# Navigation in LEGO MINDSTORMS Programming

Do more with your LEGO Mindstorms robot by wheedling the power of Java and navigation. Learn how to apply navigation theory and then build your own navigator robot.
This chapter is from the book

### This chapter is from the book 

Topics in this Chapter

• Understanding the Problem of Navigation

• Using The Navigator API

• Creating a Navigator Robot

• TimingNavigator Accuracy

## Understanding the Problem of Navigation

We'll try to imagine your living room from the perspective of a robot. Our robot Tippy, from previous chapters, is equipped with a single touch sensor that is either on or off. It also has two motors it can control, and an internal timer that functions like a stopwatch. To put yourself in the robot's shoes, imagine climbing into a yellow box about one cubic meter in size. When the box is closed everything about the outside world becomes unknown. The box is equipped with a small control panel that contains a three-way switch (forward, stop, reverse) for the left wheel, another three-way switch for the right wheel, a stopwatch, and a light bulb (Figure 7–1). The light bulb will go on only when the bumper hits an object. We will also add the presence of a benevolent giant who exists outside the box. He may, according to his whim, give you small pieces of information that can help you in your task. Furthermore, we must introduce some pseudo-physics into this scenario. If the box scrapes by an object you will not hear it. If you back into an object, you will not be jostled or feel your momentum change. If you run over some objects on the floor, you will not feel a bump. If a wheel becomes stuck in midair, rotating freely, you will be unaware of this. This is exactly how Tippy experiences the world, and it is a very dark place.

Figure 7–1 Life inside the box. (Artwork courtesy of Wil Glass.)

Now let's see how well we can navigate within this scenario. Imagine you have a piece of paper and a pencil inside this box. The first thing you can do is mark an X representing your starting point. The strategy we employ is to measure the forward and backward movement using a stopwatch. We aren't really sure how fast the robot moves, but we do know that moving forward for four seconds is twice as far as moving for two seconds. Units are not really important to us because we have no idea of the scale of the outside world. One second could represent a centimeter, a meter, or 20 inches. To us it is irrelevant, so all measurements will be measured in seconds. Now it's time to boldly go forth, so we start the watch and switch both motors to forward. It's a little scary, knowing that at any moment the box could go plunging down a flight of stairs. After a time the touch sensor light goes on, so we halt the stopwatch and record the time. In this case, we have moved forward for four seconds, so we draw a line—say four inches long—due east from the X. This is our new position. We can also indicate on our piece of paper that an object was struck at that point by placing O1 next to the point. Now, because we are trying to emulate Tippy's world, our only recourse is to back up and turn in a new direction. Backing up is no problem—we just back up an arbitrary amount of, say, one second. To rotate we drive one motor forward and one in reverse, but we need to know how many degrees we have rotated. To calculate this, we need to know how long it takes to complete one full rotation. In our case, the giant has given us a piece of paper that tells us it takes four seconds to rotate 360 degrees. Thus, we decide to rotate to the right 90 degrees, or one second. Now we can carry on our way, navigating around the room and keeping track of when we bump into objects. After a while, we might even be able to make inferences about the room, such as where doorways and walls exist. At any time we can attempt to head back to our starting point, or to one of the other points within the area, using the coordinates we have written on the paper.

So, as you can see, we can perform some rudimentary exploration with little external information. The main source of external data is the light bulb—one bit—which is either on or off, but much can be inferred from changes in this state. This doesn't guarantee we are in fact where we believe we are, though. For all we know, the giant, in one of his more puckish moods, could be holding the box off the ground and pressing the touch sensor at random intervals (Figure 7–2). Barring any anomalies such as this, we should have a pretty good idea of our location at any given time. We could even go so far as to navigate to a specific section of the house. For this to succeed we have to ensure the starting point and orientation are always the same. The external giant could even hand a piece of paper through a slot that tells us the coordinates of the kitchen, bathroom, bedroom, and living room. By receiving some external data, it is possible for Tippy to get from one known location to another.

Figure 7–2 External conditions can affect accuracy. (Artwork courtesy of Wil Glass.)

To really understand what the RCX robot sees, I heartily recommend navigating around your own living room. By involving yourself in what the robot experiences you can really start to understand how limited its perceptions of the world are. To do this you need to shut off as many senses as possible by turning off sources of sound such as a TV, a radio, an air conditioner, or a ticking clock. Better yet, use ear-plugs. You may want to blindfold yourself so you aren't subconsciously navigating by the brightness of a local light source. Please take extreme care if you decide to perform this experiment. I don't want to be responsible for intrepid roboticists falling down flights of stairs while blindfolded!

### InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

## Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

## Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

### Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

### Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

### Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

### Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

### Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

### Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

## Other Collection and Use of Information

### Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

### Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

### Do Not Track

This site currently does not respond to Do Not Track signals.

## Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

## Children

This site is not directed to children under the age of 13.

## Marketing

Pearson may send or direct marketing communications to users, provided that

• Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
• Such marketing is consistent with applicable law and Pearson's legal obligations.
• Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
• Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

## Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

## Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

## Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

## Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

## Sharing and Disclosure

Pearson may disclose personal information, as follows:

• As required by law.
• With the consent of the individual (or their parent, if the individual is a minor)
• In response to a subpoena, court order or legal process, to the extent permitted or required by law
• To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
• In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
• To investigate or address actual or suspected fraud or other illegal activities
• To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
• To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
• To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.