# Control Methods Comparison for the Real Quadrotor on an Innovative Test Stand
## 2 Design of Test Stands  
### Test Stand for Identification of Static and Dynamic Characteristics of the Actuator  
Parameters used in the derived mathematical model in our previous work [9] were obtained from the static and dynamic characteristics of the real actuators. In this work [9] several controllers were tested in simulation environment. The next logical step was to evaluate selected controllers on the real quadrotor platform. That is why this section will present the first stand, which was used to identify static and dynamic characteristics of the actuators. The main contribution of this stand is significantly lower price compared to standard test stands, without loss of the identification quality. In order to identify coefficients of the actuators, namely the thrust coefficient and the drag coefficient, the relation between the force/torque and rotor speed or PWM (pulse width modulation) sent to ESC (electronic speed controller) must be measured. Test stands used by other researchers normally consist of a six-axes torque/force sensor or load cell, a pivot arm, tachometer and actuator. In [34], the identification was focused only on a motor itself without considering an ESC. In [35], the dynamics of an actuator was concerned when the propeller or ESC was changed. The price range of the used sensors varied from 400 to 1300 \(\upmu\)m [36; 37; 38; 39; 40; 41]. A low-cost measuring test stand was designed to measure force, torque, battery voltage and current, so that complete identification including motor dynamics could be established. Mechanical construction of the test stand (Figure 1) consists of a large wooden board, an arm of the quadrotor frame, a binocular beam load cell, an optical sensor and the actuator. The wooden board is used to fix the whole construction to the ground or another base during the measurement. The arm of a DJI Flame Wheel F450 is used to ensure that the measurement will be under the same aerodynamic conditions as during the flight. The load cell, which can be found in most kitchen scales, is attached to the arm. Maximum load capacity of the load cell is 5 kg. Its output transfer sensitivity is 1 mV per 1 V of excitation voltage at full load. The binocular beam load cell incorporates a full-bridge configuration of strain gauges. Because of the two holes in the middle of the load cell, it can measure only vertical deformation and the length of the arm is not taken into account during the measurement. The usage of four strain gauges ensures compensation of the unwanted temperature effect on the output signal and increases measuring sensitivity. The approximated price of this test stand is 60 \(\upepsilon\). For the experiments, the DJI BLDC motor was used and it was firmly attached to the load cell. The BLDCM belongs to the 2212 power class. The power class determines the power of a motor and its diameter (28 mm). The voltage constant of the motor is 920 Kv and nominal current ranges from 15 to 25 A. The use of a 3- or 4-cell lithium polymer battery (LiPo) is recommended. A DJI clockwise propeller 10\({}^{\prime\prime}\)\(\times\) 4.5\({}^{\prime\prime}\) was mounted to the motor and a 3-cell LiPo battery with the nominal voltage of 11.1 V was used. The motor is powered by the 30 A ESC from DJI. The input signal for the ESC is a PWM with refresh rate range from 30 to 450 Hz. Power supply and the control signal are obtained from the DAQCB (data acquisition and control board). If a propeller with two blades is used, there are only two increments per revolution when using a tachometer. In order to obtain higher resolution an encoder was used to measure the motor speed. For this purpose, the outer side of the motor was covered with a sequence of 22 black and white stripes. The input-output DAQCB was developed to amplify the output signal from the load cell and to gather other signals. The board connects the object of measurement (ESC, BLDCM, battery) with the I/O board MF624 from Humusoft.