Circuit and servo amplifier

We choose brushless DC motor (BLDC motor) to drive the exoskeleton. BLDC motor is generally used for high positioning accuracy. Its brushless feature can avoid sparks and brush friction loss so that BLDC motor can save energy and pursuit high

efficiency.

The basic concept of BLDC motor is that replacing the carbon brush with electronics in a motor. With the electronics and control for each switch, this non-mechanical contact system can change the direction as carbon brush does. BLDC motor has the following advantages. First, it runs quietly and is suitable in some quiet places such as hospitals and schools. Second, BLDC does not generate any spark and thus can use in some flammable and explosive places. Third, BLDC is more durable because it uses controller instead of the carbon brush.

BLDC motor has three phases and a Hall position sensor so the motor commutation accurate rotor position detection accuracy is not affected by the motor speed. It does not require additional rotor position detection circuit hardware. We install the gearbox to ensure that motor have ability to supply sufficient torque to assist the human body.

5.2.2 Servo amplifier

To drive BLDC motor, we need to know rotor position by Hall sensors. We then need to input correct three phases driving signal and enough power to drive the motor.

We choose the BLDC motor, MAXON EC 60 flat [42], with gearbox as our system power unit. Its corresponding servo amplifier is ESCON module 50/5 [42].

To setup the motor, we should prescribe the motor phase. Our motor have already inserted encoder; we should set encoder information in ESCON studio to make sure that the encoder is working. To drive the motor by using ESCON, the digital I/O and analog input can be set as a function to enable or disable motor driving, to accept PWM duty from controller, and to determine the motor driving direction. For capturing data, ESCON can set analog output pin to read motor speed and current data.

ESCON can operate three different modes to control motor. In Mode 1, ESCON

acts as a voltage amplifier. ESCON can do IXR compensation in mode 1 and 2. IXR compensation is to stabilize voltage during motor rotation. Parameters of IXR compensation can be chosen as static or dynamic in mode 1. In mode 2, motor speed is controlled in closed loop using feedback from the encoder. In mode 3, motor current is controlled in a closed loop, which is useful for torque control. We compare these three modes control effect for our exoskeleton system. We then choose the proper mode to control the exoskeleton.

5.2.3 Controller circuit

We separate our electrical system into three sub-systems: action system, calibration system; stop system and graphical user interface. The action system is for moving exoskeleton. The calibration system is for calibrating the zero position. The stop system is to help user stop exoskeleton in emergency. Graphical user interface is for user to see actuation angles and calibration state.

When we need to move the exoskeleton, we design a button for user to control.

The button delivers signal of “walking”. Then National Instrument (NI) Myrio [43]

captures the feedback signal of the encoder and calculates PID algorithm. The signal that after PID calculation will pass from NI Myrio to ESCON 50/5 motor driver as PWM signal. Finally, ESCON 50/5 would control MAXON EC60 flat motor to achieve our expected gait cycle. Fig. 5-4 illustrates this process.

Encoder

Fig. 5-4 Action system structure.

We also place hall sensor at point B in Fig. 5-5. This component can make program to identify zero rotation, which can provide information to help user stand. This is important as if we lose the real zero position, the encoder signal would be not correct.

Producing a wrong signal to the algorithm will make a wrong output signal.

Fig. 5-5 Calibration system.

To avoid motor rotation over the range of gait cycle, we make two limit switches on the sliding boundary, as shown in Fig. 5-6. When stance state occurs, the cylinder will hit limit switch. Then the controller will command the driver to stop motor to prevent user from injury.

Fig. 5-6 Stop system.

B

The graphic user interface is shown in Fig. 5-7. With this interface, user can know the motion of the exoskeleton. The information comes from the right and left leg encoder signal. This signal is calculated with gear ratio for correspond to actual angle rotation. There are two lights placed at the upper left side of the interface. One is to tell user whether the limit switch has been triggered. An on light indicates that the corresponding leg reaches the stance state. The other light is to tell user whether the exoskeleton has been calibrated or not. This bulb lighting is a result from Hall sensor signal. When the user is standing, which is zero calibration state as we defined, the Hall sensor is triggered by strong magnetic flux. This is signal for reaching zero state, and the bulb is turned on. There is one button placed at upper right side. By pushing this button, the system will stop for emergency. Motor driving would be disabled for safety consideration. Then program would start to collect data before this button is pushed again. The data such as encoder signal, motor current and motor speed can help us analyze the system failure.

Fig. 5-7 Graphical user interface.