Muscle-like contraction control of tendon-sheath artificial muscle

Highlights

• A transmission model of the tendon-sheath artificial muscle is established.

• Muscle contraction properties are investigated by Hill-type muscle model.

• A muscle-like contraction control method for the artificial muscle is proposed.

• Isometric contraction and quick release experiments are conducted.

• The force–length and force–velocity properties of muscle contraction are achieved.

Abstract

The development of artificial muscles has focused on a high energy–weight ratio and soft structures, however, little work has been done towards muscle-like contraction behaviors. This unfortunately leads to a lack of comfort and safety, which is especially important for robotic applications like orthotics and exoskeletons. In this paper, we propose a contraction control method for a tendon-sheath artificial muscle to contract and relax like biological muscles. In view of the nonlinear transmission characteristics of the tendon-sheath artificial muscle, a transmission model is established and its accuracy is verified through experiments. A muscle-like contraction control method is then proposed based on the transmission model and the Hill-type muscle model. Through this method, the contraction force of the tendon-sheath artificial muscle can be adjusted according to the output displacement and velocity of the tendon-sheath mechanism estimated by the transmission model. Isometric contraction and quick-release experiments are then conducted. The experimental results demonstrate that this control method allows the tendon-sheath artificial muscle to contract with specific muscle-like force–length and force–velocity properties.

Introduction

The skeletal muscle is a biological actuator that can naturally and safely drive human movement, it has soft structures and high power density. Inspired by skeletal muscles, various artificial actuators with compliant structures have been developed for robotic applications. Many studies have shown that the elastic actuator composed of an elastic mechanism and a motor-reducer actuator has high compliance and energy efficiency. As the elastic mechanism can be placed in series or parallel and can have variable stiffness, the currently developed elastic actuators include series elastic actuators [1], [2], parallel elastic actuators [3], variable stiffness actuators [4], [5], and series–parallel elastic actuators [6]. However, the elastic actuators have poor flexibility as using rigid structures. To achieve a muscle-like compliant structure, many artificial muscles with soft structures, light weight, and small size were proposed in recent years. They are made of various materials and have different contraction mechanisms [7]. The pneumatic artificial muscle (PAM) is one of the most commonly used artificial muscles, usually consisting of a rubber or silicone bladder and a hard braided sheath. With the restraint of the outer braided sheath, the PAM can contract by inflating the inner bladder. PAMs have high power density and soft structures, and have been widely used in many robotic applications [8], [9]. However, the air pumps are too bulky for portable mechanisms.

In recent studies, smart and composite materials have shown their potential in developing artificial muscles, including shape memory alloy (SMA), ionic polymer-metal composite (IPMC), dielectric elastomer, carbon nanotubes, nylon fiber, etc. They can generate a recoverable deformation under thermal, electrical, or chemical stimulation. The SMA can deform to its original shape after being heated [10]. The ions in the IPMC can migrate under a low voltage (1–5 V), resulting in a macroscopic deformation [11]. The dielectric elastomer can produce large deformation once a high voltage is applied (up to thousands of volts) [12], [13]. The twisted carbon nanotube yarns can produce high-speed rotation or large deformation when heated or electrified [14], [15]. Similarly, the highly twisted nylon fibers, such as sewing threads or fishing lines, can generate linear or torsional motion when heated, and have a large power-to-weight ratio [16], [17]. However, these materials have some shortcomings in actuation, including slow response, high excitation voltage, and limited tension and deformation. These shortcomings make them unsuitable for robotic applications that require precision motion control and high power output.

In addition to these mentioned shortages, current artificial muscles also lack muscle contraction property. The unique active contraction property is essential for the biological muscle to be the best natural actuator to adapt and balance the external loads. To improve the safety and comfort of human–robot interaction systems (such as orthotics, exoskeleton and prosthetic devices), an artificial muscle with more muscle-like behaviors should be developed.

Related researches have shown that muscle contraction can be well described by the Hill muscle model [18] which usually consists of three units, i.e., the contraction element (CE), the series elasticity (SE), and the parallel elasticity (PE). The SE and the PE are passive elasticities regulating muscle stiffness and energy storage [19], [20], [21], and have great significance in the efficiency and safety of joint movements [22]. The CE is the core element of the Hill muscle model and can generate active muscle contractions. Muscle contraction is a complex process that converts chemical energy into mechanical energy, its most important property is the relationship between muscle force, length, and velocity. With this regard, many Hill-type muscle models using different equations as the CE to describe the active contraction property.

Inspired by the Hill muscle model, this paper studies the muscle-like contraction strategy on the tendon-sheath artificial muscle proposed in [23]. Similar to the three elements of the Hill muscle model, the tendon-sheath artificial muscle takes a motor-driven tendon-sheath mechanism as the CE to generate active contraction, and uses two springs as the SE and the PE. The tendon-sheath artificial muscle has a flexible structure and resilience similar to biological muscles. In this paper, a muscle-like contraction control method is proposed to enable the tendon-sheath artificial muscle to contract like skeletal muscles. As Fig. 1 shows, the active contraction force $F_{CE}$ of the tendon-sheath artificial muscle is adjusted by the activation $a$, the muscle contraction length $l_{CE}$, and the contraction velocity ${\dot{l}}_{CE}$. Considering the nonlinear transmission characteristics of the tendon-sheath artificial muscle and the inconvenience of measuring the force and length of the CE, the tendon-sheath artificial muscle transmission model is established to estimate the CE length and compensate for the friction loss in the force transmission.

The paper is organized as follows. First, the transmission characteristics of the tendon-sheath artificial muscle are studied, and the mathematical model is established in Section 2. Section 3 conducts experiments to validate the accuracy of the transmission model. To imitate the muscle contraction behavior, a muscle-like contraction control method based on the transmission model and the Hill muscle model is proposed in Section 4. In Section 5, two typical muscle contraction experiments are conducted to test the muscle-like contraction performance of the tendon-sheath artificial muscle. Finally, conclusions and future work are discussed in Section 6.