# Considerations for the Design of a Wheelchair Dynamometer Concerning a Dedicated Braking System
## 1 Introduction  
Brakes are integral to drive units. Their purpose is to dissipate energy, either for the purpose of speed control or in order to bring the system to a complete standstill. There are many types of brakes that may be classified in terms of the forces generated in them. Brake force (or torque) may be generated through the action of the forces of a magnetic field, the resistance of a medium (either liquid or gas) or friction. Different solutions have different functional properties and thereby have unique advantages and disadvantages. For this reason, it is necessary to choose the correct type and size of braking system for specific applications. The issue presented here is related to R&D work that focused on building a test stand for wheelchairs. This unique application requires a dedicated drive unit. Its braking system is especially important because it must meet a series of requirements arising from its operating conditions. Its function is crucially the need to generate a constant and adjustable braking torque value at a low rotation speed. Other important requirements also have to be met, such as the simplicity of control and minimal impact on the components of the measurement system. The motivation to undertake development work in this area was the requirement of a test stand with a wheelchair dynamometer. The purpose of the stand in question is to enable the measurement of biomechanical parameters related to wheelchair mobility and propulsion biomechanics. This will enable the analysis and comparison of the efficiency of the propelling mechanism of wheelchairs with regard to design, construction and various drive systems. This test stand will also enable the modeling of the kinematics of the propulsion process. It will enable the measurement of a number of parameters related to movement, such as the position of the center of gravity, the demand for drive torque or motion capture measurements, depending on various conditions of movement. The test scenario may include driving at different velocities and driving uphill, but also movement with adjustable resistance, for which a dedicated braking system is required. The developed measurement system also enables the measurement of the drive torque on the wheels of the wheelchair. This is quite important because the measurement of this value in the case of a wheelchair is difficult to implement technically. The preliminary work involved an analysis of a number of technical ways to generate braking torque. The drive unit's requirements in terms of the power transmitted and the nature of the device's operation led us to decide that the best way would be to use a mechanical friction brake (a disc brake). In particular, a servomechanism that used a BLDC motor to generate the load torque was considered. The task of this system would be to generate and hold a constant torque value independent of the rotational speed (for a certain range of rotational speed values). Additional resistance in the system that must be overcome in order to propel the wheelchair would effectively constitute the braking of the wheels. A system operating on this principle was used in an earlier version of the wheelchair dynamometer. The description and principle of its operation are found in [1; 2]. A decision was made, however, to implement the braking system differently. Generating a constant value of resistive torque (for a selected range of rotational speeds) using a servomechanism with an electric motor is both unnecessary and complicated from a technical point of view. A typical drawback of this design is disc brakes' tendency to generate great amounts of heat, and this requires the dissipation of this heat [3]. This heat generation arises from the brake pad's friction against the brake disc, a phenomenon that takes place over a relatively small contact area compared, for example, to a drum brake.