An arrangement for moving a piston in a compressed air cylinder, which arrangement comprises: - a compressed air cylinder comprising: a casing which surrounds a substantially circular cylindrical cylinder chamber, a longitudinal stem intended to transmit force from the compressed air cylinder, a piston coupled to the longitudinal stem and having a substantially circular shape and a radius corresponding to the inside radius of the cylinder chamber, the piston being placed in the cylinder chamber in such a way that a space (V1, V2) is formed on each side of the piston and that one of the spaces has a hose or pipe connected to it for supply of compressed air, and the other space has disposed in it a spring which acts between the piston and the casing of the cylinder chamber so that the spring acts to urge the piston to an extreme position (s 1) in the cylinder chamber, being a position of rest, at which the space where compressed air is supplied is minimised; - a valve device situated adjacent to the space to which compressed air is supplied, or somewhere along the compressed air hose or pipe; - a compressed air device for feeding compressed air to the space via the valve device and the compressed air hose or pipe; - a control unit connected to the valve device for controlling the supply of compressed air to the space in the compressed air cylinder via the compressed air hose or pipe; and - an aperture or valve which allows air to leave the space when a certain pressure prevails therein and through which air is allowed to leave the space, such that the aperture or valve is always in communication with the space, which arrangement is characterised in that the control unit comprises means for controlling the valve device in such a way that it supplies compressed air to the space in a constant flow, or in pulses of varying length, so that the piston and the stem coupled to said piston are thereby moved to desired positions in the cylinder chamber and also kept in the respective desired position. - a compressed air cylinder comprising: a casing which surrounds a substantially circular cylindrical cylinder chamber, a longitudinal stem intended to transmit force from the compressed air cylinder, a piston coupled to the longitudinal stem and having a substantially circular shape and a radius corresponding to the inside radius of the cylinder chamber, the piston being placed in the cylinder chamber in such a way that a space (V1, V2) is formed on each side of the piston and that one of the spaces has a hose or pipe connected to it for supply of compressed air, and the other space has disposed in it a spring which acts between the piston and the casing of the cylinder chamber so that the spring acts to urge the piston to an extreme position (s 1) in the cylinder chamber, being a position of rest, at which the space where compressed air is supplied is minimised; - a valve device situated adjacent to the space to which compressed air is supplied, or somewhere along the compressed air hose or pipe; - a compressed air device for feeding compressed air to the space via the valve device and the compressed air hose or pipe; - a control unit connected to the valve device for controlling the supply of compressed air to the space in the compressed air cylinder via the compressed air hose or pipe; and - an aperture or valve which allows air to leave the space when a certain pressure prevails therein and through which air is allowed to leave the space, such that the aperture or valve is always in communication with the space, which arrangement is characterised in that the control unit comprises means for controlling the valve device in such a way that it supplies compressed air to the space in a constant flow, or in pulses of varying length, so that the piston and the stem coupled to said piston are thereby moved to desired positions in the cylinder chamber and also kept in the respective desired position.