Snake Robot Controlled by Biomimetic CPGs
- Damien Blanchard1, Kazuyuki Aihara2, Timothée Levi2, *1GEII Deparment, IUT Bordeaux, University of Bordeaux, Talence 33400 France2Institute of Industrial Science (IIS), The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan*Corresponding author. Email: firstname.lastname@example.org
- Corresponding Author
- Timothée Levi
- https://doi.org/10.2991/jrnal.k.190220.010How to use a DOI?
- Snake robot, CPG, biomimetic, spiking neural network, locomotion
The design of biomimetic robot is one popular research. To achieve this goal, the reproduction of animal locomotion is mandatory. Animal locomotion is created by the activities of Central Pattern Generator (CPG). CPGs are neural networks capable of producing rhythmic patterned outputs without rhythmic sensory or central input. We propose a network of several biomimetic CPGs using biomimetic neuron model and synaptic plasticity. This network is implemented on a field programmable gate array. We designed one unsupervised snake robot using this network of CPG. It is composed of one head wagon followed by seven slave wagons. Infrared sensors are also embedded in the head wagon. This robot can reproduce the locomotion of one snake.
- © 2019 The Authors. Published by Atlantis Press SARL.
- Open Access
- This is an open access article distributed under the CC BY-NC 4.0 license (http://creativecommons.org/licenses/by-nc/4.0/).
The design of biomimetic robot is one popular research. To achieve this goal, the reproduction of animal locomotion is mandatory. Animal locomotion is created by the activities of Central Pattern Generator (CPG) that produces alternating bursts . Most rhythmic movements are programmed by CPG networks . CPGs are neural networks capable of producing rhythmic patterned outputs without sensory input. CPGs in animal locomotion have been studied such as swimming in salamander  or lamprey , and heartbeat system in leech [5,6].
Usually in robotic field, CPGs are designed using simple neuron models  or simple oscillators , which are not biological time scales and which are considered as bio-inspired systems rather than biomimetic systems. These models provide sinusoidal oscillations and cannot reproduce biomimetic CPGs which have variability in performance. As our aim is to replicate biological behavior, we developed biomimetic CPGs using digital neuromorphic system.
2. BIOMIMETIC CPGS IN DIGITAL NEUROMORPHIC SYSTEM
2.1. CPG Neural Network
The biomimetic CPG is based on the neural network system that controls the heartbeat of a leech . This simple network uses eight excitatory neurons with 12 inhibitory synapses, making it an ideal candidate for our applications. This neural network model has been designed by Hill et al.  and described in Figure 1.
2.2. Simplification of the Model
The network designed by Hill uses the Hodgkin–Huxley (HH) model  and complex equations for synapses. Our first work was to simplify this model by applying Izhikevich neurons  instead of HH model. Using the short-term synaptic plasticity  and the Izhikevich Regular Spiking neurons, the activity of Hill’s model was reproduced.
Even though the model was simplified, the same behavior and characteristics as the biological CPGs was successfully reproduced. The important point is to allow a variability in the characteristics of the CPGs. Percentages of variability can be controlled by adjusting the parameters of short-term synaptic plasticity.
By changing the parameters of the inhibitory synapses, it is possible to obtain different periods and cyclic ratios. This is very important especially for closed-loop experiments and to reproduce locomotion activities for robotics. A single parameter pair manages the modification of the period and of the duty cycle. The possible period interval in seconds is quite wide [0.3 and 35 s].
2.3. Implementation in Digital Neuromorphic System
Biomimetic CPGs [12–15] are implemented in a digital platform: CMOD A7-A35 Xilinx Artix-7 Field Programmable Gate Array (FPGA). The neural network implementation architecture operates on a single computation core. This real-time digital system requires few resources and low power consumption [16–19]. Table 1 summarizes the stated resources and Figure 2 describes two CPGs with different periods.
Utilization of the resources for the implementation of biomimetic CPGs in CMOD A7-A35 board
As we implemented the biomimetic CPGs in the FPGA board, we design the structure of the snake robot which embedded the FPGA board.
3. SNAKE ROBOT
The snake robots is composed of seven wagons (like spinal cord) and one locomotive (mimics the brain).
Biomimetic CPGs will control the different motors of the wagons. As the output of the FPGA board is 3.3 V with low current, a power driver for the motors and a VHDL module for converting the CPGs to Pulse Width Modulation (PWM) to control motor speed needs to be added. The PWM frequency should be between 100 Hz and 200 kHz. The duty cycle of the PWM controls the speed of the motor. Pololu_DRV8835 was used for the power driver. Motors are DG01D with a 0.8 kg.cm couple and a no load speed of 90 rpm. Figure 3 shows the 3D structure of the wagons.
All the distance between the different wagons and the possible movement angle can be tuned with mechanical structure. Figure 4 shows how the tuning is possible with the orange screws.
The design of electrical connection and components for the locomotive and for the wagon is described in Figure 5.
The locomotive system has three IR sensors for detecting the different obstacles and then send stimulus to CPGs for modifying duty cycle and period. The tuning of the duty cycle allows the movement and the tuning of period allows the speed or the stop of the robot.
Height CPGs were implemented (one for the locomotive and seven for the wagons). Enable input are connected to each wagon to choose which ones are working for simulating a lesion of the spinal cord. The CPGs are connected for allowing forward locomotion like we can find in animal locomotion . Figure 6 describes this CPG network and the forward locomotion .
The final snake robot is 59 cm long and works with real-time biomimetic CPGs and mimics the snake locomotion. Figure 7 shows the final snake robot picture.
This article describes the design of a snake robot. This robot is controlled by real-time network of biomimetic CPGs. This system can be used in robotic applications  for a closer behavior to biological animals. It can also be used in bio-hybrid robotics (biomimetic CPGs can be replaced by in vitro cell culture). We previously showed that our system can be connected to biological neurons [22,23]. Another advantage is that the real-time systems can simulate multiple CPG topologies and simulate different hypothesis and protocol treatments for biomedical applications such as spinal cord injury  and neuroprosthetics [25,26].
Authors thank Social Cooperation Program for Brain-Morphic AI to Resolve Social Issues, IIS, University of Tokyo.
Mr. Damien Blanchard
Dr. Timothée Levi
Prof. Kazuyuki Aihara
Cite this article
TY - JOUR AU - Damien Blanchard AU - Kazuyuki Aihara AU - Timothée Levi PY - 2019 DA - 2019/03/30 TI - Snake Robot Controlled by Biomimetic CPGs JO - Journal of Robotics, Networking and Artificial Life SP - 253 EP - 256 VL - 5 IS - 4 SN - 2352-6386 UR - https://doi.org/10.2991/jrnal.k.190220.010 DO - https://doi.org/10.2991/jrnal.k.190220.010 ID - Blanchard2019 ER -