计算图示机构的自由度，若含有复合铰链，局部自由度和虚约束请明确指出。<br/><img src="https://nimg.ppkao.com/STUploads/2021/4/14/Images/28832ba30bed46d88302d6496aa2cd9c.png?sign=d2c392c897db74f27b6d5a96e841c78a&t=62d4bacd"/>