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Steering with the Modern Robotics gyro – an introduction

v4 dated 1/22/16, clarified heading directions

v3 dated 1/20/16, added new signed heading

v2 dated 12/11/15

This article is a low-tech introduction to steering a robot autonomously using the new

Modern Robotics Integrating Gyro sensor. The focus here is learning, rather than simply

providing copy-and-paste code. Examples are given mostly in pseudocode which can be

adapted for languages such as Java, RobotC, C++, etc.

The MR gyro sensor gives rate of rotation about X, Y and Z axes. It also performs an on-

board calculation (integration) to give the current heading about the vertical Z axis. This

article discusses only the heading value, for steering. It’s an integer value in degrees,

initialized to zero at calibration, where zero is straight ahead (I2C cable at the back).

For simplicity, this tutorial has heading values increase with clockwise (CW) sensor

rotation, looking down from above. This is true only when using basic headings obtained

with the default/generic gyro command. See the end of Section 11 for actual directions.

The sensor should be placed horizontal (parallel to the ground), close to the robot’s center

of rotation, and as low as possible. Convenient access is not required, although it helps to

see its glowing LED to indicate the sensor is connected.

1. Heading Values

The sensor can give two types of heading value. The basic heading type ranges only

from 0 to 359 degrees, never a negative number. So, continuous CW rotation from zero

gives positive values that increase to 359 degrees, then jump back to zero.

Counterclockwise (CCW) rotation from zero immediately jumps to 359 degrees, then 358,

and so on.

The other type is a signed heading that can be positive or negative, and extends without

limit. So, counterclockwise (CCW) rotation from zero gives negative values such as -1

degree, then -2 degrees, and so on. And there’s no rollover at 360 degrees, so two full

CW rotations would result in a heading of +720 degrees.

For both types of headings, turning actions accumulate, unless the sensor is purposely

reset to zero. If the sensor rotates CW 30 degrees from zero, then rotates CCW 10

degrees, the heading value will now read +20 degrees. But when the sensor rotates

through zero, you must be aware of the adding and subtracting process. If the sensor

rotates CW 10 degrees, then rotates CCW 30 degrees, the two heading types will give

different results. The basic heading will be 340 degrees, while the signed heading will be

-20 degrees.

Just choose the type of heading that you prefer, become familiar with its math, and stick

with it. They have different characteristics for autonomous driving, as you will see below.

In general the basic heading needs more managing, covered by Sections 3 and 5.

Summary of content (9 pages)

PAGE 1
Steering with the Modern Robotics gyro – an introduction v4 dated 1/22/16, clarified heading directions v3 dated 1/20/16, added new signed heading v2 dated 12/11/15 This article is a low-tech introduction to steering a robot autonomously using the new Modern Robotics Integrating Gyro sensor. The focus here is learning, rather than simply providing copy-and-paste code. Examples are given mostly in pseudocode which can be adapted for languages such as Java, RobotC, C++, etc.
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. Target Headings Autonomous steering is typically done by comparing the current heading to a target heading. You calculate the difference, or error, and command the motors accordingly. When choosing a target heading, you must consider the type of heading value (basic vs. signed). Suppose you are working with signed values, and you want the robot to turn 90 degrees to the left. This steering destination could be represented by either a target of 270 degrees or a target of -90 degrees.
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But suppose for some reason you need or want the target heading to be designated as +270 instead of -90. With the sensor pointing roughly forward, the initial heading could be a low positive number, not converted by the above pseudocode, giving a very large error value. Standard steering math would again sharply spin the robot around the wrong way (clockwise). In this case it’s better to convert a low positive heading into a value greater than 360 degrees.
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You might think the condition should always read “while current heading less than target heading”. This works for CW rotation from zero towards a positive target. But what if the target is -90 degrees? A current heading near zero is not less than -90, the conditional is false, and the while loop will not run at all. A skipped loop can be hard to notice and frustrating to debug. So, pay attention to the choice of greater-than or less-than. It depends on your target heading (e.g.
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A shorter solution uses a do...while loop, which always executes at least once. Thus the conversion code doesn't need to appear both outside and inside the loop. ! & & $ " # 6.
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7. Power Limits The above calculations of motor power could give values outside the allowed range, possibly resulting in run-time errors or random motor commands. In general, a program needs to do something about calculated values that may give unpredictable behavior.
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9. Safety Timeouts Action loops need safety timeouts. Repeat that to yourself five times. Your program must deal with an autonomous robot getting stuck in a loop, for whatever reason. Start a system timer when entering the loop, then monitor it and take action when needed. One method is simply to add ‘time’ to the loop conditional. If the time limit is reached, the loop exits normally and the program continues.
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11. Sample Outline Putting all this together, here is a sample outline for “steer right to 90 degrees” using basic headings; sensor values are converted before use.
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12. Verifying Solutions For each planned driving action, always step through the code manually. Yes, this means with paper and pencil; ask an older person for those. Make a chart. The first column is gyro heading, the others are variables in order: currentHeading (after adjustment, if any), headingError, driveSteering, leftPower and rightPower (after high/low clipping, if any).