**4. Seal actuators**

The seal is a kind of mammal living in the ocean. It is found that the seal has a flexible body with the contracting ability, two forefeet with the walking ability, and two hindfeet losing the walking ability. When the seal moves, it grips the sand beach firstly with its forefeet. Next, the seal contracts its flexible body and pulls forward the tail and the hindfeet. Then, it releases the forefeet from the sand beach. Afterwards, the seal stretches the flexible body and pushes forward the head and the forefeet. Finally, the seal grips the sand beach again with its forefeet. After the above actions, the seal has moved forward with one step and reverts to the initial status. If the above process is repeated continuously, the seal can move forward continuously on the sand beach.

Inspired by the wriggle mode of the seal, the researchers develop the seal-type actuators driven by piezoelectric materials. The inchworm type actuator consists of two clamping components that are both intermittent clamping mechanisms that can generate intermittent clamping force as required. If one of the two intermittent type clamping components is replaced by a persistent clamping component that can only generate constant clamping force, the inchworm type actuator will be changed into a seal type actuator. Similarity with the body structures of the seal, the seal type piezoelectric actuator is composed of a feeding component, an intermittent clamping component, and a persistent clamping component, which imitate the flexible body, the forefeet, and the hindfeet of the seal respectively.

As shown in **Figure 4**, according to the relative position relationship of the feeding component, the intermittent clamping component, and the persistent clamping component, the seal actuator is classified to walker type, pusher type, and mixed type. As shown in **Figure 4(a)**, the walker type actuator is whose feeding component, intermittent clamping component and persistent clamping component are all designed on the mover. During the process of the walker type actuator running, all of the feeding component, the intermittent clamping component, and the persistent clamping component are pushed with the mover. As shown in **Figure 4(b)**, the pusher type actuator is whose feeding component, intermittent clamping component and persistent clamping component are all designed on the stator. During the process of the pusher type actuator running, none of the feeding component, the intermittent clamping component, and the persistent clamping component is pushed with the mover. As shown in **Figure 4(c)**, the mixed type actuator combines the structural features of the walker and pusher piezoelectric actuators. One of the feeding component, the intermittent clamping component and the persistent clamping component is designed on the mover, and the other components are designed on the stator. During the process of the pusher type actuator running, one of the feeding component, the intermittent clamping component and the persistent clamping component is pushed with the mover and the other components are fixed on the stator.

**Figure 4.**

*Operating principles of seal actuators [32]. (a) Walker type; (b) pusher type; (c) mixed type.*

The operating principles for the three types of seal actuators are shown in **Figure 4** and introduced as follows:

	- 1.The initial status of the walker actuator;
	- 2.The feeding component functions and pushes rightwards the intermittent clamping component on the right side of the mover;
	- 3.The intermittent clamping component on the right side of the mover operates and clamps the right side of the stator;
	- 4.The feeding component resets and pulls rightwards the persistent clamping component on the left side of the mover;
	- 5.The intermittent clamping component on the right side of the mover resets and releases from the right side of the stator. The actuator reverts to step (0).

After the above actions, the mover of the walker actuator has moved rightwards with one step. If the steps (0) to (4) are repeated continuously, the walker seal actuator can move rightwards to output long-range displacement step by step. The backward motion can be generated if the operating sequences of feeding component and the intermittent clamping component are changed.

b.The operating principle of the pusher type actuator:

1.The initial status of the pusher actuator;


After the above actions, the mover of the pusher actuator has moved rightwards with one step. If the steps (0) to (4) are repeated continuously, the pusher seal actuator can move rightwards to output long-range displacement step by step. The backward motion can be generated if the operating sequences of feeding component and the intermittent clamping component are changed.

c.The operating principle of the mixed type actuator:


After the above actions, the mover of the mixed actuator has moved rightwards with one step. If the steps (0) to (4) are repeated continuously, the mixed seal actuator can move rightwards to output long-range displacement step by step. The backward motion can be generated if the operating sequences of feeding component and the intermittent clamping component are changed. Generally, the electric signals to drive the seal actuator are usually the rectangular wave signal, the trapezoidal wave signal or the triangular wave signal.
