# Modern Robotics Errata

Errata below are for the hardcover book published by Cambridge University Press, the preprint version of the book posted on May 3, 2017, the practice exercises, and the linear algebra refresher appendix. Thanks to everyone who provided corrections to earlier versions!

## Contents |

## Book: Significant corrections

### Chapter 2

- Proposition 2.2 says that Grubler's formula "provides a lower bound on the number of degrees of freedom" of a mechanism to account for the case where the joint constraints are not independent. This statement is misleading, as it assumes a linear analysis of mobility. In fact, some mechanisms may have
**fewer**dof than implied by Grubler's formula by a higher-order analysis. A more correct statement is that Grubler's formula yields the correct number of dof in the "generic" case (where the joint constraints are independent and a linear analysis of mobility suffices), but a mechanism may have more dof (e.g., if the joint constraints are not independent everywhere) or fewer dof (e.g., the zero-dof four-bar linkage where the sum of the lengths of three links equals the length of the fourth link). This comment also applies to the sentence immediately after Example 2.6, the fifth bullet of the Summary in Chapter 2.6, and the second-to-last sentence of the third paragraph of Chapter 1's summary of Chapter 2.

### Chapter 3

- In the caption of Figure 3.3, it should read and .
- Under "Representing a configuration" in Chapter 3.3.1.2, the text says "
*T*_{bc}= (*R*_{bc},*p*_{bc}) represents {b} relative to {c}" but it should say "represents {c} relative to {b}". - There is a typo in the final matrix,
*T*_{ce}, of Example 3.19. The term should be . (Also, there is no need to use*T*_{ad}in this example.) - In the displayed equation just after Equation (3.76), the left-hand side should be (the brackets are missing).
- Exercise 3.16(i) asks for "the representation" but it should say "a representation" since
*q*is not unique.

### Chapter 4

- Example 4.1, last row of the table (screw axis for joint 3): The linear component
*v*_{i}should be (0, −*L*_{2},0) (the − sign is missing).

### Chapter 5

- Equation (5.7): The first two terms on the right-hand side of the equation should be .

- Exercise 5.2(b): The wrench applied by the last link, expressed in the {s} frame, is 5 N in the direction, with zeros in all other components. (The wording seems to imply that the force is through the tip of the last link, which is incorrect.)

- Exercise 5.16 and Figure 5.26: The robot is referred to as PRRRRR, but it is PRPRRR.

### Chapter 6

- Chapter 6.3, first line after Eq (6.7): the matrices and are referred to as twists, but these are the se(3) matrix representations of the twists.

### Chapter 8

- In Equation (8.14), should be replaced by .

- Figure 8.5 says the volume of the rectangular parallelepiped is
*a**b**c*but it should be*h**l**w*.

- End of first sentence after Eq (8.26) in Section 8.2.1 should read "columns of
*R*_{bc}correspond to the eigenvectors of ." (The word "eigenvalues" should be replaced by "eigenvectors.")

- In the parenthetical text a few lines after Equation (8.94): "...there is an equality constraint..." should instead be "...there is also an inequality constraint..."

- Caption of Figure 8.10: The operating region is light gray and the continuous operating region is dark gray.

- Exercise 8.6(a): The expression has the indices switched; the correct expression is .

- Exercise 8.7: The expression should be written:

### Chapter 10

- Equation (10.4) should read (the plus sign in the book should be a minus sign).

- First line of chapter 10.6.4: The equation should be (plus sign should be a minus sign). Similarly, in the last sentence of chapter 10.6.4, the plus sign should be a minus sign.

### Chapter 11

- In the displayed equation after Equation (11.18), the vector
*X*_{e}(*t*) is a six-vector. The bottom three elements are written correctly, but the top three elements, an angular velocity, are written instead in their 3x3*s**o*(3) form. Also, the term written*R*^{T}(*d*) should be written*R*^{T}(*t*).

- In Equation (11.33), the right-hand side should be zero (
*c*= 0).

- Figure 11.24, the Robonaut 2 series elastic actuator, 4th sentence of the caption: The words "outer" and "inner" should be switched, so the new sentence reads "The inner ring of hole mounts connects to the harmonic gearhead output, and the outer ring of hole mounts is the output of the SEA, connecting to the next link."

### Chapter 12

- In Example 12.4, the elements of the wrenches are erroneously written in the order (
*f*_{x},*f*_{y},*m*_{z}); they should be written in the order (*m*_{z},*f*_{x},*f*_{y}), i.e., ( − 2,0,1),(1, − 1,0),(1,1,0), respectively.

### Appendix C

- Section C.3, Equation (C.5): All instances of ϕ
_{i − 1}should be replaced by ϕ_{i}.

- In Equation (C.12), two instances of
*M*_{i − 1}should be replaced by*M*_{i}.

## Book: Minor typos, etc., no danger of misunderstanding

### Chapter 1

- (online version only) Description of Chapter 6: "jont positions" should be "joint positions."

### Chapter 2

- Chapter 2.2.2, Example 2.3: "Substituting" is misspelled.

### Chapter 3

- Near the end of Chapter 3.6 Software, the function "AxisAng" should be written "AxisAng6."

### Chapter 5

- Figure 5.15(b): The circular arrow indicating the rotation of joint 3 is slightly misplaced.

- Exercise 5.25: This problem instructs you to use the space Jacobian, so you should use it for the manipulability ellipsoids in this problem (even though it is generally preferred to use the body Jacobian for manipulability ellipsoids).

### Chapter 6

- (online version only) Figure 6.7: θ
_{0}should be θ^{0}(five times) and θ_{1}should be θ^{1}(one time).

### Chapter 7

- (online version only) Last line of first paragraph of Chapter 7.1: "valuees" should be "values."
- Just below caption for Figure 7.8: There is an extraneous dot.

### Chapter 8

- (online version only) Chapter 8.1.1, last paragraph: is improperly typeset.

### Chapter 12

- Caption of Figure 12.14, second-to-last sentence: "rotation if possible" should be "rotation is possible."
- (online version only) Example 12.11: The sentence "... can remain standing if there is a consistent solution if there exist ..." should read simply "... if there exist ..."

### Chapter 13

- Equation (13.29):
*y*_{p}should be*y*_{P}.

## A partial list of errata contributors

Thanks to the following people who provided corrections, starting from the preliminary version of the book posted in October, 2016:

H. Andy Nam, Eric Lee, Yuchen Rao, Chainatee Tanakulrongson, Mengjiao Hong, Kevin Cheng, Jens Lundell, Elton Cheng, Michael Young, Jarvis Schultz, Logan Springgate, Sofya Akhmametyeva, Aykut Onol, Josh Holcomb, Yue Chen, Mark Shi, AJ Ibraheem, Yalun Wen, Seongjae Jeong, Josh Mehling, Felix Wang, Drew Warren, Chris Miller, Clemens Eppner, Zack Woodruff, Jian Shi, Jixiang Zhang, Shachar Liberman, Will Wu, Dirk Boysen, Awe Wang, Ville Kyrki, John Troll, Andrew Taylor, Nikhil Bakshi