1 brake braking force measurement principle 1.1 The motor drives the roller set through the torque box to drive the car wheel to rotate. When the wheel brakes, the reaction force between the tire and the roller causes the torque box to oscillate, through the force measuring lever at the front of the torque box and the tensile force applied on it. The sensor outputs the braking force value through an electrical signal, and after being processed by the instrument's microprocessor, it is displayed accurately by the digital tube. 1.2 The main drum is driven by the motor, and its rotation speed is basically fixed at the timing of the voltage balance. The third roller is rotated by the tire and slows down as the rotation speed of the tire decreases. The line speed is always equal to the line speed of the tire. On the third roller, six long holes are placed next to both ends of the bracket, and an inductive speed sensor is installed below the hole. The gap between the probe and the long hole is maintained at 0.5-1.5mm. Every time the third roller rotates, the sensor outputs 6 pulse signals. The instrument samples this signal at any time and compares it with the drum's linear velocity. When the predetermined set value is reached, a stop signal is sent to stop the two motors at the same time, so as to ensure that the maximum braking force is measured and that the car tires are not stripped by the drum. 2 stepper motor computer control An ordinary stepping motor drive device is composed of a logic circuit and an amplifying circuit, the design is complicated, and once the circuit is determined, it is very difficult to change the control scheme. Computer technology opens up new ways for the control of stepper motors. Because the stepper motor driver itself is a digital device, it is best suited for digital control. The microprocessor can use the software program to complete various control schemes of the stepping motor, such as the functions of speed, jog, forward and reverse. In the hardware design, there are points of interface direct control and interface indirect control. Direct interface control means that the microprocessor output port is directly connected to the driver. At this moment, the output port of the microprocessor serves as the role of the ring distributor. Various energizing methods, function conversions, pulse generation and counting of the motor can be implemented by software. Interface indirect control is the use of special pulse distributor circuit, such as the PMM8713 dedicated integrated block to replace the motor power control part of the microprocessor interface, the microprocessor burden is relatively reduced, you can control the motor outside the work. In terms of software design, the procedure used to control the operation of the stepper motor is very simple. Taking the direct interface control as an example, the microprocessor first determines the direction of the motor and how many steps it takes to run the data. The data is placed in various input ports; the microprocessor reads the input port data according to the instructions, and then outputs the command to change the power status. At this time, the number of steps to be run is reduced by one, waiting for a certain period of time T; the next step is to change the power-on state until the end of the operation. Among them, the waiting time T is usually called the delay time, and the power-on state is stored as data in the data memory. For the indirect interface circuit, the microprocessor program is simpler. It only needs to output a pulse of a certain width to the outside at a given frequency. Changing the delay time T will cause the operating frequency of the motor to change, so that the motor can be realized. Speed ​​control. 3 establish a stepper motor speed curve 3.1 The theoretical open-loop control of the speed curve has the advantage of simplicity and low cost. Phase control pulses can be generated by the microprocessor. The designer needs to know what limitations it has on timing. For example, the maximum stepping frequency of a specific load torque, the time when the motor accelerates the load inertia, and the like. In open-loop control, the load position has no feedback to the control circuit. Therefore, the stepper motor must correctly respond to each excitation change. If the excitation changes too quickly and the motor cannot move to the new desired position, the actual load position will have a permanent error with respect to the controller's expected position. The simplest open-loop control method is the one in which the step frequency is constant. The motor rotates at this frequency before reaching the target position. The phase control signal is turned on by the “start†signal, causing the motor to run at a stepping frequency equal to the clock frequency: the “stop†signal turns off this clock and stops the motor. The target position is sent to the subtraction counter and the number of steps executed is recorded in this counter. The clock pulse is sent to the timing generator and the subtraction counter at the same time. Thus, the phase excitation changes at a constant clock frequency and the subtraction counter records the instantaneous position of the motor relative to the target. When the load reaches the target, the content of the subtraction counter is reduced to zero. The "stop" signal of the clock uses this "zero" produce. If the constant clock frequency is adjusted too high, the motor cannot accelerate the load inertia to the corresponding step frequency; the system either does not work at all or loses the step at the beginning of the trip. From standstill, the highest stepping frequency at which the motor can respond without desynchronization is called the "startup frequency." Similarly, the "stop frequency" is the highest step frequency at which the system control signal is suddenly turned off and the motor does not pass through the target position. In actual work, the starting frequency is often obtained through experiments. Because the starting frequency of the stepper motor system is much lower than its maximum operating frequency, in order to reduce the positioning time, the motor is often operated at a speed close to the highest speed by acceleration. As the target position approaches, in order to stop the motor smoothly, the stepping frequency is gradually reduced to the stop frequency again. The stepping frequency changes during the whole process of the motor moving from the initial position to the target position. If the curve is expressed, the "speed curve" is obtained. Perfect open-loop control can make the system very close to its maximum operating frequency. 3.2 Formulas Derive the six quantities associated with the velocity profile. They are: downtime time(s), maximum velocity velocity (Hz), rise/fall velocity ramp alpha (Hz/s), step angle delta (degrees), Shutdown frequency (Hz), diameter D (mm). The stepper motor will go through a step angle delta every step, so the meaning of each step corresponds to the distance = delta180°πD2D. It is not difficult to see from the above formula. Therefore, it is easy to think of the total number of steps, to determine the speed curve, as long as the coordinates of the three turning points can be determined. If the speed curve is a two-fold line, it means that two of them coincide. These coordinates are calculated only to show the curve on the interface. Actually transmitted to the lower computer is three steps. Let the coordinates of the three turning points be (X1, Y1), (X2, Y2), (X3, Y3). Assume that the speed curve is a tri-fold line. Then Y1=Y2=velocity, Y3=shutdown. Because the area under the curve is the corresponding number of steps, the following three equations are established: Step number 1=Step number 2=Step number 3=And step number 1+Step number 2+Step number 3=totalSteps Then we get a system of equations: This solution will determine the speed curve. Substituting the coordinates into the step formula yields 3 steps. If the speed curve is a two-fold line, there will be X2 4 conclusions It can be clearly seen from the speed curve theory that when the stepping motor can reach the maximum starting frequency; when it is stopped, the maximum braking force can be taken, and at the same time, the tire stripping phenomenon does not occur.
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