## 1 Introduction
This paper was written in response to an engineering need to optimise the design of the motion apparatus of a CPM device using modern CAD/CAE tools and Generative Design methods, which are a form of artificial intelligence that use the power of the cloud and machine learning to speed up the whole process starting from design to execution [11; 12]. The optimisation methodology proposed in this paper will lead to a reduction of the mass of the kinematic chain components of the motion apparatus which will translate into a bespoke selection of the correct electrical actuator. This results in lower operating costs. It also extends the life of the equipment and reduces the risk of equipment failure, thereby reducing service costs. State of the Art of Scientific Research in the Field of CPM Devices Design  
The use of the CPM device in kinesitherapy immediately after surgery and during the subsequent recovery period after knee arthroscopy or endoprosthetic is a common practice in many medical facilities. Continuous passive motion is designed to improve the ROM of the knee joint. This will consistently reduce pain. It is assumed that by bending the limb in a range of 90 to 106 degrees, it is possible to perform basic activities, including movements such as descending stairs, rising from a toilet or low chair, and tying shoes [13; 14; 15; 16]. The existing body of knowledge on CPM research is mainly concerned with the validity of the use of CPM devices and determining the possible benefits for the patient's health. The vast majority are scientific papers, mainly of a strictly medical nature [17; 18; 19; 20; 21]. They look at the health effects of rehabilitation using a CPM device. As far as scientific work in engineering and technical sciences is concerned, in the case of CPM rehabilitation devices offered on the market, the research is not published, most likely due to the protection of the intellectual property of the solution. Therefore, the synthesis of knowledge on CPM design can only be based on selected articles and selected patents filed in international patent databases. Analysing selected solutions from patent databases, it is clear that the majority of CPM device designs are based mainly on the use of nut-screw-type linear guides driven by an electric actuator to move the positioner. Examples of these are presented in the following papers [22; 23; 24; 25]. Another group are CPM systems where the actuator is directly coupled to the knee flexion mechanism (in the flexion axis) [26; 27]. In these studies, the authors describe solutions that are usually offered by rental companies that specialize in rehabilitation equipment and operate in Poland. Among them are the products of the ARTROMOT family [28], Chattanooga OptiFlex3 CPM machine [29], and Jace K200 The Zero Machine CPM machine provided by Jace System [30]. In the above-mentioned CPM systems, the movement of the motion apparatus is carried out following the patents in question, either using a screw and nut system or using a direct drive in the flexion axis of the knee joint. There are also proposals for technical solutions based on the mechanism of a modified articulated quadrilateral [31], CPMs driven by a set of drive cables coupled by pulleys [32], or a driving rotating member in the form of a disc [33]. The technical solution of the motion apparatus presented in this study is unique. This is due to how the positioner motor of the CPM device is driven. A slider-crank mechanism coupled to an electric actuator was used [10]. The following Section 2 presents the technical details and the research carried out to optimise the CPM's positioner movement apparatus.