Patent Publication Number: US-9845101-B2

Title: Pushcart

Description:
This is a continuation of International Application No. PCT/JP2015/074585 filed on Aug. 31, 2015 which claims priority from Japanese Patent Application No. 2014-178730 filed on Sep. 3, 2014. The contents of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to pushcarts that have right and left wheels, and particularly relates to pushcarts configured to control and drive the wheels. 
     Two-wheel riding vehicles configured to drive and control the wheels have been known (for example, see Patent Document 1). A two-wheel riding vehicle disclosed in Patent Document 1 includes a base, a boarding platform supported on the base with a spring, a motor provided on the base, and right and left wheels driven by the motor. This two-wheel riding vehicle further includes a rate gyroscope, a base tilt sensor, a boarding platform tilt sensor, a motor rotation angle sensor, and a control unit. 
     The control unit performs self-control using a wheel-type inverted pendulum stabilization control method based on signals of the rate gyroscope, the base tilt sensor, and the motor rotation angle sensor. In addition, the control unit detects weight movement of a user in a front-rear direction with the signal of the base tilt sensor so as to perform forward-backward movement control, and detects the weight movement of the user in a right-left direction with the signals of the boarding platform tilt sensor and the base tilt sensor so as to perform travelling direction control. 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-94898 
     BRIEF SUMMARY 
     With the two-wheel riding vehicle of Patent Document 1, in the case where only one of the two wheels falls into a groove and thereafter comes out of the groove, forces largely different from each other are temporarily exerted on the right and left wheels. In the case where motors for separately driving the right and left wheels are controlled by a general PI control operation, even if one of the wheels comes out of the groove into which the stated wheel has fallen and the loads exerted on the right and left wheels have come to be approximately equal to each other, output of each integral operation with respect to the right and left wheels remains largely different from each other. This raises a risk that a user cannot make the two-wheel riding vehicle travel in a straight line temporarily. 
     The present disclosure provides a pushcart that a user can make travel in a straight line as intended even if the loads exerted on the right and left wheels temporarily become largely different from each other. 
     A pushcart according to the present disclosure includes a main body, a first wheel, a second wheel, a first wheel driver, a second wheel driver, a control unit, and a wheel angular velocity detector. The first wheel is provided at the left side of the main body in a travelling direction. The second wheel is provided at the right side of the main body in the travelling direction. The first wheel driver rotates the first wheel about a rotational shaft of the first wheel. The second wheel driver rotates the second wheel about a rotational shaft of the second wheel. The wheel angular velocity detector detects an angular velocity of each of the first wheel and the second wheel about the rotational shafts thereof. The pushcart of the disclosure performs feedback control, at least using an integral operation, on the angular velocities of the first wheel and the second wheel about the rotational shafts thereof. The control unit calculates a weighted average of an integral element with respect to the angular velocity of the first wheel about the rotational shaft thereof and an integral element with respect to the angular velocity of the second wheel about the rotational shaft thereof, and then separately controls the first wheel driver and the second wheel driver based on the weighted average. 
     With this configuration, by adjusting the weights in the calculation of the weighted average, output of the integral operation with respect to the first wheel and output of the integral operation with respect to the second wheel can be appropriately averaged. This makes it possible to make the pushcart travel in a straight line as intended by the user even if loads exerted on the first and second wheels are significantly different from each other temporarily. 
     In the pushcart according to the disclosure, the control unit may make the weighted average approach an arithmetic average as a difference between an angular velocity command value of the first wheel and an angular velocity command value of the second wheel becomes smaller. With this configuration, in the case where a user attempts to make the pushcart travel straight in the travelling direction, the output of the integral operation with respect to the first wheel and the output of the integral operation with respect to the second wheel become equal to each other. Because of this, even if the loads exerted on the first wheel and the second wheel are significantly different from each other temporarily, the user can make the pushcart travel in a straight line. Further, in the case where the user attempts to turn the pushcart (attempts to revolve in a yaw direction), the respective integral operations with respect to the first wheel and second wheel become independent of each other. This makes it possible for the user to turn the pushcart as intended. 
     The pushcart according to the disclosure may be configured as follows. That is, the pushcart of the disclosure includes a braking operation reception portion that receives a braking operation with respect to the first wheel and the second wheel. The control unit makes the weighted average approach the arithmetic average as an operation amount of the braking operation becomes larger. With this configuration, in the case where the user attempts to stop the pushcart, the output of the integral operation with respect to the first wheel and the output of the integral operation with respect to the second wheel become equal to each other. Because of this, even if the output of the integral operation with respect to the first wheel and the output of the integral operation with respect to the second wheel are largely different from each other before the braking operation, the user can stop the pushcart without necessarily turning the pushcart in the yaw direction. 
     According to the present disclosure, even if the loads exerted on the right and left wheels become largely different from each other temporarily, the user can make the pushcart travel in a straight line as intended. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a left side view of a pushcart according to a first embodiment. 
         FIG. 2A  is a front view of the pushcart according to the first embodiment. 
         FIG. 2B  is a plan view of the pushcart according to the first embodiment. 
         FIG. 3  is a block diagram illustrating a hardware configuration of the pushcart according to the first embodiment. 
         FIG. 4  is a control configuration diagram of a control unit according to the first embodiment. 
         FIG. 5A  is a schematic plan view illustrating operations of a pushcart in an existing configuration. 
         FIG. 5B  is a schematic plan view illustrating operations of the pushcart according to the first embodiment. 
         FIG. 6  is a block diagram illustrating a configuration of a pushcart according to a second embodiment. 
         FIG. 7  is a control configuration diagram of a control unit according to the second embodiment. 
         FIG. 8  is another control configuration diagram of the control unit according to the second embodiment. 
         FIG. 9  is a block diagram illustrating a configuration of a pushcart according to a third embodiment. 
         FIG. 10  is a control configuration diagram illustrating part of a control unit according to the third embodiment. 
         FIG. 11  is an exterior appearance perspective view of a baby carriage according to a fourth embodiment. 
         FIG. 12  is a left side view of the baby carriage according to the fourth embodiment. 
         FIG. 13  is a front view of the baby carriage according to the fourth embodiment. 
         FIG. 14  is a rear view of the baby carriage according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A pushcart  10  according to a first embodiment of the present disclosure will be described.  FIG. 1  is a left side view of the pushcart  10 ,  FIG. 2A  is a front view of the pushcart  10 , and  FIG. 2B  is a plan view of the pushcart  10 . 
     The pushcart  10  includes a main body  11  formed in a shape which is relatively long in a vertical direction (a Z direction in the drawing) and is relatively short in a depth direction (a Y direction in the drawing) as well as in a right-left direction (an X direction in the drawing). In a lower portion of the main body  11  downward in the vertical direction, a pair of main wheels  12  is respectively attached to right and left ends thereof in a travelling direction. The main wheels  12  are configured of a left wheel  12 A and a right wheel  12 B. 
     The left wheel  12 A (first wheel) is provided at the left side of the main body  11  in the travelling direction (a positive direction of the Y axis). The right wheel  12 B (second wheel) is provided at the right side of the main body  11  in the travelling direction. The left wheel  12 A rotates about a rotational shaft (axle) of the left wheel  12 A having a central axis in the right-left direction. The right wheel  12 B rotates about a rotational shaft of the right wheel  12 B having a central axis in the right-left direction. Note that the main wheels  12  do not rotate relative to the main body  11  when viewed in the vertical direction. To rephrase, the main wheels  12  do not change their orientations relative to the main body  11 . 
     The main body  11 , formed of two rod-like members respectively linked to the main wheels  12  and connected at the upper portion thereof, is rotatable about the shafts of the main wheels  12  in a pitch direction. However, it is not necessary for the main body  11  to be formed of two rod-like members like in this example; the main body  11  may be formed of a single rod-like member, or formed of a thin plate-like member. In the vicinity of the lower portion of the main body  11 , there is disposed a box  16  storing a control board, a battery, and the like. Note that, in reality, a cover is attached to the main body  11  so that the board and the like inside the main body  11  are not seen from the exterior. 
     A holding section  15  is formed in a cylinder shape being long in the right-left direction, is bent near the right and left ends in a reverse direction (toward a rear side) with respect to the travelling direction, and then extends toward the rear side. With this, the position at which a user U holds the holding section  15  can be shifted toward the rear side, thereby making it possible to widen a space at the feet of the user U. 
     Support members  13  formed in a thin plate-like shape and extending toward the rear side are linked to the rotational shafts of the main wheels  12 . The support members  13  are connected to the rotational shafts of the main wheels  12  in a rotatable manner in the pitch direction so as to extend in parallel to a road surface. 
     Auxiliary wheels  14  are each linked to a lower surface of the support member  13  on the opposite direction side relative to a side where the support member  13  is linked to the rotational shaft of the main wheel  12 . This allows both the main wheel  12  and the auxiliary wheel  14  to make contact with the road surface. The support member  13  extends toward the rear side of the travelling direction farther than the main wheel  12 . As a result, the main wheel  12  having a relatively large inside diameter is disposed on the front side in the travelling direction, which makes it easy to ride over a step. The support member  13  may be disposed in a mode in which it is extended toward the front side in the travelling direction farther than the main wheel  12  so that the auxiliary wheel  14  is disposed on the front side in the travelling direction farther than the main wheel. With the mode in which the support member  13  extends toward the front side farther than the main wheel  12 , the space at the feet of the user U can be widened. 
     Although  FIGS. 1 and 2  illustrate a state in which the auxiliary wheels  14  are in contact with the road surface, the pushcart  10  can be self-standing by performing inverted pendulum control even if only the main wheels  12  are in a state of making contact with the road surface. 
     Further, in this example, the support members  13  and the auxiliary wheels  14  both in the number of two are so provided as to be linked to the respective rotational shafts of the right and left main wheels  12 ; however, the support members  13  and the auxiliary wheels  14  may be provided in a mode in which each of the number of the support members  13  and the number of the auxiliary wheels  14  is one or more than two. Note that, by the support members and auxiliary wheels being linked to the rotational shafts of the right and left main wheels  12  as shown in  FIGS. 2A and 2B , the space at the feet of the user U can be widened. 
     In the holding section  15 , a user interface (I/F)  27  including a power switch and the like is provided. The user U can push the pushcart  10  forward in the travelling direction by holding the holding section  15 . Alternatively, by placing his or her forearms or the like on the holding section  15  so as to press from above without necessarily holding the holding section  15 , the user U can also push the pushcart  10  forward in the travelling direction by making use of friction generated between the holding section  15  and the forearms or the like while placing the forearms or the like on the holding section  15 . 
     A hardware configuration and operations of the pushcart  10  will be described next.  FIG. 3  is a block diagram illustrating the configuration of the pushcart  10 . The pushcart  10  includes a control unit  21 , a ROM  22 , a RAM  23 , a left wheel driver  24 A, a right wheel driver  24 B, a left wheel rotary encoder  25 A, a right wheel rotary encoder  25 B, a main body rotary encoder  26 , and the user interface (I/F)  27 . 
     The control unit  21  is a functional unit configured to integrally control the pushcart  10 , and realizes various kinds of operations by reading out programs stored in the ROM  22  and executing the above programs in the RAM  23 . 
     The left wheel driver  24 A (first wheel driver) is a functional unit configured to supply power to the left wheel  12 A by driving a motor for rotating the rotational shaft attached to the left wheel  12 A, and drives the motor of the left wheel  12 A based on an output signal of the control unit  21  so as to rotate the left wheel  12 A about the rotational shaft of the left wheel  12 A. The right wheel driver  24 B (second wheel driver) is a functional unit configured to supply power to the right wheel  12 B by driving a motor for rotating the rotational shaft attached to the right wheel  12 B, and drives the motor of the right wheel  12 B based on an output signal of the control unit  21  so as to rotate the right wheel  12 B about the rotational shaft of the right wheel  12 B. 
     The left wheel rotary encoder  25 A detects a rotational angle of the left wheel  12 A about the rotational shaft of the left wheel  12 A, and outputs the detection result to the control unit  21 . The right wheel rotary encoder  25 B detects a rotational angle of the right wheel  12 B about the rotational shaft of the right wheel  12 B, and outputs the detection result to the control unit  21 . The main body rotary encoder  26  detects an intersection angle which is an angle formed by the main body  11  and the support member  13 , and outputs the detection result to the control unit  21 . Hereinafter, the above-mentioned intersection angle is referred to as a pitch angle. The pitch angle may be detected by a potentiometer rather than only by the rotary encoder. 
       FIG. 4  is a control configuration diagram of the control unit  21 . The control unit  21  separately controls the left wheel driver  24 A and the right wheel driver  24 B. The control unit  21  performs feedback control on the angular velocities of the left wheel  12 A and the right wheel  12 B about the rotational shafts thereof using PI control. The control unit  21  includes a PI control section  31 A, a PI control section  31 B, a differential element  35 A, a differential element  35 B, and a wheel angular velocity command generator  36 . 
     The wheel angular velocity command generator  36  calculates a wheel angular velocity command value ωtr based on a pitch angle θh of the main body  11  detected by the main body rotary encoder  26  and a pitch angle command value θhr. The pitch angle command value θhr is a target value for the pitch angle θh of the main body  11 . For example, in the case where the pushcart  10  is on a level road surface, when the main body  11  is so controlled as to be vertical to the road surface, the pitch angle command value θhr is set to 90 degrees. The wheel angular velocity command value ωtr is a target value for the angular velocity of the main wheel  12  about the rotational shaft of the main wheel  12 , and is determined so that the pitch angle θh will come to be the pitch angle command value θhr. In the pushcart  10 , the target value for the angular velocity of the left wheel  12 A about the rotational shaft thereof and the target value for the angular velocity of the right wheel  12 B about the rotational shaft thereof are equal to each other. The wheel angular velocity command value ωtr is calculated, for example, by using a formula ωtr=K A  (θhr−θh), where K A  is a proportional gain. 
     The differential element  35 A differentiates a rotational angle θt 1  of the left wheel  12 A detected by the left wheel rotary encoder  25 A to calculate an angular velocity ωt 1  of the left wheel  12 A about the rotational shaft of the left wheel  12 A. The differential element  35 B differentiates a rotational angle θt 2  of the right wheel  12 B detected by the right wheel rotary encoder  25 B to calculate an angular velocity ωt 2  of the right wheel  12 B about the rotational shaft of the right wheel  12 B. The left wheel rotary encoder  25 A and the differential element  35 A, and the right wheel rotary encoder  25 B and the differential element  35 B correspond to a wheel angular velocity detector of the present disclosure. 
     The PI control section  31 A performs PI control taking an angular velocity deviation value ωe 1 =ωtr−ωt 1  as a control deviation. The PI control section  31 B performs PI control taking an angular velocity deviation value ωe 2 =ωtr−ωt 2  as a control deviation. The left wheel driver  24 A applies torque to the left wheel  12 A in accordance with the output of the PI control section  31 A. The right wheel driver  24 B applies torque to the right wheel  12 B in accordance with the output of the PI control section  31 B. 
     The PI control section  31 A includes a proportional operation portion  32 A, an integral operation portion  33 A, and a coefficient processing portion  34 A. The PI control section  31 B includes a proportional operation portion  32 B, an integral operation portion  33 B, and a coefficient processing portion  34 B. The proportional operation portion  32 A calculates a proportional term pe 1 =K p ωe 1  taking the angular velocity deviation value ωe 1  as a control deviation. The proportional operation portion  32 B calculates a proportional term pe 2 =K p ωe 2  taking the angular velocity deviation value ωe 2  as a control deviation. Here, K p  is a proportional gain. The integral operation portion  33 A calculates an integral term ie 1 ′ by multiplying a time integral of the angular velocity deviation value ωe 1  by an integral gain K I . The integral operation portion  33 B calculates an integral term ie 2 ′ by multiplying a time integral of the angular velocity deviation value ωe 2  by the integral gain K I . 
     The coefficient processing portion  34 A calculates a·ie 1 ′ taking the integral term ie 1 ′ as input. The coefficient processing portion  34 B calculates a·ie 2 ′ taking the integral term ie 2 ′ as input. A coefficient “a” represents, as explained later, a level of averaging of an integral term ie 1  and an integral term ie 2 . To rephrase, the coefficient a represents a level of sharing of the integral operation portion  33 A and the integral operation portion  33 B. The coefficient a is set within a range of 0.5≦a≦1.0. In the first embodiment, the coefficient a is set to 0.5. 
     The PI control section  31 A calculates the integral term ie 1  by obtaining an arithmetic average of the integral term ie 1 ′ and the integral term ie 2 ′. The PI control section  31 B calculates the integral term ie 2  by obtaining an arithmetic average of the integral term ie 1 ′ and the integral term ie 2 ′. The integral term ie 1  and the integral term ie 2  are expressed in a time region by the following formula.
 
[Formula]
 
 ie 1( t )= ie 2( t )=0.5· K   I   ∫ωe 1( t ) dt+ 0.5· K   I   ∫ωe 2( t ) dt    (1)
 
     The first term of Formula (1) corresponds the integral term ie 1 ′, and the second term of Formula (1) corresponds to the integral term ie 2 ′. The integral term ie 1 ′ corresponds to an “integral element with respect to an angular velocity of a first wheel about a rotational shaft thereof” of the present disclosure. The integral term ie 2 ′ corresponds to an “integral element with respect to an angular velocity of a second wheel about a rotational shaft thereof” of the present disclosure. The integral term ie 1  and the integral term ie 2  correspond to a “weighted average” of the present disclosure. The PI control section  31 A outputs a sum of the proportional term pe 1  and the integral term ie 1 . The PI control section  31 B outputs a sum of the proportional term pe 2  and the integral term ie 2 . 
     As discussed above, the pushcart  10  performs inverted pendulum control and controls its posture so that the pitch angle θh of the main body  11  is maintained at the pitch angle command value θhr. Further, in the case where tilting the main body  11  is continued so that a difference between the pitch angle θh and the pitch angle command value θhr becomes a value of not zero, the pushcart  10  keeps rotating the main wheels  12  about the rotational shafts thereof so as to maintain the pitch angle θh at the pitch angle command value θhr. With this, the pushcart  10  moves forward or backward. 
       FIG. 5A  is a schematic plan view illustrating operations of a pushcart  40  in an existing configuration.  FIG. 5B  is a schematic plan view illustrating operations of the pushcart  10 . Arrows extending toward the front side of each pushcart indicate a travelling direction of the pushcart in the case where only the left wheel  12 A fell into a groove when a user attempted to move the pushcart straight in the travelling direction, and thereafter the left wheel  12 A has come out of the groove. A length of the arrow on the left wheel  12 A side represents magnitude of the integral term ie 1 , while a length of the arrow on the right wheel  12 B side represents magnitude of the integral term ie 2 . Note that in  FIGS. 5A and 5B , part of the configuration of the pushcart is omitted. 
     The pushcart  40  is configured in the same manner as the pushcart  10  except that the PI control section  31 A and the PI control section  31 B are independent of each other, in other words, the coefficient a is set to 1. In the case where the left wheel  12 A falls into a groove and a large load is exerted on the left wheel  12 A, the angular velocity ωt 1  of the left wheel  12 A becomes slow so that an absolute value of the angular velocity deviation value ωe 1  becomes large. Then, the angular velocity deviation value ωe 1  of this time is accumulated in the integral term ie 1 . With this, even when the left wheel  12 A has come out of the groove and started its normal rotation about the rotational shaft thereof, the integral term ie 1  maintains a large value. Meanwhile, because a normal load is exerted on the right wheel  12 B and the PI control sections  31 A,  31 B are independent of each other, the integral term ie 2  maintains the normal value. This makes the toque applied to the left wheel  12 A larger than the torque applied to the right wheel  12 B. As a result, the pushcart  40  turns right (revolves clockwise in the yaw direction) against the intention of the user. Because the angular velocity command value ωtr is common to the left wheel  12 A and the right wheel  12 B, the proportional terms pe 1  and pe 2  work so as to suppress the turn of the pushcart  40 . However, at the time when the pushcart  40  starts the turn, the integral terms more contribute to the turn of the pushcart  40  than the proportional terms. 
     In the case of the pushcart  10 , as expressed by Formula (1), the integral term ie 1  equals the integral term ie 2  regardless of the angular velocity deviation value ωe 1  or the angular velocity deviation value ωe 2 . Further, as discussed above, the proportional term pe 1  and the proportional term pe 2  work so as to suppress the turn of the pushcart  10 . As such, the pushcart  10  travels in a straight line as intended by the user after the left wheel  12 A has come out of the groove. When the pushcart  10  travels in a straight line and the angular velocity ωt 1  and the angular velocity ωt 2  become equal to each other, the integral term ie 1  and the integral term ie 2  become the same in terms of numerical values as in the case where the PI control section  31 A and the PI control section  31 B are independent of each other. With this, the angular velocity ωt 1  and the angular velocity ωt 2  are so controlled as to be the angular velocity command value ωtr. 
     In the first embodiment, as expressed by Formula (1), the integral term ie 1  and the integral term ie 2  are equal to each other. That is to say, even if the loads exerted on the left wheel  12 A and the right wheel  12 B significantly differ from each other temporarily, the integral terms are averaged so that the deviation in the integral terms is dispersed. With this, even in the case where the loads exerted on the left wheel  12 A and right wheel  12 B significantly differ from each other and thereafter they become substantially equal to each other, the pushcart  10  can be controlled to travel in a straight line as intended by the user. In other words, the pushcart  10  can travel in a straight line as intended by the user even when the pushcart  10  has returned to a state of normal travelling from a state in which different loads were exerted on the left wheel  12 A and the right wheel  12 B. 
     Second Embodiment 
     A pushcart  50  according to a second embodiment of the present disclosure will be described.  FIG. 6  is a block diagram illustrating a configuration of the pushcart  50 . The pushcart  50  includes a control unit  51  in place of the control unit  21  of the first embodiment, and includes, in addition to the configuration of the first embodiment, a turn angular velocity command reception portion  58 . The turn angular velocity command reception portion  58  receives an operation to turn the pushcart  50  carried out by a user, and outputs a turn angular velocity command value ωc. The turn angular velocity command value ωc is a target value for an angular velocity of the pushcart  50  in the yaw direction. 
       FIGS. 7 and 8  are control configuration diagrams of the control unit  51 . The control unit  51  includes a PI control section  61 A, a PI control section  61 B, and a wheel angular velocity command generator  66  in place of the PI control section  31 A, the PI control section  31 B, and the wheel angular velocity command generator  36  of the first embodiment. The wheel angular velocity command generator  66  calculates a wheel angular velocity command value ωtr 1  and a wheel angular velocity command value ωtr 2  based on the pitch angle θh of the main body  11 , the pitch angle command value θhr, and the turn angular velocity command value ωc. The wheel angular velocity command value ωtr 1  is a target value for the angular velocity of the left wheel  12 A about the rotational shaft thereof. The wheel angular velocity command value ωtr 2  is a target value for the angular velocity of the right wheel  12 B about the rotational shaft thereof. The wheel angular velocity command value ωtr 1  and the wheel angular velocity command value ωtr 2  are so determined as to realize the pitch angle command value θhr and the turn angular velocity command value ωc. 
     The PI control section  61 A performs PI control taking an angular velocity deviation value ωe 1 =ωtr 1 −ωt 1  as a control deviation. The PI control section  61 B performs PI control taking an angular velocity deviation value ωe 2 =ωtr 2 −ωt 2  as a control deviation. The PI control section  61 A includes a coefficient processing portion  64 A in place of the coefficient processing portion  34 A of the first embodiment. The PI control section  61 B includes a coefficient processing portion  64 B in place of the coefficient processing portion  34 B of the first embodiment. 
     A coefficient “a” of the coefficient processing portion  64 A and the coefficient processing portion  64 B is set, as shown in  FIG. 8 , based on a difference Δωtr=|ωtr 1 −ωtr 2 |, which is a difference between the wheel angular velocity command value ωtr 1  and the wheel angular velocity command value ωtr 2 . The coefficient a becomes larger in a step-like pattern as the difference Δωtr becomes larger, for example. Then, in the case where the difference Δωtr is smaller than a threshold, that is, in the case where the user attempts to move the pushcart  50  in a straight line, the coefficient a becomes 0.5. In the case where the difference Δωtr is larger than the threshold, that is, in the case where the user attempts to turn the pushcart  50 , the coefficient a becomes 1.0. The coefficient a may change linearly with respect to the difference Δωtr. 
     The coefficient processing portion  64 A calculates a·ie 1 ′ taking the integral term ie 1 ′ as input. The coefficient processing portion  64 B calculates a·ie 2 ′ taking the integral term ie 2 ′ as input. Here, input-output of the coefficient processing portion  64 A and input-output of the coefficient processing portion  64 B are expressed in a time region. 
     The PI control section  61 A calculates the integral term ie 1  by obtaining a weighted average of the integral term ie 1 ′ and the integral term ie 2 ′. The PI control section  61 B calculates the integral term ie 2  by obtaining a weighted average of the integral term ie 1 ′ and the integral term ie 2 ′. The integral term ie 1  and the integral term ie 2  are expressed in a time region by the following formulas.
 
[Formulas]
 
 ie 1( t )= a ( t )· K   I   ∫ωe 1( t ) dt +(1− a ( t ))· K   I   ∫ωe 2( t ) dt    (2)
 
 ie 2( t )= a ( t )· K   I   ∫ωe 2( t ) dt +(1− a ( t ))· K   I   ∫ωe 1( t ) dt    (3)
 
     The first term of Formula (2) and the second term of Formula (3) correspond to the integral term ie 1 ′, while the second term of Formula (2) and the first term of Formula (3) correspond to the integral term ie 2 ′. The coefficient a is a weight in the calculation of the weighted average. As discussed above, as the difference Δωtr becomes smaller, the coefficient a approaches 0.5. Accordingly, as the difference Δωtr becomes smaller, the weighted average approaches the arithmetic average. 
     When the coefficient a is 0.5, the integral term ie 1  becomes equal to the integral term ie 2 . When the coefficient a is 1.0, the integral term ie 1  becomes equal to the integral term ie 1 ′ and the integral term ie 2  becomes equal to the integral term ie 2 ′. In other words, the PI control section  61 A becomes independent of the PI control section  61 B. In the manner as discussed above, by adjusting the value of the coefficient a, a level of averaging of the integral terms can be controlled, to rephrase, a level of dispersion of the deviation in the integral terms can be controlled. 
     In the second embodiment, in the case where a user attempts to make the pushcart  50  travel straight in the travelling direction, the coefficient a becomes 0.5 and the integral term ie 1  becomes equal to the integral term ie 2 . Because of this, the user can move the pushcart  50  in a straight line even after the loads exerted on the left wheel  12 A and the right wheel  12 B have become significantly different from each other temporarily. In the case where the user attempts to turn the pushcart  50 , the coefficient a becomes 1.0 and the PI control section  61 A and PI control section  61 B become independent of each other. With this, the angular velocity ωt 1  and the angular velocity ωt 2  are so controlled as to become the wheel angular velocity command value ωtr 1  and the wheel angular velocity command value ωtr 2 , respectively. This makes it possible for the user to turn the pushcart  50 . 
     Third Embodiment 
     A pushcart  70  according to a third embodiment of the present disclosure will be described.  FIG. 9  is a block diagram illustrating a configuration of the pushcart  70 . The pushcart  70  includes a control unit  71  in place of the control unit  51  of the second embodiment, and includes a braking operation reception portion  79  in addition to the configuration of the second embodiment. The braking operation reception portion  79  is provided, for example, on the holding section  15 . The braking operation reception portion  79  receives a braking operation with respect to the main wheels  12 , and outputs a braking operation amount “b”. The braking operation amount corresponds to an “operation amount of a braking operation” of the present disclosure. 
       FIG. 10  is a control configuration diagram illustrating part of the control unit  71 . The control unit  71  includes a wheel angular velocity command generator  76 , a coefficient processing portion  74 A, and a coefficient processing portion  74 B in place of the wheel angular velocity command generator  66 , the coefficient processing portion  64 A, and the coefficient processing portion  64 B of the second embodiment. 
     The wheel angular velocity command generator  76  calculates a wheel angular velocity command value ωtr 1  and a wheel angular velocity command value ωtr 2  based on the pitch angle θh of the main body  11 , the pitch angle command value θhr, the turn angular velocity command value ωc, and the braking operation amount b. The wheel angular velocity command value ωtr 1  and the wheel angular velocity command value ωtr 2  approach 0 as the braking operation amount b becomes larger, and become 0 when the braking operation amount b is at its maximum, that is, the stated command values become 0 when the user attempts to stop the pushcart  70 . 
     The coefficient a of the coefficient processing portion  74 A and the coefficient processing portion  74 B is set based on the braking operation amount b. For example, the coefficient a approaches 0.5 as the braking operation amount b becomes larger, and becomes 0.5 when the braking operation amount b is at its maximum. In other words, as expressed by Formulas (2) and (3), the weighted average approaches the arithmetic average as the braking operation amount b becomes larger. 
     In the third embodiment, in the case where a user attempts to stop the pushcart  70 , the coefficient a becomes 0.5 and the integral term ie 1  becomes equal to the integral term ie 2 . With this, the user can stop the pushcart  70  without necessarily the pushcart  70  being revolved in the yaw direction even if the integral term ie 1  and the integral term ie 2  significantly differ from each other before the braking operation. Note that, because the integral terms are averaged, the pushcart  70  slightly moves forward or backward in some case when being stopped. However, the user can freely change the pitch angle θh of the main body  11 , whereby the user can operate the pushcart  70  so that the pushcart  70  does not move forward or backward without necessarily having a clear consciousness. 
     Fourth Embodiment 
     A baby carriage according to a fourth embodiment of the present disclosure will be described. The stated baby carriage is an example of a pushcart of the present disclosure.  FIG. 11  is an exterior appearance perspective view of a baby carriage  80 .  FIG. 12  is a left side view of the baby carriage  80 .  FIG. 13  is a front view of the baby carriage  80 .  FIG. 14  is a rear view of the baby carriage  80 . The baby carriage  80  includes a main body  81 . The main body  81  is a frame-like member extending substantially in the vertical direction. 
     A pair of main wheels  12  is supported in a rotatable manner at an end portion on the lower side of the main body  81 . An auxiliary support member  83  is provided substantially at the center of the main body  81  so as to stick out toward the travelling direction side of the baby carriage  80 , and a pair of auxiliary wheels  84  is supported in a rotatable manner at an end portion of the auxiliary support member  83 . As such, in the baby carriage  80 , the pair of main wheels  12  are rear wheels and the pair of auxiliary wheels  84  are front wheels. A diameter of each main wheel  12  is longer than a diameter of the auxiliary wheel  84 . 
     An upper portion  811  of the main body  81  is slightly slanted toward the opposite side of the travelling direction of the baby carriage  80 , and a cylinder-shaped holding section  85  is provided at an end portion on the upper side of the main body  81 . In the holding section  85 , there are provided a user interface including a power switch and the like, and a holding force detector (either of them not shown). The holding force detector detects a force of a user (a person who pushes the baby carriage) holding the holding section  85  (holding force). The holding force detector is, for example, a contact sensor including a piezoelectric device or the like configured to detect a pressure force against the holding section  85 . 
     A seat  91  where a baby is seated is provided substantially at the center of the main body  81 . A backrest  92 , a sun shade  93 , and a front bar  94  are provided between a pair of frames of the upper portion  811  of the main body  81 . The backrest  92  is arranged along the frame of the upper portion  811  of the main body  81 . The sun shade  93  is so arranged as to cover an upper portion of the backrest  92 . The front bar  94  is formed in a substantially U shape, and both ends of the front bar  94  are attached to the frame of the upper portion  811  of the main body  81 . A box  16  is provided under the seat  91 . Inside the box  16 , there are stored a battery for supplying drive voltages to respective portions of the baby carriage  80 , a control board, and the like. 
     A hardware configuration and operations of the baby carriage  80  are the same as the hardware configuration and the operations of the pushcart  10  of the first embodiment (see  FIGS. 3 and 4 ). Note that, however, the baby carriage  80  includes the holding force detector in place of the main body rotary encoder  26  of the pushcart  10 . The wheel angular velocity command generator of the baby carriage  80  calculates a wheel angular velocity command value ωtr based on the holding force detected by the holding force detector. 
     The baby carriage  80  may be configured as follows. That is, the frame of the upper portion  811  of the main body  81  is attached to a frame of a lower portion  812  of the main body  81  in a rotatable manner in the pitch direction. The baby carriage  80  includes a main body rotary encoder for detecting an angle (pitch angle) formed by the frame of the upper portion  811  of the main body  81  and the frame of the lower portion  812  of the main body  81 . The wheel angular velocity command generator of the baby carriage  80  calculates a wheel angular velocity command value ωtr based on the pitch angle θh of the main body  81  detected by the main body rotary encoder and the pitch angle command value θhr, like in the pushcart  10  of the first embodiment. 
     Like the pushcart  50  of the second embodiment, the baby carriage  80  may operate in response to a turning operation by the user. Further, like the pushcart  70  of the third embodiment, the baby carriage  80  may operate in response to a braking operation by the user. 
     Although, in the above-described embodiments, the left wheel driver  24 A and the right wheel driver  24 B are controlled through PI control, the disclosure is not limited thereto. In the present disclosure, the left wheel driver  24 A and the right wheel driver  24 B may be controlled through PID control. 
     Further, although, in the above embodiments, the weighted averaging is carried out at the output side of the integral operation portion  33 A and the integral operation portion  33 B, the disclosure is not limited thereto. In the present disclosure, the weighted averaging may be carried out at the input side of the integral operation portions in the case where the coefficient a is set to a constant value. 
     REFERENCE SIGNS LIST 
     U USER 
       10 ,  40 ,  50 ,  70  PUSHCART 
       11 ,  81  MAIN BODY 
       12  MAIN WHEEL 
       12 A LEFT WHEEL (FIRST WHEEL) 
       12 B RIGHT WHEEL (SECOND WHEEL) 
       13  SUPPORT MEMBER 
       14 ,  84  AUXILIARY WHEEL 
       15 ,  85  HOLDING SECTION 
       16  BOX 
       21 ,  51 ,  71  CONTROL UNIT 
       22  ROM 
       23  RAM 
       24 A LEFT WHEEL DRIVER (FIRST WHEEL DRIVER) 
       24 B RIGHT WHEEL DRIVER (SECOND WHEEL DRIVER) 
       25 A LEFT WHEEL ROTARY ENCODER 
       25 B RIGHT WHEEL ROTARY ENCODER 
       26  MAIN BODY ROTARY ENCODER 
       27  USER I/F 
       31 A,  61 A PI CONTROL SECTION 
       31 B,  61 B PI CONTROL SECTION 
       32 A PROPORTIONAL OPERATION PORTION 
       32 B PROPORTIONAL OPERATION PORTION 
       33 A INTEGRAL OPERATION PORTION 
       33 B INTEGRAL OPERATION PORTION 
       34 A,  64 A,  74 A COEFFICIENT PROCESSING PORTION 
       34 B,  64 B,  74 B COEFFICIENT PROCESSING PORTION 
       35 A DIFFERENTIAL ELEMENT 
       35 B DIFFERENTIAL ELEMENT 
       36 ,  66 ,  76  WHEEL ANGULAR VELOCITY COMMAND GENERATOR 
       58  TURN ANGULAR VELOCITY COMMAND RECEPTION PORTION 
       79  BRAKING OPERATION RECEPTION PORTION 
       80  BABY CARRIAGE 
       83  AUXILIARY SUPPORT MEMBER 
       91  SEAT 
       92  BACKREST 
       93  SUN SHADE 
       94  FRONT BAR