A manipulator suitable for tasks in remote places which are not easily accessible to operators comprises an arm composed of a series of arm modules each having an articulate and an address decoder and being constructed by the arm modules of an identical construction to attain an easy maintenance and high flexibility of the manipulator. The operation of the arm modules is controlled a control system which has a plurality of articulate angular-position commanding circuits and a time-sharing control circuit.

BACKGROUND OF THE INVENTION 
2. FIELD OF THE INVENTION 
The present invention relates to a manipulator and, more particularly, to a 
remote controlled manipulator which is suited for various tasks in places 
which are not easily accessible to operators. 
2. Description of the Prior Art 
Japanese Patent Publication No. 37307/1972 discloses a known manipulator 
which makes use of wire ropes for transmitting the driving power, while 
Japanese Patent Laid-Open Publication No. 73463/1977 discloses a 
manipulator in which concentric driving cylindrical tubes are used for 
power transmission. In either case, the power transmission means extends 
through an interior of the manipulator arm and is connected to the 
respective shafts for articulating the adjacent arm sections. Therefore, 
the disassembly of the manipulator articulate portion for the purpose of, 
for example, inspection requires demounting and remounting of the power 
trnamsission means onto the articulate portion, which makes the work 
difficult and time-consuming. 
Japanese Patent Laid-Open Publication No. 6272/1979 discloses a manipulator 
composed of a plurality of modules each of which can be detachably onto 
the next one without disassembly of other modules in order to improve the 
maintainability thereof. This publication, however, discloses merely the 
basic concept for a module-type manipulator constituted by detachable 
modules, and fails to disclose practical and specific means for embodying 
such module mounting and demounting. 
The operation rate of the manipulator used in a remote controlled work in 
places where the operators do not have access to readily, e.g., a high 
radioactive level area in a nuclear power plant, underwater, cosmic space 
and so forth, is affected by the maintainability of the manipulator 
itself. In particular, in a high radioactive level area, the manipulator 
requires to be assembled and disassembled in a short time. The task in 
such area, however, is quite severe and difficult even for the experts. 
In general, all of arm sections, articulate mechanisms, and power 
transmission means incorporated in a manipulator are changed according to 
the degree of freedom thereof and/or the intended use thereof. This in 
turn requires the articulate portions to be designed and constructed 
separately and independently. Consequently, the cost of the manipulator is 
greatly affected by the number of articulates. 
In case that it is required to modify an existing manipulator so as to 
increase arm flexibility by increasing the number of degree of freedom, 
e.g., in the case where the manipulator is required to extend to an object 
by detouring or clearing an obstacle. Judging from the degree of freedom, 
such modification, however, becomes materially equivalent both in 
difficulty and cost to the designing and the manufacturing of an entirely 
new manipulator having a different construction, which undesirably limits 
the adaptability of the manipulator to various specific working demands. 
On the other hand, the module-type manipulator composed of modules each of 
which can be detachably mounted onto the next one suffers from the 
following disadvantage in respect of the maintainability thereof. Namely, 
the cost of the manipulator is raised undesirably because of the necessity 
for the preparation of modules for maintaining the respective detachable 
modules. In addition, the provision of a plurality of power transmission 
lines, such as wire ropes or drive tubes, in the arm causes various types 
of inconvenience, e.g. an increase in the weight, complication in the 
construction and difficulties in the maintenance due to an increase of 
coupling elements between adjacent modules. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a manipulator 
having a higher flexibility and an improved maintainability. 
Another object of the present invention is to provide a manipulator in 
which a plurality of arm section swing angles in the articulate portions 
can be controlled by a single driving shaft unit and then the construction 
of the arm and articulate portion is very much simplified. 
In view of the above, the present invention aims at a provision of 
multi-articulate manipulator composed of a plurality of arm modules each 
having an articulate, wherein the arm modules is exchangeable one another 
and is constructed to be the same arrangement as another one. 
To this end, the present invention provides a multi-articulate manipulator 
composed of a plurality of arm modules having respective articulates, 
wherein each arm module is provided with an articulate address decoder and 
an articulate control circuit adapted to control a movement of the arm 
module in respect of the associated articulate portion, these circuits 
being connected to command circuits for commanding the articulate 
positions to be desired by means of address lines, data lines and a 
time-sharing control circuit. 
These and other objects, features and advantages of the present invention 
will become clear from the following description taken in connection with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a first embodiment of the manipulator of the 
invention, generally denoted by a reference numeral 1, has a 
multi-articulate construction composed of a plurality of arm modules 2a to 
2f articulated to each other to form an arm 2, and a gripper 3 articulated 
to an end of the distal arm module 2f. The proximal arm module 2a is 
connected to a base 4 which is mounted on a carrier 5 and then the 
manipulator 1 can be carried to any desired working place. 
In the described embodiment, the arm modules 2a to 2f have an identical 
construction. The construction of the arm module 2b will be explained by 
way of example. As will be seen from FIGS. 2 and 3, the arm module 2b has 
a hollow arm portion 11b and an articulate portion 12b which is connected 
to the arm portion 11b for swinging about an axis of an articulate shaft 
13b. A drive shaft 14b extends through an interior of the arm portion 11b. 
Bevel gears 15b and 16b are attached to both ends of the drive shaft 14i 
b. The bevel gear 16b engages a bevel gear 17 which is fixed to an 
articulate shaft 25b. As will be seen from FIG. 1, the arm modules 2a to 
2f are so arranged that the axes of the articulate shafts 13a to 13f 
extend orthogonally to each other. The gripper 3 is coupled to the 
articulate portion 12f of the distal arm module 2f. 
As will be best seen from FIGS. 2 and 3, the drive shafts 14a to 14f 
extending through the arm modules 2a to 2f are operatively connected in 
series through bevel gears 15a-15f, 16a-16f and 17a-17f. 
Referring back to FIG. 1, a motor 7 mounted within the base 4 has an output 
shaft 9 one end of which a bevel gear 10 is fixed to, which drives the 
bevel gear 15a through a bevel gear 18 (see FIG. 3), so that the torque of 
the output shaft 9 of the motor 7 is transmitted to the drive shaft 14a 
and further to the series of drive shafts 14b to 14f through meshing bevel 
gears. 
As shown in FIG. 1, the control system of this manipulator has an address 
line 110 and a data line 111 which constitute a bi-directional signal 
line, as well as a motor control line 112 and an encoder signal line 113. 
The address line 110 and the data line 111 are connected to the arm 
modules 2a-2f and further to the gripper 3 through the base 4. The motor 
control line 112 and the encoder signal line 113 are connected, 
respectively, to the motor 7 and the encoder 8 which are mounted within 
the base 4. 
The arm module will be more specifically explained with reference to FIG. 3 
showing the arm module 2b. 
The arm portion 11b has a concaved frusto-conical inner peripheral end 
surface. The outer peripheral end of the articulate portion 12b opposite 
to the frusto-conical end of the arm portion 11b has a convexed 
frusto-conical surface. These frusto-conical surfaces complements each 
other so that the concaved and convexed frusto-conical surfaces of 
adjacent arm modules closely fit each other. Thus, the proximal 
frusto-conical concaved inner peripheral end surface of the arm portion 
11b of the arm module 2b closely fits the mating frusto-conical convexed 
peripheral end surface on the articulate portion 12a of the adjacent arm 
module 2a having the same construction as the arm module 2b. Similarly, 
the frusto-conical surface on the distal end of the articulate portion 12b 
fits the mating frusto-conical surface on the proximal end of the arm 
portion 11c of the adjacent arm module 2c having the same construction as 
the arm module 2b. The drive shaft 14b is born by bearings 19b, 20b in the 
arm portion 11b and carries bevel gears 15b and 16b at both ends thereof 
as explained before. The articulate 25b is mounted at the distal end of 
the arm portion 11b for swinging about an axis 100b extending 
perpendicularly to the axis 102. The bevel gear 15b engages with the bevel 
gear 17a of the arm module 2a, while the bevel gear 17b engages with the 
bevel gear 15c of the arm module 2c. The bevel gears 17a and 17b have an 
identical construction. Similarly, the bevel gears 15b and 15c have an 
identical construction. 
The articulate shaft 13b is coaxial with the shaft 25b and is fixed at its 
one end to the articulate portion 12b while at the other end is mounted on 
the arm portion 11b through a bearing 26b. The articulate portion 12b is 
swingably mounted onto the arm portion 11b through a bearing 27b. The axis 
of the bearing 26b and the articulate shaft 13b coincide with the axis 
100b of the shaft 25b, so that the articulate portion 12b is swingable 
about the axis 100b, in directions of an arrow 101b. 
A clutch 23b and a brake 24b are mounted on the portion of the arm portion 
11b adjacent to the articulate shaft 13b. As will be seen from FIG. 4, the 
clutch 23b has an input shaft 28b and an output shaft 29b which are 
connected to the shaft 25b and the articulate shaft 13b respectively 
through keys 31b and 32b. The clutch 23b also has a housing 33b fixed to a 
housing 34b of the brake 24b which in turn is fixed to the arm portion 
11b. The articulate shaft 13b extends through the center of the brake 24 
and is coupled through a key 36b to the input shaft 35b of the brake 24b 
which is coaxial with the articulate shaft 13b. The clutch 23b and the 
brake 24b are of electromagnetic type and are adapted to be turned on when 
the electric power is not supplied thereto. Namely, when the electric 
power is not supplied to the clutch 23b, the input shaft 28b and the 
output shaft 29b are operatively coupled to each other. Similarly, when 
the electric power is not supplied to the brake 24b, the input shaft 35b 
is braked by the brake 24b. 
As shown in FIG. 3, an address decoder 40b and a control circuit 41b are 
provided within the arm portion 11b. The address decoder 40b is connected 
to the address line 110 and also to the control circuit 41b through a 
control line 114b. The control circuit 41b is connected to the data line 
111, and also to the clutch 23b and the brake 24b through an output line 
115b for the clutch and an output line 116b for the brake. Terminals 42b 
are provided on ends of the address line 110 and the data line 111 
respectively, while terminals 43b are provided on the other ends thereof. 
The terminals 42b are fixed to a surface on the arm portion 11b fitting 
the articulate portion 12a of the adjacent arm module 2a, while the 
terminals 43b are fixed to a surface of the articulate portion 12b fitting 
the arm portion 11c of the adjacent arm module 2c. The terminals 43b and 
42b have mutually engageable constructions. Thus, the terminals 43b engage 
with the terminals 42c on the arm module 2c having an identical 
construction to the terminals 42b, while the terminals 42b are coupled to 
terminals 43a on the arm module 2a having the same construction as the 
terminals 43b. 
The abovementioned coupling manner between the adjacent arm modules is 
applied to another arm modules 2a, 2c, 2d, 2e and 2f. However, the 
connection between the distal end arm module 2f and the gripper and the 
connection between the proximal end arm module 2a and the base 4 are made 
through specific coupling manners which are basically the same as the 
abovementioned coupling manner between adjacent arm modules. 
More specifically, the coupling between the arm module 2a and the base 4 is 
achieved through a construction basically identical to that employed for 
the connection between adjacent arm modules, namely, through mutual 
engagement between the complemental frusto-conical surfaces. The arm 
portion 45 is fixed to the base 4 and supports a shaft 46 through bearings 
47 and 48. The motor 7 is attached to the base 4 through a pedestal 49. 
The bevel gear 18 fixed to the shaft 46 engages both the bevel gears 10 
and 15a such as to form a gear train through which the torque of the motor 
7 is transmitted to the drive shaft 14a. Terminals 50 are provided on a 
distal end inner peripheral surface of the arm portion 45 and connected to 
the terminals 42a. The address line 110 and the data line 111 are 
connected to the terminals 50. 
As shown in FIGS. 6 and 7, the gripper 3 is coupled to the end of the 
articulate portion 12f of the distal arm module 2f. A bevel gear 52 is 
fixed to one end of the drive shaft 53 and meshes with the bevel gear 17f. 
The drive shaft 53 is supported within a housing 55 through a bearing 54. 
The other end of the drive shaft 53 is coupled to the input shaft of a 
clutch 56 fixed to the housing 55. An output shaft 58 of the clutch 56 
extends through a brake 57 and is connected to the input shaft (not shown) 
of the brake 57. The brake 57 is fixed to the housing 55 through the 
housing 59. The construction and the operation of the clutch 56 and the 
brake 57 are not described because they are materially identical to those 
of the clutches and brakes on the arm modules. The bevel gear 60 is fixed 
to the end of the output shaft 58 and engages the gear units 61 and 62 
which are mounted through bearings 63 on a post 64 fixed to the housing 
55. As will be clearly seen from FIG. 7, each of the gear units 61 and 62 
has a bevel gear 61a, 62a and a spur gear 61b, 62b which are provided on a 
single piece member. The bevel gears 61a, 62a engage a common bevel gear 
60, while the spur gears 61b, 62b mesh spur gears 65, 66 which are 
provided on shafts 67 and 68, respectively. The shafts 67 and 68 are 
swingably mounted through bearings 69 onto posts 70, 71 fixed to the 
housing 55. Link members 72 and 73 are fixed at ones of their ends to the 
shafts 67 and 68 while the other ends of these link members are provided 
with fingers 75, 76 through bearings 74. Link members 77 and 78 are 
provided at both of their ends with shafts 79 and 80 which are mounted 
through bearings 74 to the housing 55 and the finger 75, respectively. The 
link members 72 and 77 constitute a parallelogram link mechanism. An 
identical parallelogram link mechanism is formed by the link members 73 
and 78. Consequently, the fingers 75 and 76 are moved towards and away 
from each other while keeping the parallel relation therebetween, in 
accordance with the rotation of the shafts 67 and 68, thereby gripping and 
releasing an object. More specifically, a rotation of the drive shaft 58 
causes a rotation of the bevel gear 60 which in turn drives the gears 61 
and 62 in the opposite directions. Consequently, the spur gears 65 and 66 
meshing with these gears 61 and 62 also rotate in the opposite directions. 
This in turn drives the shafts 67 and 68 in the opposite directions so 
that the fingers 75 and 76 are moved towards or away from each other while 
keeping their gripping surfaces 81 and 82 in parallel with each other. 
Whether the fingers move towards or away from each other depends on the 
direction of rotation of the drive shaft 58. Namely, when the drive shaft 
58 is rotated clockwise as viewed from the right side in the drawing, the 
fingers 75 and 76 are moved away from each other. On the other hand, when 
rotated counter-clockwise, the fingers move towards each other. Needless 
to say, the drive shaft 58 is driven by the motor 7 through the drive 
shafts 14a to 14f which are drivingly connected each other by the bevel 
gears 15a-15f, 16a-16f, 17a.varies.17f, 10, 18 and 50. 
Referring to FIG. 8, the control system 6 of this manipulator has seven 
articulate angular position commanding circuits 85a to 85g and a time 
sharing control circuit 86 to which these circuits 85a to 85g are 
connected through the respective data lines 115. The commanding circuits 
85a 85f are adapted to command the corresponding angular positions of the 
articulates of the arm modules 2a to 2f, while the commanding circuit 85g 
commands the position of the fingers of the gripper 3. Although omitted 
from FIGS. 6 and 7, an address decoder 40g, an articulate position control 
circuit 41g and associated signal lines are accommodated within the 
gripper 3. 
The construction and operation of each constituent of the control system 
shown in FIG. 8 will be explained hereinunder. 
The required posture of the manipulator 1 at a certain moment is given in 
the form of command values of angular positions of the articulates issued 
from the articulate angular-position commanding circuits 85a-85g. The 
signals issued from the articulate angular position commanding circuits 
85a-85g are delivered to the timesharing control circuit 86. The 
time-sharing.control circuit 86 is adapted to successively output the 
address codes Ac and the articulate control data Dc for respective arm 
modules 2a to 2f to the address line 110 and the data line 111, 
periodically. 
In general, the bit number k of the address signal is required to satisfy 
the following logarithmic function with the base 2: 
EQU k&gt;log.sub.2 (n+1) (1) 
where, n represents the number of degrees of freedom of the manipulator. In 
the case of the described embodiment where the manipulator has 7 degrees 
of freedom, so the required address bit number k has to be three or more 
(k.gtoreq.3. However, since no redundant address exists in the case of 
k=3, the bit number k is selected to be four (4) so as to attain a greater 
adaptability or flexibility to any increase in the number of articulates. 
Table 1 shows an example of the address codes allotted for respective arm 
modules. 
TABLE 1 
______________________________________ 
Address Address 
Arm module decoder code 
______________________________________ 
Arm module 2a 40a 0001 
Arm module 2b 40b 0010 
Arm module 2c 40c 0011 
Arm module 2d 40d 0100 
Arm module 2e 40e 0101 
Arm module 2f 40f 0110 
Gripper 40g 0111 
No appointment of 0000 
articulate 
Address for extension 1**** 
______________________________________ 
*: 0 or 1 
The described embodiment has 8 addresses for extension so that the degree 
of freedom is extendable to the maximum value 15, i.e. 2.sup.k -1=2.sup.4 
-1=15. If a degree of freedom exceeding 15 is necessary, the address bit 
number should be determined in accordance with the above formula (1) so as 
to realize such degree of freedom 
As shown in FIG. 9, the address decoder 40i (i: a.about.g) is composed of 
an address setting circuit 87, a buffer 88 and an AND circuit 89. The 
address setting circuit 87 has a plurality of switches which are adapted 
to be selectively turned on and off to produce peculiar address bit 
patterns A.sub.M which are delivered to the AND circuit 89 through a 
signal line 120. The buffer 88 is connected to the address line 110 and is 
connected to the AND circuit 89 through the signal line 120. The outputs 
from the AND circuit 89 is delivered to the control line 114i. 
The address code Ac from the time sharing circuit 86 to the address line is 
stored in the buffer 88 and is delivered to the AND circuit 89 through the 
control line 121. The AND circuit 89 computes the logical AND between the 
address bit pattern A.sub.M and the address code A.sub.C and deliveres a 
bit signal corresponding to the following output S to the control line 
114i. 
EQU A.sub.M .circle.X A.sub.C =0.fwdarw.S=1 
EQU A.sub.M .circle.X A.sub.C =0.fwdarw.S=0 
In the described embodiment, the address setting means is so pre-setted 
that the address bit patterns A.sub.M for respective modules correspond to 
the patterns shown in Table 1. 
As shown in FIG. 10, the articulate control circuit 41i (i: a.about.g) is 
composed of the following parts: namely, a buffer 90 to which the data 
line 111 and signal lines 122, 123 are connected; an AND circuit 91 to 
which the signal line 122, the control line 114i and the signal line 124 
are connected; an AND circuit 92 to which the signal line 123, the control 
line 114i and the signal line 125 are connected; and drivers 93, 94 to 
which are connected signal lines 124, 125 and 115, 116, respectively. 
Assume here that the bit pattern of the 2-bit data Dc delivered to the data 
line 111 is expressed as follows: 
EQU D.sub.C =(d.sub.C, d.sub.B) 
where, d.sub.C and d.sub.B represent the control signals for the clutch 23 
and the brake 24, which take, respectively, a level "1" in the "on" state 
thereof and a level "0" in the "off" state thereof. The control signals 
d.sub.C and d.sub.B are delivered to the AND circuits 91 and 92 through 
the signal lines 122 and 123, so that the logical ANDs of these control 
signals d.sub.C and d.sub.b and the control signal S from the control line 
114i are produced by these AND circuits. The driver 93 operates to turn 
the clutch on when the output from the associated AND circuit 91 is "1", 
while the driver 94 operates to release the brake when it receives the 
output "1" from the associated AND circuit 92. 
The operation of the articulate control circuit 41 is summarized as below: 
On a condition: S=1 and d.sub.C =1.fwdarw.clutch ON (connected) 
On a condition: S=1 and d.sub.B =1.fwdarw.brake OFF (release) 
The clutch is "OFF" and the brake is "ON" when these conditions are not 
satisfied. 
Consequently, the articulate control circuit in the arm module having the 
address pattern A.sub.M matching the address code A.sub.C delivered to the 
address line 110 is validated alone. In consequence, the brake and the 
clutch associated with this arm module are driven to actuate the 
articulate of this arm module according to the data D.sub.c delivered to 
the data line 111. The amount of angular movement of the articulate is 
computed out on the basis of the variance in the encoder 8 during the 
period in which the same address bit pattern A.sub.M is maintained, which 
is inputted to the time-sharing circuit 86. 
As explained before, the address code A.sub.D is successively changed after 
a predetermined period so as to conform with the address bit patterns of 
successive articulates. Assume here that the address pattern A.sub.D has 
been maintained in conformity with the address bit pattern A.sub.M 
peculiar to one of the articulates. Then, immediately after the change of 
the address code, the amount of operation of this articulate is fed back 
to the articulate command control circuit of this articulate and is 
compared with the articulate angular-position command so as to produce a 
next command. After a predetermined time interval, the articulate control 
circuit of this articulate is validated again to effect a further control 
of the articulate position. 
As described above, the angular positions of the articulates of the arm 
modules are controlled and then the position of the distal end of the 
manipulator and the opening and closing position of the gripper are 
controlled. 
Although not shown, electric power supply lines are provided to supply 
respective circuits with electric power from a power source accommodated 
in the control system 6. 
In the embodiment described hereinbefore, the arm modules are arranged such 
that the articulate shafts of adjacent arm modules extend orthogonally 
each other. This, however, is not exclusive and the orientation of the arm 
modules 2a to 2f may be varied in various ways. For instance, FIG. 11 
shows a modification in which the arm modules 2a, 2b, 2c are articulated 
such that their articulate axes 13a, 13b, 13c extend in parallel, while 
the arm modules 2d, 2e, 2f are articulated such that the articulate axes 
13d, 13e, 13f thereof extend in parallel one another and extend 
perpendicular to the articulate axis 13c. With such a construction, the 
gripper can approach the object by detouring an obstacle 95 as 
schematically shown in FIG. 12. 
As has been described, according to the invention, the construction of the 
arm and articulates can be very much simplified because the angular 
positions of all articulates can be controlled by means of a single drive 
shaft unit. 
FIG. 13 shows another embodiment having a construction of the arm modules 
2a to 2f different from the construction of those shown in FIG. 3. Namely, 
this embodiment employs a motor 7b and an encoder 8b in place of the 
clutch 23b, brake 24b, drive shaft 14b, bevel gears 15b, 16b, 17b and 
other related elements, while the control line 114b leading from the 
address decoder 40b is substituted by lines 131b, 132b. Furthermore, an 
additional buffer 96b is used and a driver 97b is provided in place of the 
articulate control circuit 41b. In this embodimnet, two address bit 
patterns concerning the motor 7b and the encoder 8b are used as the code 
peculiar to the address code 97b. The address bit patterns are determined, 
for example, as shown in Table 2 below. 
TABLE 2 
______________________________________ 
Module Address Address bit 
number decoder Device pattern 
______________________________________ 
Arm module 2a 
40a Motor 0001 
encoder 1001 
Arm module 2b 
40b Motor 0010 
Encoder 1010 
Arm module 2c 
40c Motor 0011 
Encoder 1011 
Arm module 2d 
40d Motor 0100 
Encoder 1100 
Arm module 2e 
40e Motor 0101 
Encoder 1101 
Arm module 2f 
40f Motor 0110 
Encoder 1110 
Gripper 3 40g Motor 0111 
Encoder 1111 
No appointed 0000 
articulate 1000 
______________________________________ 
The motor 7b includes a reduction gear (not shown) fixed to the arm portion 
11b. The output shaft of the motor 7b is connected to the articulate shaft 
13b. The encoder 8b is fixed to the motor 7b so as to detect the angle of 
rotation of the motor output shaft. 
The driver 97b is enabled when the signal "1" is delivered thereto through 
the control line 131b, and delivers an output to the motor 7b, which 
corresponds to the signal transmitted through the data line 111. On the 
other hand, the buffer 96b latches the signal from the encoder 8b and 
delivers a latch signal to the data line 111 when the signal "1" is 
delivered through the control line 132b. 
As will be understood from the foregoing description, this embodiment is 
distinguished from the preceding embodiment in that the angular positions 
of articulates are controlled through delivery of data concerning both the 
motors 7b and the encoders 8b. 
According to this embodiment, since the motors and the encoders are mounted 
on the respective articulate shafts, the independent servo system can be 
constituted with the respective are modules. Accordingly, even if the 
magnitude of the torque to be required for the respective arm modules is 
substantially different from one another, the angular position of the 
articulate portion of the arm module can be controlled rapidly because the 
change of the torque magnitude is not required between the arm modules. 
According to the present invention, one or more articulates can be 
selectively controlled even through the control lines are common, so that 
the manipulator can be composed of a plurality of arm modules having an 
identical construction. This in turn allows free replacement or exchange 
of modules, rearrangement of degrees of freedom and increase or decrease 
of the degrees of freedom without requiring any change in the design. In 
addition, only one spare arm unit suffices as a common spare part for all 
arm modules.