3.2.1 Bionic Analysis of the System

The Bio-Cycle was inspired by the cheetah as well as other four-legged animals As a member of the big cat family, the cheetah is the best runner in the animal world. When running at a high speed, the cheetah jumps forward by treading the ground with both of its forelegs at the same time and alternately pushing with both of its hind legs. Figure 3.3 shows the two states in a cycle of the cheetah's run. In this mode, the cheetah's powerful legs provide it with great strength and enable it to run at a speed of more than 100 km per hour Different from running, when walking at a normal speed, the cheetah alternately uses one side of its foreleg and hind leg together to walk first, and then the other side, as shown in Figure 3.4.

To imitate this running motion, we designed the Bio-Cycle with a four-drive structure In the bionic riding mode of cheetah-like running, the user's legs are treading and two hands are pushing forward at the same time This makes the user's body stretch adequately Almost all muscle groups of the upper limbs, lower limbs, and abdomen are balanced and exercised all over Combined with a virtual reality scene and sound system, it can make the user feel as if he were running freely in the wild

Furthermore, because the Bio-Cycle can be driven by either hands or legs or any combination of any hand and leg, users will be able to carry out a variety of exercise motions that cannot be achieved by a traditional fitness cycle Theoretically, there are fourteen running modes for the Bio-Cycle and the main exercise modes include alternate riding, stair-climbing, and bionic riding, as shown in Table 3.1. Also, the exercise motions can be freely changed any time so that the user can realize aerobic and anaerobic training.

3.2.2 Mechanism Scheme-Based Multidrive Modes

The mechanism scheme of a multidrive system is mainly applied with a set of overrunning clutches, which is a special kind of mechanically controlled clutch In the mechanical transmission, when the relative rotation speed or direction changes, the clutch will transfer to the other mode; for example, from engage to disengage Taking a two-drive unit as an example, when drive A is rotating in the direction as shown in Figure 3.5, overrunning clutch A engages to make the driven device rotate in the same direction At the same time, overrunning clutch B automatically disengages (overrun) to put drive B out of action Conversely, when drive B is rotating in the direction shown, clutch B engages to make the driven device rotate, and clutch A disengages (overrun).

Because the handles of the Bio-Cycle can carry out a 360-degree rotation motion, which differentiates them from the fixed handles designed for the common fitness cycle, a transmission system to transfer the power created by the hands to the spindle and to combine four independent transmission units to drive the fitness cycle is designed.

The common transmission mode for fitness cycles is belt driven or chain driven, but it is usually applied when the transmission distance is fixed. As to the supporting bar of the handles, the function of distance adjustment between the upper limbs and lower limbs should be considered to adapt to different users Problems of the transmission process such as torque, stability, and noise should also be considered. Therefore, the torque transmission system uses a spur bevel gear with a spline shaft Considering the volume of the gear and its impact on the whole design, the ratio of this gear transmission is selected to be 1:1

As shown in Figure 3.6, the power offered by hands is transmitted to bevel gear 1 by the respective overrunning clutch. Bevel gear 2 is fixed on the external spline shaft and bevel gear 3 is fixed on the internal spline. To adapt to different users, the spline shaft is used to change the transmission distance between the hands and feet without affecting the transfer efficiency. By the transmission of bevel gear and spline shafts, output power is transmitted to the spindle efficiently. The power of the users' feet can also be transmitted to the spindle through overrunning clutches Thus, the purpose of simultaneously driving with both hands and feet can be achieved

3.3.1 Design of the Pedal Crank and Hand Crank

As shown in Figure 3.7, the structure of the pedal crank corresponds to a crank-rocker mechanism. The pedal crank l₁ is the crank that can do a cycle rotation, the shank of user l₂ is the connecting rod, leg l₃ is the rocker, and the body of Bio-Cycle l₄ is a pedestal that is fixed and unmovable. Therefore, leg l₃ is the active part for this crank-rocker mechanism As is well known, when people exercise on a bike, the distance that the pedal crank covers in one round is equivalent to the walking step of an average person (Xu 2004). Thus, the most optimal diameter for a crank is equal to the walking step length of people in a natural state. If we do a trigonometrical analysis, we obtain an inequality of h > k , l₃ > l₁, k > li. Generally, the walking step length of adults is about 300-400 mm in a natural state. Therefore, the crank length of l₁ ranges from 150 to 200 mm. To satisfy most users, the median value of 170 mm is applied as the optimum.

This new fitness cycle's hand crank can also carry out a 360-degree rotation motion The length of the pedal crank is designed to be equivalent to the step of people in their natural state, and the bionic riding motion, which is driven by both hands and legs, imitates the running methods of animals; thus the hand crank's length should be kept the same as the pedal crank length; that is, 170 mm.

3.3.2 Design of the Saddle Pole

There are two main parameters for the saddle pole: the tilt angle and the length adjustment range.

1. Analysis of the tilt angle

When the user rides a fitness cycle, the relationship of the plane location of the user's thigh, shank, pedal crank, and saddle pole is shown in Figure 3.8a (N. Y. Zhang 2001). The limit location as shown in Figure 3.8b is taken to determine the tilt angle of the saddle pole; that is, the location where the user's legs are straight and the pedal crank is horizontal. The first human template provided by the GB/T 15759-95 standard form is adopted (General Administration of Quality Supervision, Inspection and Quarantine of China, 1995). That is, the female minimum perineum height h = 673 mm is selected as the design standard; therefore, most users will not need to unbend their legs and almost all will be able to tread the pedal crank to the bottom of its motion. According to physiological characteristics, angle a should not be greater than 90 degrees (Das and Behara 1998); a = 90 degrees was chosen

Then, the tilt angle of the saddle pole ß is

β=arctan(h/l)=arctan(673/170)=75.8∘ (3.1)

According to the working condition of the saddle, the ideal angle of ß is 90 degrees. The smaller ß is, the higher the requirement of material strength is on the pole. The seat is designed to move horizontally, and the effect of the seat moving backwards horizontally is similar to increasing the tilt angle of the saddle pole. Therefore, ß = 75 degrees is used.

2. Analysis of the length adjustment range

The lower limb movement is equivalent to a crank-rocker mechanism. As shown in Figure 3.9, part of the bike O₁0₂ stands for the frame, pedal crank L₀ stands for the crank for the mechanism, shank L₂ stands for the connecting rod, and thigh L₁ stands for the rocker The rocker constitutes the active part of the crank-rocker mechanism The comfort angles of human joints correspond to a shoulder joint of 35-90 degrees, an elbow joint of 95-180 degrees, and a knee joint of 60-130 degrees (Hu, Jiang, and Wu 1997).

When the feet are at the top dead center, that is, when the crank and shank are in line, as shown in Figure 3.9a, the knee joint has the minimum value; that is, 9_2min = 60 degrees. According to the cosine theorem, the minimum value L_min of the saddle pole can be obtained.

Lmin2=L12+(L0+L2)2−2L1(L0+L2)cosΘ2min (3.2)

When the crank and O₁0₂ are in line, as shown in Figure 3.9b, the knee joint has the maximum value, that is, 9_2max = 130 degrees, and the maximum value L_max of the saddle pole can be obtained.

(Lmin+L0)2=L12+L22−2L1L2cosΘ2max (3.3)

3.3.3 Handle Design

The handles of the Bio-Cycle are designed to adjust the height to the saddle. The adjusting method of rotating the support bar around the shaft is also part of this fitness cycle. This enables most users to exercise in comfort.

The greater the height difference between the handles and seat, the worse the manipulation, and the greater the pressure on the arms. In order to make the fitness cycle more comfortable, the handle should not be too low—it should be designed to be higher than the seat.

In the horizontal direction, the space of the handle must be in the size range of the user's upper limbs. It should not be too far from the saddle; oth-erwise, it will exceed the maximum forward angle of 45 degrees at which the human body maintains a comfortable sitting posture The distance should not be more than that of the largest upper limbs of the first human example. The forward angle of almost all the users is less than 45 degrees and they can exercise in a comfortable range. Thus, the first human example with the forward angle of 45 degrees is used as a design standard to determine the maximum tilt angle of the support bar

According to the first human example, thigh length L₁ is 402 mm and shank length L₂ is 313 mm. Then the length of the saddle pole can be retrieved from the above analysis: 448 mm < L < 479 mm. In order to obtain the maximum tilt angle, L_min = 450 mm is used as the length of the saddle pole. The other data can be retrieved from the first human example: shoulder height L₃ is 518 mm and upper limb length L₄ is 455 mm. The hand crank L₀ is 170 mm. Under the cosine theorem, as shown in Figure 3.10, the maximum tilt angle a_min is about 37 degrees.

3.4.1 Analysis of Driving Characteristics

The motion of the pedal crank can be divided into four areas in one cycle (Figure 3.11; He et al. 2005; Li and Liu 2005; Wu, Yuan, and Wu 2007). For traditional fitness cycles, the pedal can only fit the treading motion, and the feet do not produce a driving effect while lifting because the two pedals are fixed at a 180-degree angle. Thus, when the foot on one side is treading, the pedal will drive the foot on the other side to lift at the same time. Finally, a circular motion is completed

For the Bio-Cycle, the driving force in a cycle of motion has its own characteristics. Because they are driving separately, the two feet need to produce a driving force in both treading and lifting motions The driving force in the whole circular motion is always positive and has two peak values as shown in Figure 3.12. One peak value is in area II for treading motion, and another peak value is in area IV for lifting motion. The lifting motion is rarely used in normal human sport, so the lifting peak value is much smaller than the treading peak value and will cause discomfort while riding Therefore, the range of such cyclical fluctuation must be reduced.

In addition, the Bio-Cycle can achieve many motion modes, and different motions have different driving characteristics. So when the user changes from one riding mode to another, the balance between resistance and former driving force will be broken This will cause the user an uncomfortable feeling and the phenomenon of no-load running

3.4.2 Principle of the Compound Resistance System

In order to adapt the driving characteristics, three kinds of resistance methods are considered in the system The compound resistance system includes the following:

• Basic resistance refers to the exercise intensity, the level of which can be selected by the user

• Position resistance R_p is normally used when using the bionic riding mode to counteract the cyclical fluctuation, and a designed cam mechanism is used to adjust the position resistance automatically.

• Speed resistance R_s is an adjustment parameter to avoid driving fluctuation, which always occurs when speeding up

Rc=fι1Rb+fι2Rp+fι3Rs (3.4)

where f_ij (j = 1,2,3) stands for the weighing factor based on different motion modes, j represents different resistances, and i refers to different exercise modes; f can be acquired through experiments.

For the driving fluctuation caused by the change of the riding mode, a method to avoid the phenomenon of no-load running must be considered

The speed of the inertia wheel goes through two stages. In the first stage, the driving force is constant and the output power of the user increases from 0 to P_max. The speed of the inertia wheel can be brought up to co_p. In the second stage, the output power will keep the maximum value P_max. According to formulas (3.5) and (3.6), when the speed increases from w_p to co_max, the driving momentum M will decrease because of the reduction of the inertia force J Δó.

(fΩ=PmaxM (3.5)

M=J((Ω.−Δ(Ω.)+Mr (3.6)

When the speed reaches co_max, the inertia wheel rotates in a constant speed, the inertia force decrease to 0, and the driving momentum M = M_r. The phenomenon of no-load running will occur when M < M_min, where M_min is the limit value of the occurrence of no-load running. Therefore, the limit speed co¡ can be obtained.

(fΩl=PmaxMmin (3.7)

In order to avoid the phenomenon of no-load running, when co<co„ the speed resistance can be increased by controlling the angle of the steering engine.

A flowchart of the automatic resistance control system is shown in Figure 3.13.

3.4.3 Detection of Exercise Motions

The system of exercise motions can be determined by the relative position of arms and legs. The typical exercise motion of the Bio-Cycle is alternate rid-ing and bionic riding. These two motions can be classified by the system of exercise motions An infrared photoelectric switch has been used as a sensor and a code wheel has been designed.

For the upper limbs, one sensor has been used and an interstice of the code wheel has been designed as a circular arc of about 45 degrees, as shown in Figure 3.14a. The signal is 11 when two hands are pushing or pulling simultaneously.

For the lower limbs, motions such as alternate riding or bionic riding must be determined, so the position of the pedal crank should be determined accurately. Three sensors for one side have been used and the code wheel has been designed as shown in Figure 3.14b. Two legs are considered to be synchronous when the phase difference of the legs is less than 45 degrees. Two legs are considered to be alternately riding when the phase difference of the legs is between 135 and 180 degrees. The cor-responding signal for the different places of the pedal crank is shown in Table 3.2.

According to the arrangement of the sensors and the design of the code wheel, an algorithm for the determination of exercise motions has been achieved, and a flowchart on the determination of exercise motions is shown in Figure 3.15.