaces which reﬂect light well such as mirrors. In addition, the target color and transparency should

also be considered when investigating this type of sensor because these properties affect the absorption of

light by the target.

Optical triangulation sensors are high resolution and typically offer ranges up to one half meter. Higher

ranges can be achieved, albeit with signiﬁcantly increased cost.

12.2.1.6.2 Optical Time-Of-Flight

Optical time-of-ﬂight sensors detect the position of objects by measuring the time it takes for light to

travel to the object and back. As in the case of optical triangulation sensors, time-of-ﬂight sensors also

contain an emitter and a receiver. The emitter is a laser or an LED while the receiver is a photodiode. The

emitter generates one or more pulses of light toward a target. Some of the light is reﬂected off the target

and captured by the photodiode. The photodiode generates a pulse when it receives the light, and the time

difference between the emitted pulse and the received pulse is determined by the sensor electronics. The

distance to the target is then calculated based on the speed of light and the time difference.

Most time-of-ﬂight sensors have measurement ranges of several meters. However, laser based time-of-

ﬂight sensors can have a range of several miles if a series of pulses from the emitter is used. The accuracy

of these sensors is not as high as optical triangulation sensors but the range is typically greater.

12.2.1.6.3 Analog Photoelectric

Photoelectric sensors are noncontact displacement sensors which measure the position of objects by

measuring the intensity of reﬂected light from the object. Again, an emitter and a receiver are used. An

LED is used as the emitter and a phototransistor or photocell is used as the receiver. Photoelectric sensors

are most often used as digital proximity sensors, but one category, diffuse-mode proximity, can be used as

an analog sensor for measuring distance. In this design, light generated by the emitter is directed toward

the target. Light reﬂected from the target is captured by the receiver. The analog output from the receiver

is proportional to the intensity of the light received from the target. This output will be proportional to

the distance of the target from the receiver.