With the continuous integration of China's economy with the world, Taiwan, China, has been in a monopoly position in the electric treadmill industry, and has been gradually weakened by the electric running manufacturers in mainland China. According to the incomplete statistics of Chengdu Sports Products Expo in 2006 and 2007, there are more than 100 enterprises producing electric treadmills in China. In the electric treadmill products, the home treadmill occupies the largest market share, and the home treadmill with practical performance, low price determines the characteristics of the treadmill drive with low power DC motor, due to the speed of the DC motor and the armature The output voltage is proportional to, therefore, the motor speed control problem has actually become a DC power supply design problem.

In recent years, with the emergence of new power electronic power devices, the control mode of DC motor has changed greatly. The use of fully controlled switching power components for pulse width modulation (pulse width modulation, referred to as PWM) has become one of the main DC motor speed control methods. This speed control method has the advantages of high switching frequency, stable operation at low speed, excellent dynamic performance and high efficiency.

1 Power system design

The design of the treadmill power control object is the DC motor, the purpose is to achieve the motor speed control, the brief working principle is the utility of the full bridge rectifier, filter to achieve AC/DC conversion, and then use the power FET as the main switching element, through the TL494 to control the power FET on-off time, so as to control the motor armature voltage to achieve speed control.

1) Main circuit design

The main circuit of the power supply we designed is shown in Figure 1. The design uses four rectifier diodes 6A10 to form a rectifier bridge, and then through the capacitor C1 filter, to achieve AC/DC conversion. Then the PWM signal sent by the PWM controller circuit controls the on-off of the power field effect tube IRFP460 to change the voltage on the DC motor armature. In the figure, Rq is a sampling resistor for current protection. The voltage drop of the resistor directly reflects the current of the main loop. Contact J1 -1 is closed after the electric treadmill starts to run.

2) Control circuit

The control circuit includes a PWM signal generation circuit and an over-current and over-temperature protection circuit when abnormal.

(1)PWM signal generation

According to the application data of TL494, the frequency of the sawtooth wave can be calculated by the following formula:

Reference range of Rt and Ct values in formula (1):

Rt = 5 ~ lOOkΩ,Ct = 0.001~0.1 μF. According to the switching frequency l5kHz of the post-stage power FET recommend, Rt is 7.5 kΩ and Ct is 0.01 μF in this design. The circuit for generating the PWM signal from TL494 is shown in Figure 2.

The error amplifier and the control amplifier are used in parallel in the system. The non-inverting end of the amplifier inputs the speed given signal and the inverting end inputs the speed feedback signal to form closed-loop control. Resistance-capacitance correction is connected between pins 2 and 3. The 13-pin grounding makes TL494 not use push-pull output. In this mode, the internal trigger does not work, and the two outputs the same PWM signal, and the frequency is the same as the sawtooth wave oscillation frequency.

The speed given signal connected to TL494 is an analog signal generated by the single chip microcomputer according to the digital pulse signal generated by the user's operation on the panel. The speed feedback signal is obtained by the digital pulse signal of the photoelectric sensor, which is installed on the code disc coaxial with the motor, and its pulse frequency directly reflects the rotating speed of the motor. Similarly, the digital pulse signal is also converted into an analog signal, the conversion circuit from digital pulse to analog circuit is not repeated because of the space problem.

(2) Protection circuit

The protection of the motor generally includes overvoltage, undervoltage, overcurrent, overheating, etc. Considering the actual situation of the site and controlling the cost of the product and many other factors, the more important overheating protection and overcurrent protection functions are designed. The specific protection circuit is shown in Figure 3.

The value of the current sampling resistor Rq on the main circuit in fig. l is determined according to the current limiting value required by electric treadmills with different power, and constantan wires of different lengths are selected in actual use. Rw is a thermistor, and its heat conducting part is fastened with a diameter φ3 screw and a power field effect tube. We use a negative temperature system RT338006 with a resistance value of lOkΩ at 25 ℃ and 3.02 kΩ at 60 ℃.

Under normal circumstances, the current on the main circuit is only about 2A, and the voltage reflecting the current signal is not enough to turn on VT1. Under normal circumstances, the resistance value of thermistor Rw is greater than R10, the non-inverting terminal voltage of comparator B is less than the inverting terminal voltage, and the output is low level. The voltage at the inverting input terminal of comparator A is greater than the voltage at the non-inverting input terminal, and the output is also at a low level. Therefore, the transistor VT2 is cut off, and the PWM signal output by TL494 passes through VT3 and VT4 to control the on-off of the power FET, thereby controlling the speed of the motor.

When overcurrent occurs, the voltage drop on Rq increases until VT1 is turned on, and the voltage at the inverting input terminal of comparator B drops to close to 0V. At this time, the voltage at the non-inverting input terminal is higher than that at the inverting input terminal, outputting a high level to turn on VT2, thus blocking the PWM signal output by TL494, cutting off the power FET, and losing power to realize motor protection. In fact, when overcurrent occurs, the voltage drop on Rq increases and its value is also reflected at the non-inverting terminal of comparator A. At this time, the conduction of VT1 also leads to the decrease of the voltage at the inverting terminal of comparator A. This effect also causes comparator A to output a high level. The signal also ensures the conduction of VT2 through the following OR gate. Therefore, from the circuit form, overcurrent protection realizes double protection.

For overheat protection, once the power FET is overheated, the temperature of the thermistor will also rise. When the resistance value of the thermistor is less than 3kΩ (about 60 ℃), the voltage at the non-inverting input terminal of comparator B is greater than the voltage at the inverting input terminal, and the comparator outputs a high level. The PWM signal output by TL494 is also blocked to make the motor lose power and realize protection.

2 Concluding remarks

The electric treadmill power supply designed in this paper has the characteristics of simple speed control mode, simple circuit structure and low cost. At the same time, the power supply also has protection measures such as overcurrent and overheating. In the past three years, the power supply has been used by many domestic electric running manufacturers. Customers have reported that the motor runs smoothly and has reliable protection performance. It is an ideal control power supply for low-power DC motors and has good use value.