# Design of a Planar Cable-Driven Parallel Robot for Non-Contact Tasks
## 2 Design Strategies
### Actuation Unit  
The most appropriate type of actuation is chosen depending on the \(EE\) motion pattern and \(WS\) size, defined by the task [1,33]. A rotary actuation system generally consists of a servo-actuated winch where the cable is coiled onto a cylindrical drum. This solution is easy to implement but can result in low robot position accuracy since it introduces several errors when estimating cable lengths [34]. Lengths are computed by measuring the rotation of the motor and, thus, they depend on the drum design and cable arrangement on it. Linear actuation systems reduce cable-length estimation errors, but they usually limit the size of the achievable \(WS\)[35], or increase cable wear [34]. A solution could be to adopt a rotary actuation system designed to be as close as possible to the model intended for its control. The main drawback of the rotary actuation, if a simple lifting winch is considered, is the fact that the cable exit direction from the drum varies, even unpredictably, as the motor rotates: this results in a non-linear transmission ratio between the motor angle and the cable length, which is usually undesirable. A constant transmission ratio can be obtained (i) by avoiding the cable overlapping onto the drum surface, (ii) by suitably grooving the drum to accommodate the cable, which is also desirable for reducing cable wear [36], and (iii) by constraining the cable to exit the drum in a fixed, known, direction. There are several solutions in the literature to achieve such desired design requirements (Figure 1). It is possible to rototranslate the drum [16; 37; 38] so that the cable exit point from the drum, and consequently its direction, is kept fixed with respect to the winch frame, while the cable is coiled and uncoiled. This is usually achieved by employing screw/nut joints to convert the rotational motion of the motor into rotational and translational motion of the drum, and allowing the drum to translate by mounting it on a passive prismatic joint. In [38] the drum houses a nut and two pairs of cylindrical bearings. Two rods, which are coupled with the motor shaft, parallel to the drum axis, but mounted with a radial offset with respect to the drum, pass through the bearings, thus transmitting the rotational motion, but allowing the drum to translate. The translation is caused by the drum nut being joined with a screw, which is fixed on the winch frame: a cross section of the drum system is shown in Figure 1a. The main advantages of this solution are compactness and mechanical simplicity, but this design suffers from one main drawback, namely the rods used for transmitting the motor rotations pass through the drum, thus the latter diameter needs to be large. An example of this design is shown in Figure 1b, where a timing belt is used to transmit motion from the motor to the drum, as in [37]. Figure 1: Winch models. (**a**) Cross section of a rototranslating drum. (**b**) Rendering of a winch with rototranslating drum kinematically equivalent to the ones in [37; 38]. (**c**) Close-up view of a winch of the Ipanema _CDPR_ family [14]. (**d**) Winch with translational motor-drum. As an alternative, in [14] an auxiliary cable guiding device equipped with a pulley (_spooling helper_) continuously follows the variable cable exit point on the rotating drum (Figure 1c) by translating parallel to the drum axis, to ensure that the direction of the cable connecting the drum and the spooling helper is constant. The spooling helper is mounted on a support equipped with a nut and a pair of bearings, and thus slides with respect to fixed rods, driven by a screw, which is ultimately connected to the drum with a synchronous belt. Given its footprint, this solution has the primary benefit of allowing very long cables to be stored onto the drum. Still, an additional mechanical transmission, such as a timing belt, is always required to transmit drum motion to the screw.