We have constructed an insect-computer hybrid legged robot using a living beetle (Mecynorrhina torquata; Coleoptera). The protraction/retraction and levation/depression motions in both forelegs of the beetle were elicited by electrically stimulating eight corresponding leg muscles via eight pairs of implanted electrodes. To perform a defined walking gait (e.g., gallop), different muscles were individually stimulated in a predefined sequence using a microcontroller. Different walking gaits were performed by reordering the applied stimulation signals (i.e., applying different sequences). By varying the duration of the stimulation sequences, we successfully controlled the step frequency and hence the beetle's walking speed. To the best of our knowledge, this paper presents the first demonstration of living insect locomotion control with a user-adjustable walking gait, step length and walking speed.
biobot; biomechanics; electrical stimulation; insect–computer hybrid robot; legged robot; walking gait.
© 2016 The Author(s).
Insect platform for the legged robot and its front leg anatomy. (
a) Beetle's front leg anatomy. The protraction and retraction muscles (inside the prothorax) enable the leg to swing forwards and backwards about the thorax–coxa joint. ( b) The levation and depression muscles (inside the coxa) enable levation and depression motions of the femur about the coxa–trochanter joint. Red crosses mark the sites of stimulation electrodes implanted into each muscle.
Experimental set-up for the walking gait investigation. (
a) Three-dimensional motion-capturing system used for recording and storing the three-dimensional position information of the markers attached on the beetle. ( a-1) Six T40 s VICON ® cameras of 4 megapixels (2336 × 1728) resolution operating at 100 fps for motion recording. ( a-2) The VICON ® server was used to reconstruct, store and export the marker position information collected by the cameras. ( a-3) The computer was custom-programmed in MATLAB ® for the walking gait analysis. ( b) Two reflective markers were attached to each of the beetle's front legs (L1, L2, R1 and R2), representing the tibia segments. Two additional reflective markers (B1 and B2) were attached to the beetle's body, allowing tracking of the walking direction and body orientation. ( c) The collected three-dimensional motion data of both tibia segments and the body segment were displayed as three independent segments. To represent the tibia sections of the right and left legs and the beetle's body, we linked markers R1 and R2, L1 and L2, and B1 and B2, respectively.
Circuit diagram of the insect–computer hybrid robot. (
a) The CC2530 microcontroller connected to the external circuit for stimulation of one muscle. The PWM wave generation was controlled via the I/O pins (channels 1–8) and the GND pin of the microcontroller. The circuit consists of an LED (for stimulation status indication), an optocoupler (acting as a switch for the muscle stimulation circuit), a 470 Ω resistor (for fast release of the residual charge in the optocoupler) and a 1.5 V battery (for power supply for muscle stimulation). Channels 1–8 separately controlled the eight identical external circuits that stimulated the eight muscles in the beetle's two front legs. ( b) A beetle with 16 stimulation electrodes (two electrodes per muscle) implanted into the eight muscles (controlling protraction, retraction, levation and depression motions in both front legs). Channels 1–8 generated stimulation signals in a predefined sequence for the walking control of the beetle.
Experimental set-up of the walking speed versus step frequency analysis. (
a) Snapshot of the insect–computer hybrid robot during galloping with both front legs at their first AEPs. ( b) One step later, with both front legs at their second AEPs. Red crosses indicate the pixel coordinates of the articulation connecting the beetle's left tibia and tarsus, used to calculate the step length. The beetle's walking distance was determined from the distance travelled by the beetle's horn (indicated by the red cross on the horn). The pixel distance between AEP 1 and AEP 2 was calculated by Pythagoras' theorem. The pixel distance was then converted to the actual distance (in centimetres) travelled by the leg by calibration with a 30 cm ruler.
Stick diagram for walking gait analysis of an intact beetle. Lateral view of the motion trajectory of the tibia segment of the beetle's front leg. The time between two consecutive sticks was 10 ms. The horizontal and vertical axes indicate the forward and upward distances travelled by the tibia segment, respectively. The progression from one blue stick (AEP) to the next blue stick (the consecutive AEP) indicates a walking step. Levation (depression) motions are indicated by an increase (decrease) in the marker's vertical position.
Time intervals of the four motions, normalized by the corresponding step duration (
N = 5 beetles, n = 25 steps). The four motion types (protraction, retraction, levation and depression) were performed at different timings to generate the cyclic power and return strokes during voluntary walking. First, retraction and depression (red and green bars, respectively) were executed concurrently during the power stroke (comprising the first 61 ± 22% of a complete walking step). During the following return stroke, protraction (blue bar) was executed throughout, whereas levation (orange bar) was switched to depression (purple bar) at 78 ± 15% of the normalized step duration.
Demonstration of sequential leg motion control in tripod and galloping gaits. Videos were shot from the beetle's ventral view for clear viewing of the resultant leg motions during the predefined stimulation sequence (table 1). (
a-1)–( a-4) and ( b-1)–( b-4) are the leg motions under stimulation sequences 1–4 during tripod and galloping walking control, respectively. LED lights near the beetle's head indicate the on–off status of the corresponding stimulation channel.
Average normalized walking speed (body lengths per second as a function of step frequency (
N = 6 beetles, n = 90 data points). The walking speed study yielded 90 data points (nine data points per step frequency in each walking gait). Five continuous steps (the first and last steps were not counted) were used to obtain one average walking speed data point. The black bars indicate the standard deviations. For a given step frequency, the insect–computer hybrid robot progressed faster during galloping than during tripod walking. In both gaits, the average normalized walking speed increased with each doubling of the step frequency from 0.125 to 1 Hz, and then decreased from 1 to 2 Hz. The blue and red numbers indicate the percentage change of the average normalized walking speed at each doubling of the step frequency during tripod and galloping walking, respectively. See also the electronic supplementary material, movie S1.
Average normalized step length versus step frequency (
N = 6 beetles, n = 150 data points). Blue and red numbers indicate the percentage change of the average normalized step length at each doubling of the step frequency during tripod and galloping walking, respectively. In both gaits, the average normalized step length decreased with increasing step frequency, except for the doubling from 0.25 to 0.5 Hz. Note that the step length was measured as the linear distance between consecutive AEPs of the front legs. See also the electronic supplementary material, movie S1.
Repeatability test: cycles of leg motion induced by the electrical stimulation of the retraction and protraction muscles. A leg's motion (over a duration of 20 s) tracked in the repeatability test. Custom-developed MATLAB
® code was used to automatically detect the angular positions of extreme retraction (green circles) and extreme protraction (red circles) with respect to the maximum protraction position reached during the 30 min stimulation (angular position = 0°). The beetle's leg keeps moving between the extreme retraction and protraction positions every second due to the applied electrical stimulation. The difference between adjacent retraction and protraction positions is the leg's angular displacement.
Repeatability test: the average and standard deviation of four beetles' front leg angular displacement (
N = 4 beetles, n = 1800 data points). Every beetle was stimulated for more than 30 min per day for 7 consecutive days. The four different colours of the columns indicate the four tested beetles. The standard deviations are indicated by the black bars. The mean and standard deviation of the leg's angular displacement of each beetle on each day were calculated from more than 1800 data points (refer to figure 10 for the data points allocation method), which resulted from the more than 30 min stimulation (as the protraction and retraction muscles of the beetles were stimulated alternatively for 1 s).
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