Abstract:
A robot including a controller and servos, each acting as an actuator, a supply voltage generator circuit is connected to a transmission line between a controller, which controls using the RS485 communication protocol, and the servos. The reference voltage generator circuit maintains one of a pair of differential signals (or an inversion signal) at a constant potential between a high level and a low level. As a result, a TTL communication scheme servo can be mounted and controlled, in addition to a RS485 scheme servo. A supply voltage generator circuit is formed of a Zener diode and a voltage divider circuit. Without being limited to incorporation at some midpoint on the transmission path, the supply voltage generator circuit can be incorporated in a controller or a hub for splitting a transmission path.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of Japanese Patent Application No. 2007-263925 filed on Oct. 10, 2007, which is fully incorporated herein by reference. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to a robot that performs predetermined operations in a remote control mode, or based on operation commands which are sent from a personal computer (PC), or based on a program which runs automatically when predetermined conditions occur. More particularly, the present invention relates to a robot where both a RS485 communication scheme servo system, which operates with differential signals produced in response to control signals, and a transistor-transistor logic (TTL) communication scheme servo system can be used in a mixed mode. 
         [0004]    There have been many recent developments in the field of hobby robots such as biped walking humanoid robots, with a number of hobby robot competitions being held. Like the human body, robots of this type have movable joints for wrists, elbows, shoulders, neck, waist, knees, and ankles. Servos, acting as actuators, which have rotating parts, are mounted on each joint. 
         [0005]      FIG. 12  is a schematic diagram illustrating a humanoid robot. The robot has joint parts  1 - 6  and a body part  7 . With reference also to  FIGS. 13-16 , servos  10  disposed in joints  1 - 6  are controlled synchronously, based on control signals from a controller  20 , to execute sequential operations such as walking. The whole of the robot is formed by combining a number of frame parts  8  together. The head, body, legs, arms and the like are also formed by assembling frame parts  8  together. Each frame part  8  has cut-out sections  8 a to provide space to accommodate the servos  10  while also providing weight savings and ease of assembly. 
         [0006]    As shown in  FIGS. 13(   a )- 14 , each frame part  8  includes a reception part  8   a,  on the end of which a servo  10  is mounted. Each frame part  8  is journaled using a rotating disc  12  and stopper  13 , which can rotate with reference to the rotational shaft  11  of the servo  10 , thus forming joints  1 - 6 . For each of the joints  1 - 6 , one or two servos  10  are disposed in conformation with the movement of one or two of three axes, namely, pitch axis, roll axis, and yaw axis. 
         [0007]    The body  7  includes a controller  20  and a power source, here a secondary battery contained therein. As shown in  FIG. 15 , the controller  20  and servos  10  are connected together using a transmission path  15  which includes signal lines, a ground line, and power lines. The controller  20  outputs control signals based on received operation commands to control the servos  10 . 
         [0008]    As shown in  FIG. 16 , the controller  20  comprises a control circuit  21  for producing control signals for the servos  10 , a power source  22 , and a power voltage converter circuit, or regulator,  23  for converting the power source voltage of the power source  22  into, for example,  5  volts, adapted for the controller circuit  21 . The voltage of the power source  22  depends on the type of secondary battery. For example, plural sets of cells are combined together to use seven to twelve-volt batteries. Moreover, the control circuit  21  includes an arithmetic part  24 ; a memory  26 , including a rewritable ROM such as an EEPROM, for storing programs corresponding to operation commands or the setting values of each servo  10 , or a RAM for temporarily storing communication data; an interface  25   a  for receiving operation commands received by a receiver  27 ; and an interface  25   b  for interacting with the servos  10 . 
         [0009]    For receiving an operation command, the interface  25   a  is linked to a personal computer  29  and uses the RS232C serial communication scheme. When radio communication is performed using a dedicated controller, such as a transmitter  28 , the receiver  27  is connected to the control circuit  21  via the interface  25   a.  The transmitter  28  transmits operation commands to the receiver  27 . This system may employ, for example, the 2.4 GHz band Bluetooth® communication scheme. Bluetooth is a registered certification mark of Bluetooth SIG Inc. 
         [0010]    The operation commands are created by coding basic actions including “rise”, “walk”, “crouch”, “straddle”, “turn head”, and the like, and are transmitted to the control circuit  21  through the personal computer  29  or the transmitter  28 . In contrast, the control circuit  21  programs a predetermined sequence of operations such as “walk”, “rise” and the like, and then stores them in the memory  26 . 
         [0011]    In order to control the robot, the transmitter  28  or the personal computer  29  transmits operation commands to the control circuit  21 . The control circuit  21  controls the rotational angle and speed of each servo  10 , according to the type of operation command, thus realizing the desired movement, for example, “walking”. In addition, the robot also has an automatic execution mode for executing a sequence of operations by executing a program sequentially. The controller  20  has three axis acceleration sensors (not shown) to detect the robot&#39;s own position. For example, when the robot falls down, the rising operation differs in the face-up state and in the face-down state. In such a case, when the controller receives the operation command for “rising”, the controller determines its own current position so that the rising operation can be carried out according to each position. 
         [0012]    The RS485 half-duplex communication scheme may be adopted for communication between the controller  20  and the servos  10 . As for the servos  10  previously described, when attempting to emulate the motion of a human joint, one robot will require twenty or more servos  10 . However, the use of the RS485 scheme allows joints to be connected to forty or more servos. Some communications between the controller  20  and the servos  10  employ the TTL scheme which uses two-valued signals comprising a high (H) level and a low (L) level. The servo  10  has sensors that measure information on the rotational angle, the current flowing through the servo, and the temperature of the servo  10 . 
         [0013]    This information is fed back to the controller  20  to reflect the control of the controller  20 . For that reason, like the controller  20 , each servo  10  includes transmission/receiving drivers and a power voltage converter circuit in addition to a signal processing controller. In the configuration of the above-mentioned robot, splitting the transmission line facilitates routing of conductor wiring. For that reason, hubs (not shown), each of which divides a single transmission path into plural paths, are disposed at desired points in a robot, thus facilitating the routing of a transmission line. Such a robot is disclosed in Japanese Patent Application No. 2006-135552 and “Robot Life, March 2007”, pp. 136-139, issued by NESTAGE Co., Ltd., the contents of which are hereby incorporated by reference. 
         [0014]    The RS485 communication scheme, which uses differential signals, is virtually immune to noise and to the effect of the resistance of a single cable, that is a transmission path, or the load resistance of a servo. This feature allows a large number of servos to be incorporated and moreover, advantageously facilitates the routing of cables. However, the RS485 scheme driver is expensive, thus increasing the cost of servos as a whole. In contrast, the TTL communication scheme is adversely affected by the resistance of the transmission path and by the load resistance of servos. The distal end of the transmission path is more susceptible to the influence of noise. Therefore, unlike the RS485 scheme, the TTL scheme has the problem that the length of a single transmission path must be short and that the number of connectable servos is small. 
         [0015]    However, many types of servos of different torque and size have been conventionally used for hobby purposes other than in robots, for example, in model aircrafts. The TTL scheme allows those servos to be used in a variety of applications relatively easily. The TTL scheme can also keep down the cost of servos because of the reduced cost of the driver. Moreover, there is the advantage that a wide selection of products is available. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention is made to overcome the above-mentioned problems. The problem of the present invention is to provide such a design that RS485 scheme servos and TTL scheme servos can be used in a mixed mode, even when the robot controller is designed to support the RS485 scheme. 
         [0017]    According to the present invention, there is provided a robot which comprises a robot body having plural joints; servos mounted to the respective joints; a controller for controlling servos using differential signals; a transmission path connected between the controller and the servos, and having first and second signal lines, each acting as a differential signal line; a ground line and a power line for supplying a power source voltage; and a power source for supplying electric power to the controller and the servos. The servos are moved based on control signals from the controller to move the robot as desired. The robot further comprises a reference voltage generator circuit disposed on the transmission path for holding at least one of the differential signals, which are output from the controller and fluctuate between the potential of the power line and the potential of the ground line, at a constant potential, this being an intermediate potential between the two potentials. 
         [0018]    The reference voltage generator circuit may be situated in the same cabinet as the controller or in the hub that divides a set of transmission lines into plural sets of transmission lines. Moreover, in an embodiment of the present invention, the reference voltage generator circuit includes a first resistor and a Zener diode, which are connected serially between the ground line and the power line, and a second resistor connected between a second signal line and the juncture of the first resistor and the Zener diode. The Zener diode holds a differential signal at a constant potential. In another embodiment, the reference voltage generator circuit may include a first resistor, a diode and a Zener diode, which are connected serially between the ground line and the power line, a second resistor connected between a second signal line and the juncture of the diode and the Zener diode, and a third diode connected between the first signal line and the juncture of the first resistor and the diode. The second signal line is held at a constant potential or at an intermediate potential between the potential of the power line and the potential of the ground line. The reference voltage generator circuit may be formed of a voltage divider circuit. 
         [0019]    Furthermore, there is provided a hub used in a transmission line for a robot according to the present invention. The hub is applied to a robot including a robot body having plural joints, servos mounted on the respective joints, and a controller for controlling the servos using differential signals. The hub connects the controller and the servos and is used in a transmission path including first and second signal lines (each acting as a differential signal line), a ground line, and a power line for supplying a power source voltage. The hub divides control signals output from the controller into plural transmission paths. The hub comprises a reference voltage generator circuit, which has plural connectors connected in common to the first and second signal lines, the ground line, and the power line, respectively; a first resistor and a Zener diode, connected serially between the ground line and the power line; and a second resistor connected between the second signal line and the juncture of the first resistor and the Zener diode. 
         [0020]    According to another aspect of the present invention, there is provided a reference voltage generator circuit used in a transmission line for a robot. The reference voltage generator circuit comprises a robot body having plural joints, servos disposed on the respective joints, a controller for controlling the servos using differential signals, a first resistor and a Zener diode, which are connected serially between the ground line and the power line, and a second resistor connected between a second signal line and the juncture of the first resistor and the Zener diode. The Zener diode maintains the differential signal at a constant potential or an intermediate potential between the potential of the power line and the potential of the ground line. 
         [0021]    In the present invention described above, the reference voltage generator circuit is disposed on the transmission path between the robot controller and servos. Therefore, even if the controller is compatible with the RS485 scheme, the present invention can use, in a mixed mode, both RS485-compatible servos and servos that are compatible only with control signals formed of TTL scheme pulse signals. As a result, the types of adaptable servo can be increased markedly. Suitable servos can be used in accordance with the requirements of each joint. 
         [0022]    Other objects and advantages of the invention will be apparent from the following description. In the description, reference is made to the accompanying drawings which form a part thereof, and in which there is shown by way of illustration a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a graphical representation of an embodiment of a robot control system according to the present invention; 
           [0024]      FIG. 2  is a schematic diagram illustrating the connecting relationship between a reference voltage generator, a controller and servos used in a robot according to the present invention; 
           [0025]      FIG. 3  is a schematic diagram illustrating a reference voltage generator used in a robot according to the present invention; 
           [0026]      FIGS. 4(   a )-( c ) are graphical representations of signal statuses between a controller and a servo in a robot according to the present invention; 
           [0027]      FIGS. 5(   a )-( c ) are graphical representations of signal statuses between a controller and a servo in a robot according to the present invention; 
           [0028]      FIGS. 6(   a )-( c ) are graphical representations of signal statuses between a controller and a servo in a robot according to the present invention; 
           [0029]      FIG. 7  is a schematic diagram illustrating one embodiment of a circuit for a reference voltage generator in a robot according to the present invention; 
           [0030]      FIG. 8  is a schematic diagram illustrating another embodiment of a circuit for a reference voltage generator in a robot according to the present invention; 
           [0031]      FIG. 9  is a graphical representation of another embodiment of a robot system including a hub having a main hub and secondary hubs according to the present invention; 
           [0032]      FIG. 10  is a schematic diagram illustrating an embodiment of the main hub of  FIG. 9  according to the present invention; 
           [0033]      FIG. 11  is a schematic diagram illustrating an embodiment of a circuit of the main hub of  FIG. 9  according to the present invention; 
           [0034]      FIG. 12  is a front view of a robot constructed in accordance with one aspect of the present invention; 
           [0035]      FIG. 13(   a ) is a front view of an exemplary frame part used in the robot of  FIG. 12 ; 
           [0036]      FIG. 13(   b ) is a side view of the frame part of  FIG. 13(   a ); 
           [0037]      FIG. 14  is a fragmented partially exploded perspective view illustrating the attachment of a servo system to the frame part of  FIG. 13(   a ); 
           [0038]      FIG. 15  is a graphical representation illustrating a connection between a conventional controller and servos; and 
           [0039]      FIG. 16  is a graphical representation illustrating the entire configuration of a conventional controller. 
       
    
    
       [0040]    Before an exemplary embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    The present invention will be explained in detail by referring to the attached drawings.  FIG. 1  shows a transmission path  15  between a controller  20  and first and second servos  10 ,  31  in a robot according to the present invention. A reference voltage generator circuit  30  is disposed on the transmission path  15  between the controller  20  and the servos  10 ,  31 . Other structures in the controller  20  and the communication scheme between the controller  20  and a personal computer or radio are conventional as detailed above. 
         [0042]      FIG. 2  shows the connection relationship between the controller  20 , the reference voltage generator circuit  30  and the servos  10 ,  31 , with the respective parts close to an actual transmission path  15 . The controller  20  is preferably a conventional RS485 scheme controller. The transmission path  15  between the controller  20  and the servos  10 ,  31  comprises four lines—two signal lines A, B, or first and second signal lines, respectively, each carrying a differential signal, the ground line GND, and a power line Vcc for supplying a power source voltage. 
         [0043]    A terminal resistor (not shown) may be inserted, if required, at the end of the signal line to match the impedance of the transmission node with the impedance of the receiving node, so that reflection loss can be decreased. In the robot of the exemplary embodiment, the terminal resistor is not inserted because of the short transmission path and small reflection effect. 
         [0044]    The first servo  10  operates in a conventional RS485 communication scheme. The second servo  31  operates in the TTL scheme. In the present example, both servos  10 ,  31  are used in a mixed manner. The second servo  31  is connected only to signal line A, acting as a first signal line. In the set of four transmission lines, the second servo  31  uses three signal lines including the ground line GND, the power line Vcc, and the signal line A. 
         [0045]    Referring now to  FIG. 3 , one embodiment of the reference voltage generator circuit  30  is shown. In the reference voltage generator circuit  30 , a first resistor R 1  and a Zener diode D 1  are connected serially between the power line Vcc and the ground line GND. The anode of the Zener diode D 1  is connected to the ground line GND. A second resistor R 2  has one end connected to the signal line B, or a second signal line for transmitting an inversion signal, and the other end connected to the cathode of the Zener diode D 1 . 
         [0046]    The Zener diode D 1  biases the potential of the signal line B. When a control signal is not transmitted, the Zener diode D 1  holds the potential of the signal line B at 2 volts. When the control signal is being transmitted, the potential of the signal line B varies according to the control signal. 
         [0047]    The second resistor R 2  has a value of several kilohms (e.g., 3.3 kΩ) to control the current flowing through the Zener diode D 1 . The first resistor R 1  is used to reduce switching noise due to the Zener diode D 1 . The resistor R 1  is designed so as to allow the flow of some current through the Zener diode D 1 , even at an L level. This feature allows the potential of the signal line B when the control signal is transmitted, to be held at a constant value, even without the first resistor R 1 . 
         [0048]    Referring also to  FIGS. 4(   a )-( c ), explanation will be made as to the waveform of the signals between the controller  20  and the first servo  10 . The function of the reference voltage generator circuit  30  will also be further explained. The controller  20  produces differential signals shown in  FIG. 4(   a ). The first servo  10 , connected to the two signal lines A and B, receives the differential signals. The first servo  10 , which employs the RS485 communication scheme, can read out control signals H and L as logical values, as shown in  FIG. 4(   c ). 
         [0049]    The second, i.e., TTL scheme, servo  31  is connected to the signal line A, but not to the signal line B. For that reason, when the controller  20  transmits a control signal to the second servo  31 , the servo  31  reads only the signal on the signal line A, shown in  FIG. 5(   b ), of the differential signal transmitted from the controller  20 , as shown in  FIG. 5(   a ). Thus, the H level corresponds to five volts while the L level corresponds to the ground level (about zero volts). The servo can easily recognize the control signal as a logical value of H or L, as shown in  FIG. 5(   c ). 
         [0050]    In contrast, when the second servo  31  transmits a control signal to the controller  20 , the transmission driver of the second servo  31  outputs a control signal via the signal line A, as shown in  FIG. 6(   a ), but does not output any signal onto the signal line B. The receiving driver of the controller  20 , in the RS485 scheme, requires any definite control signal on the signal line B. However, even when the signal line B does not receive any control signal, the reference voltage generator circuit  30  biases the voltage by two volts, as shown in  FIG. 6(   b ). Accordingly, the controller  20  can recognize the received signal from the servo  31  in the RS485 scheme as an H level signal of five volts or an L level signal of two volts. Thus, the controller  20  can recognize the received signal as having a logical value of H or L, as shown in  FIG. 6(   c ). As a result, bidirectional communications can easily be performed between the controller  20  and the second servo  31 . 
         [0051]    In the reference voltage generator circuit  30  using the Zener diode D 1 , the power voltage depends on the type of secondary battery or the number of cells. Even when the potential of the power line Vcc is different, the bias potential can be held at a constant value. In the above embodiment, the reference voltage generator circuit  30  is disposed on the transmission path between the controller  20  and the servos. However, the same effect can be obtained even in the case where the reference voltage generator circuit is disposed between servos or on the termination of the transmission path. 
         [0052]    When the transmission line becomes longer and a large number of servos are connected to a single transmission line, the effect due to the load resistance of the servo in the previous stage appears on the servo at the distal end. That effect causes the potential to boost the potential of the signal line B fixed by the reference voltage generator circuit. In contrast, the signal line A is affected by the voltage drop due to the resistance of the transmission line, so that the difference in potential between the H level and the L level is inevitably decreased. A servo may be disposed near the distal end of the transmission line and between the second, i.e., TTL scheme, servo  31  and the controller  20 . In such a case, since the second servo  31  in the TTL scheme is particularly susceptible to the influence of noise, even when noise is induced onto the transmission path, differential signals are received with noise in phase occurring on the two signal lines A and B. As a result, the noise resistance characteristic can be improved. 
         [0053]      FIG. 7  shows another embodiment of the reference voltage generator circuit according to the present invention. The robot configuration, the controller, the transmission line, and the like are applied similarly to those described in the previous embodiment. In the reference voltage generator circuit  40  shown in  FIG. 7 , a diode D 2  is disposed between the resistors R 1  and R 2  inserted between the power line Vcc and the signal line B, with the anode of the diode D 2  connected to the resistor R 1 . A resistor R 3  is connected between the signal line A and the anode of the diode D 2 . 
         [0054]    In the present invention, the resistor R 3  preferably functions as a load resistance of 2.2 kΩ, which is set at a lower value than that of the resistor R 2 . In such a configuration, when any signal is not transmitted to the transmission path, the signal line A can be set in such a way that the potential is always higher than the potential of the signal line B by the forward bias component of the diode D 2  (for example, 0.6 volts). Accordingly, there is no case where such a received signal is determined as having an L logical value. This feature has the advantage that even if the process is resumed from the wrong decision about reception of the start bit signal, and an erroneous decision is made as a result, no unnecessary process is executed. 
         [0055]    In the previous embodiment, the Zener diode D 1  is used so as to handle variations in the power source voltage of seven to twelve volts. However, if the power source voltage is determined in advance, a voltage divider circuit  50 , which comprises resistors R 4  and R 5  and a capacitor C 1 , can bias the potential of the signal line B, as shown in  FIG. 8 . The resistance values of the resistors R 4 , R 5  are adjusted according to the potential applied to the power line Vcc and the bias potential to the signal line B. 
         [0056]    In the previous embodiment, the reference voltage generator circuit may be disposed at a desired point along the transmission path. However, the reference voltage generator circuit may be incorporated in the cabinet, together with the controller. Alternatively, the reference voltage generator circuit may be incorporated in a hub, disposed on the transmission path, which branches a single signal line into plural signal lines. 
         [0057]    As shown in  FIG. 9 , the hub is disposed on the transmission path  15  to which the controller  20  and the servos  10 ,  31  are connected. The transmission path  15  is divided inside the hub  60 . A plurality of servos  10 ,  31  are connected to the hub  60 . The reference voltage generator circuit is actually incorporated and disposed in the upper portion of the body or the waist of a robot. A single hub has a plurality of reference voltage generator circuits corresponding to respective limbs or body parts, for example, one leg including an ankle joint, a knee joint, and a hip joint, one arm including a shoulder, an elbow, and a wrist, and the like. The hubs  60  are disposed on a transmission path  15  with good routing, depending on the number of ports of the hubs and the number and position of the servos to be used. 
         [0058]    The hub  60  comprises a main hub  60   a , disposed near the power source  22 , and secondary hubs  60   b , disposed in other portions. The main hub  60   a  is disposed in the body of a robot and is connected to the power via a switch. The main hub  60   a  supplies a power source voltage to the controller  20  and the servos  10 ,  31  via the transmission path  15 . The reference voltage generator circuit is disposed in the main hub  60   a.    
         [0059]    The specific configuration of the main hub  60   a  will be explained with reference to  FIGS. 10 and 11 .  FIG. 10  is a schematic diagram illustrating the main hub  60   a . The main hub  60   a  comprises the reference voltage generator circuit  61  connected to a transmission path, including two signals lines A and B, the ground line GND, and the power line Vcc, and plural ports  62 , each being formed by branching the transmission path. This is connected on the one hand to the controller  20  and other hubs  60   a  via the transmission path, and on the other to the servos  10 ,  31  via the port  62 . 
         [0060]    The circuit configuration will be explained more specifically with reference to  FIG. 11 . In the reference voltage generator circuit  61  shown in  FIG. 11 , the first resistor R 1  and the Zener diode D 1  are connected serially between the power line Vcc and the ground line GND. The anode of the Zener diodes D 1  is connected to the ground line GND. The second resistor R 2  has one end connected to the signal line B, which transmits an inversion signal, and the other end connected to the cathode of the Zener diode D 1 . In the present embodiment, the Zener diode D 1  biases the potential of the signal line B. The Zener diode D 1  holds the potential of the signal line B at 2 volts when the control signal is not transmitted. When the control signal is being transmitted, the potential of the signal line B varies according to the control signal. 
         [0061]    The second resistor R 2 , which has a value of several kilohms (3.3 kΩ), controls the current flowing through the Zener diode D 1 . The first resistor R 1  reduces the switching noise due to the Zener diode D 1 . Current is flowing slightly through the Zener diode D 1  even at an L level. Deletion of the first resistor R 1  does not influence the effect of holding the signal line B at a constant potential when the control signal is transmitted. As is apparent, the reference voltage generator circuit  61  is similar to the reference voltage generator circuit  30  shown in  FIG. 3 . Like the previous embodiment, the Zener diode D 1  clips the potential of the signal line B to a constant value and the controller  20  uses a differential signal in the RS485 scheme. However, in mixed mode, the above-mentioned configuration can use both the servo  10 , which uses a differential signal as a control signal, and the servo  31 , which uses the TTL scheme. 
         [0062]    As described above, by disposing the reference voltage generator circuit  61  in the main hub  60   a , parts such as the substrate configuring the main hub  60   a  can be shared. Compared with arrangements where additional reference voltage generator circuits are disposed on the transmission path, the number of parts and installation spaces can be reduced advantageously. The reference voltage generator circuit  61  may be mounted on the secondary hub  60   b , but the secondary hub  60   b  is somewhat large. Accordingly, arrangement of space must be considered in the layout design. 
         [0063]    The reference voltage generator circuits  40  and  50 , shown in  FIGS. 7 and 8 , have the same effect as that of the reference voltage generator circuit  61  in the hub  60 . As described above, the reference voltage generator circuit is disposed independently on the transmission line, is housed in the same cabinet together with the controller  20 , or is disposed in the hub on the transmission line. However, the reference voltage generator circuit can be effectively disposed at any point on the transmission between the first and second servos  10 ,  31  and the controller circuit. 
         [0064]    The reference voltage generator circuit has been applied to a humanoid robot. However, the reference voltage generator circuit may be applied to quadruped animal-type robots, fish-type robots, dinosaur-type robots, monstrous beast-type robots, centipede-type robots, or snake-type robots. The reference voltage generator circuit can be particularly effective when applied to snake-like robots, which have long transmission paths and a large number of servos connected to a single transmission path. 
         [0065]    In the forgoing embodiments, explanation has been made as to a negative logical circuit in which an H level is the normal status. However, the present invention is applicable to a configuration in which an L level is the normal status, and this is changed to an H level when a signal is input. In the explanation, the signal line B that transmits an inversion signal corresponds to a second signal line. However, the reference voltage generator circuits  30 ,  50  and  61 , shown respectively in  FIGS. 3 ,  8  and  11 , can use the signal line A as the second signal line to read out the control signal as an H or L logical value. Moreover, when the signal line A is used as the second signal line, the reference voltage generator circuit  40 , shown in  FIG. 7 , cannot be applied to a logical circuit with an H level as the normal status, but is suitable for a logical circuit with an L level as the normal status. 
         [0066]    The present invention includes a controller using the RS485 scheme which outputs a differential signal as a control signal in serial communications. Accordingly, the present invention is applicable to robots including a servo, acting as an actuator, which uses a differential signal as a control signal, and a servo using a signal in the TTL scheme as a control signal. 
         [0067]    The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.