Sandin P.E. Robot mechanisms and mechanical devices illustrated

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276 Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices

LAYOUTS

With the possible exception of the whisker switch, the limit switch types

discussed above almost always require some method of extending their

reach and/or protecting them and the object being sensed from damaging

each other. There are many ways to do this. The next several figures

show various basic layouts that have their own benefits and problems.

In every sensor/actuator system, there is a time lag between when the

switch is tripped and when the actuator reacts. This time lag must be

taken into account, especially if the switch or object could be damaged.

Object, in this case, can mean something in the environment, or some-

thing attached to the robot that is designed to detect things in the envi-

ronment. If the time lag between contact and reaction cannot be made

short enough, the layout must provide some other means of preventing

disaster. This is done by using one of three methods.

Figure11-7 Direct sensing combined with direct hard stop

Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices 277

• A hard stop that is strong enough to withstand the stopping force

(and yet not damage the object) can be placed just after the trip point

of the switch.

• The layout can allow the object to pass by the switch, tripping it but

not being physically stopped by anything. The robot’s stopping

mechanism is then the main means of preventing harm.

• The travel of the sensor’s lever or button, after the sensor has been

tripped, can be made long enough to allow sufficient time for the

robot to stop.

Let’s take a look at each layout.

Combination Trip (Sense) and Hard Stop

This is probably the simplest layout to implement. The switch directly

stops the sensed object (Figure 11-7), which means the switch must be

strong enough to withstand repeated impacts from the thing being

sensed. Alternatively, there is a separate hard stop that is in line with the

switch that absorbs the force of the impact after it has been tripped

(Figure 11-8). Using a switch with a long throw eases implementation,

and nearly any mechanical limit switch can be made to work with this

layout, though the button and lever designs are usually best.

Figure 11-8 Direct sensing with

separate hard stop

278 Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices

By-Pass Layouts

The by-pass layout shown in Figures 11-9 and 11-10 relieves the switch

of taking any force, but, more importantly, is less sensitive to slight vari-

ations in the positions of the switch and the sensed object, especially if a

switch with a long throw is used. Removing the hazard of impact and

reducing sensitivity make this layout both more robust and less precise.

With careful design, however, this layout is usually a better choice than

the previous layout because it requires less precision in the relationship

between the hard stop and the switch’s lever or button. Remember that

the object being sensed can be anything that is close to the robot, includ-

ing the ground.

This layout and its derivatives are the basis of virtually all mechanical

timers. They are still found in dishwashers, washing machines, and any

device where turning the knob results in an audible clicking sound as the

arm or button on the switch jumps off the lobe of the cam. They can be

stacked, as they are in appliances, to control many functions with a sin-

Figure 11-9 By-pass linear

Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices 279

gle revolution of the timer. They can also be used as a very course

encoder to keep track of the revolutions or position of the shaft of a

motor or the angle of a joint in a manipulator.

Reversed Bump

The reversed bump layout shown in Figure 11-11 is a sensitive and

robust layout. The switch is held closed by the same springs that hold the

bumper or sense lever in the correct position relative to the robot. When

an object touches the bumper, it moves the sense arm away from the

switch, releasing and tripping it. A high quality switch is tripped very

early in the travel of the sensing arm, and as far as the switch is con-

cerned, there is no theoretical limitation on how far the bumper travels

after the switch has been tripped. For this reason, it is an effective layout

for sensing bumps.

Figure11-10 Rotating cam

280 Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices

BUMPER GEOMETRIES AND SUSPENSIONS

The robot designer will find that no matter how many long and short

range noncontact sensors are placed on the robot, at some point, those

sensors will fail and the robot will bump into something. The robot must

have a sensor to detect collisions. This sensor may be considered redun-

dant, but it is very important. It is a last line of defense against crashing

into things.

The sensor must be designed to trip quickly upon contacting some-

thing so that the robot’s braking mechanism can have the maximum time

to react to prevent or reduce damage. To be perfectly safe, this sensor

must be able to detect contact with an object at any point on the outer

surface of the robot that might bump into something. This can be done

with a bumper around the front and sides of the robot, if the robot only

goes forward. Robots that travel in both directions must have sensors

around the entire outer surface. It is important that the bumper be large

Figure 11-11 Reversed bump

Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices 281

enough so that it contacts the object before any other part of the robot

does, otherwise the robot may not know it has hit something. Some robot

designs attempt to get around this by using a measure of the current

going to the drive wheels to judge if an object has been hit, but this

method is not as reliable.

A bumper, though seemingly simple, is a difficult sensor to implement

effectively on almost any robot. It is another case in which the shape of

the robot is important as it directly affects the sensor’s design and loca-

tion. The bumper is so tricky to make effective as to be nearly impossible

on some larger robots. Unfortunately, the larger the robot, the more

important it is to be able to detect contact with things in the environment,

since the large robot is more likely to cause damage to itself or the things

it collides with. In spite of this, most large teleoperated robots have no

collision detection system at all and rely on the driver to keep from hit-

ting things. Even large autonomous robots (robots around the size of

R2D2) are often built with no, or, at most, very small bumpers.

Simplifying any part of the robot’s shape, or its behaviors, that can

simplify the design of the bumper is well worth the effort. Making the

shape simple, like a rectangle or, better yet, a circle, makes the bumper

simpler. Having the robot designed so that it never has to back up means

the bumper only has to protect the front and possibly the sides of the

robot. Having the robot travel slowly, or slowing down when other sen-

sors indicate many obstacles nearby means the bumper doesn’t have to

respond as fast or absorb as much energy when an object is hit. All these

things can be vital to the successful design of an effective bumper.

There are several basic bumper designs that can be used as starting

points in the design process. The goal of detecting contact on all outer

surfaces of the robot can be achieved with either a single large bumper,

or several smaller ones, each of which with its own sensor. These smaller

pieces have the added benefit that the robot’s brain can get some idea of

where the body is hit, which can then be useful in determining the best

direction to take to get away from the object. This can be done with a sin-

gle piece bumper, but with less sensitivity.

A clever design that absolutely guarantees the bumper will completely

cover the entire outer surface of the robot is to float the entire shell of the

robot and make it the bumper, mounted using one of the techniques

described later. Place limit switches under it to detect motion in any

direction of this all-in-one bumper/shell. This concept works well for

small robots whose shells are light enough not to cause damage to them-

selves but may be difficult to implement on larger robots.

Not only is it helpful to know the location of the bump, it is even bet-

ter to be able to detect bumps from any direction, including from above

282 Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices

and below. This is due to the possibility that the robot might try to drive

under something that is not quite high enough, or try to drive up onto

something and get the bottom edge of the bumper stuck, before it trips

the sensor. Both of these cases are potential showstoppers if the robot has

no idea it has hit something. This is where a bumper compliant and sen-

sitive to bumps coming from any direction is very helpful. If there is a

chance the robot will be operating in an environment where this problem

will arise, this additional degree of freedom, with sensing, makes the

bumper’s suspension system more complex but vital. Let’s start by look-

ing at the simplest case, the one-dimension sensing bumper.

SIMPLE BUMPER SUSPENSION DEVICES

The one-dimension (1D) bumper only detects bumps that hit the bumper

relatively straight on, from one direction. Although this may seem too

limiting, it can be made to work well if there are several smaller

bumpers, each with their own 1D sensor. Together they can sense a large

area of bumps from many directions. There are also layouts that are basi-

cally 1D in design, but, by being compliant, can be made to sense bumps

from arbitrary directions.

Since straight-on or nearly straight-on bumps are the most common

and produce the largest forces, it is better to use a design that allows the

bumper to have the longest travel in that direction. Bumps can be

detected around the sides of the robot without as much motion from the

bumper. This is why a compliant 1D bumper suspension can be used for

2D detection. There are many ways to attach bumpers that are basically

1D bumpers, but that can also function as 2D bumpers.

Some of these methods, or variations of them, can be used as is, with

no additional devices required. Usually, though, a secondary device must

be incorporated into the layout to positively locate the nominal position

of the bumper. This facilitates repeatable sensing by the limit switch. The

spring-centered plate layout is shown in Figure 11-14. The moving plate

is so loosely positioned it requires a vibration damper or it will wobble

constantly.

The V-groove centering block shown in Figure 11-12, is a basic

method of realigning the bumper after encountering a bump, but there

are several others that work nearly as well. The V-groove layout is essen-

tially two reversed bump limit switch layouts at 90° to each other. It is

therefore effective for bumpers designed to detect bumps from straight or

nearly straight on.

Chapter 11 Proprioceptive and Environmental Sensing Mechanisms and Devices 283

Three Link Planar

A very useful and multipurpose mechanical link-

age is something called a four-bar link. It consists

of four links attached in the shape of a quadrilat-

eral. By varying the lengths of the links, many

motions can be generated between the links. A

3D version of this can be built by attaching two

planes (plates) together with three links so that

the plates are held parallel, but can move relative

to each other.

This could be called a five-bar link, since there

are now essentially five bars, but the term five-

bar link refers to a different mechanism entirely.

A better name might be 3D four-bar, or perhaps

three link planar. Figure 11-13 shows the basic

idea. If the base is attached to the chassis, and the

top plate is attached to the bumper or

bumper/shell, a robust layout results. This system

is under-constrained, though, and requires some

Figure 11-12 Tension spring

V-groove layout

Figure 11-13 Three link planar