Control of Automated Guided Vehicle with PLC SIMATIC ET200S CPU

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The principle of Hall-effect encoder is based on output voltage change of sensor in response to a change of magnetic field. The magnetic field is changed by rotation of shaft mounted permanent magnet. The magnetic field is sensed by two Hall-effect sensors TLE 7805L which are inthe phase of 90 degrees. The sensors produce 3 impulses for a motor turn. The gear ratio of planetary gear unit is i=91,66:1 and therefore we get 275 impulses for output shaft turn.

Figure 2. Connector cabling of the quadrature encoder performance

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The direction of rotation of the quadrature encoder depends on output phase sequence (shifted by 90 degrees). The outputs of both TLE 7805L sensors are connected with open collector and therefore appropriate pull-up resistor is necessary. This resistor is used to provide stable logic value 1 when the transistor is closed.The pull-up resistor is connected between TLE 7805L sensor output and supply voltage. The logic 1 level depends on supply voltage level.

Figure 3. TLE 7805L output connection schematic

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2.2. Motor Driver Design

Our main task is to control direction and rotational speed of wheels. In our case we use signals directing wheel rotation with 24V amplitude generated by 4DO digital output module, which is connected to PLC ET200S, so it is necessary to transform directioning signals to the TTL logic level first. We also transform PWM signals with 24 V amplitude generated by 2PULSE module. We use PWM signals to control the speed of wheel rotation. In H-bridge design we used MOSFET transistors as high power components because of their high durability during high current flows and lowerdrain-source resistance in closed statein contrary to the bipolar transistors. For the upper half of H-bridge we used P-channel power MOSFET transistors IRF4905 and for lower N-channel power MOSFET transistors IRF3205. The logical part of H-bridge that evaluates the direction of rotation, comprisesTTL logic circuits 74HCT08quad 2-input AND gate and the 74HCT04 hexinverter. The principal schematics of connected TTL logic circuits to H-bridge is shown on Figure 4.

Figure 4. TLE 7805L output connection schematic

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To limit voltage spikes we added two capacitors to motor power supply C=4700µF. We also added capacitor C=100 nF to limit high frequency noise from the brushed DC motors. The power supply part of motor driver board is designed to adjust battery voltage U=24V to TTL logic demands. We used power regulator LM7805 which stabilizes battery voltage to U=5V. The motor driver PCB is shown in the following figure.

Figure 5. Motor driver PCB

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2.3. Complete AGV Construction

Figure 6. AGV model

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Our AGV consists of four main parts.

2.3.1. The First - control System Part

The main part is modular decentralized periphery SIMATIC ET200S with integrated CPU. Our used configuration comprises following parts:

Ÿ CPU IM 151-8(F) PN/DP,

Ÿ Power module PM-E DC24. 48V

Ÿ Digital input module 4DI DC24V HF

Ÿ 2x Counter module 1 COUNT 24V/100 kHz

Ÿ 2x Digital output module 4DO DC24V/0,5A HF

Ÿ PWM module 2 PULSE

Ÿ Fail-safe 4 output digital module 4F-DO DC24V/2A

Ÿ Fail-safe 4/8 input digital module 4/8F-DI DC24V.

The PM-E DC24..48V power module provides the supply voltage for all the electronic modules in the voltage group. The 1 COUNT counter modules count both quadrature encoder impulses and evaluate the rotational speed of motors. For control of direction both motors digital output signal of the 4DO digital output module is used. The 2 PULSE PWM module controls rotational speed of both motors by pulse width modulation. The 4/8F-DI fail-safe digital input module processes signals generated from four microswitches installed on safety bumper.

2.3.2. The Second - power-sensor Part

AGV model part consists of following parts:

Ÿ 2x brushed DC motor Micromotors E192-2S.24.91

Ÿ 2x quadrature encoders with TLE 7805L sensors

Ÿ Motor driver DPS (2 x H-bridge)

Ÿ 4x microswitch ZIPPY VMN-10H-03B0-B-Z

Our AGV model power and sensor part uses differential drive chassis. Geared motor shafts are connected directly to driven wheels and stability is ensured by supporting omnidirectional wheel. The motor driver evaluates signals from the control system part and determines each motor performance and direction of rotation of each wheel. The rotational speed of both motors is sensed by quadrature encoders. The Hall-effect sensors TLE7805 generate rectangular impulses with amplitude of 24V. To stop AGV in case of collision we used two safety bumpers which are connected to four microswitches ZIPPY, which in case of collision switch on automatically. The vehicle stops after assessing the signals by 4/8F-DI fail-safe module.

2.3.3. The Third – communication Part

Communication part consists of client module SCALANCE W747-1RR. This module estabilishes wireless communication between the vehicle and human machine interface HMI in IWLAN network using PROFINET communication protocol. This module has one radio range which enables operation in 2,4 GHz or 5GHz frequency range. Furthermore the module also supports Rapid Roaming function. The Rapid Roaming function enables the moving client modules to switch between different access points and ensures interruption-free transmission of the wireless signals

2.3.4. The Fourth – power Supply of AGV

We used two sealed lead acid batteries Long WP1236W. These batteries provide supply voltage 12V with capacity of 9Ah. They are connected into series, so we got supply voltage 24V necessary to supply PLC, motor driver and motors. At maximum performance AGV is capable of performing for 6 hours.

The complete AGV construction is shownin the following figure.

Figure 7. Block schematic of AGV model

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3. Control Program

Control program was programed in LAD, FBD and STL programming languages. It was developed in engineering system Step 7. Main cyclic block OB1 was used in the program. Next, OB100 warm reset block, which is initialized after start of PLC makes initialization operations, setting of PI controller coefficients and reset of variables. The next used block is OB35 periodic interrupt block for PI controllers with 10ms period of execution. Since we use the combination of safety PLC, mobile panel with emergency stop and PROFISAFE profile, micro-switches, our program implements safety program, too. This part handles with immediate stop of vehicle in a case of accident, lost connection between panel and PLC or after pressing the safety button. OB34 block is used for safety part of the program, because OB35 block has very low period of interruptions for providing the safety function.

As the first, we introduce function blocks of organization block OB1. The main goal of FB1 “pulse_start” and its data block DB1 “2PULSE” is PWM controlling. This control program processes values of 2 PULSE module and sending level of PWM into output addresses. Blocks FB7 “countL” and FB8 “countR” are used for measuring of both motors velocities. FB5 function block “Control_of_ motors” implements control algorithm for differential drive chassis. Inputs for this block are tangential velocity of central point in percent and radius of vehicle in centimeters. FB5 block also consists of other functional blocks, which process given functions or adjustment. FB2 block “Calculation of wheels velocities”, as it is clear from the heading, counts velocities of wheels and adjustment of input range velocities of central point. The input is in percent and the limit is from -100% to 100%. During the design of FB5 block program we had to describe differential motor chassis. Simple design of differential chassis is shown on next figure.

Figure 8. Description of differential drive

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From figure we can extrapolate these equations:

(1)

(2)

(3)

After deriving velocities v_Land v_R from (2) and embedding them into equation (1) we get final equations for velocities of left and right wheel with all known variables and constants.

(4)

It is important to take into account the case when R=0. Zero turning radius means turning around the center of the vehicle. This leads to dividing by zero in (4). We solved this problem by giving velocity of right and left wheel according to central point velocity. For example, if we want positive central point velocity and turning radius R=0, we must use positive central point velocity for the left wheel and negative velocity for the right wheel. The vehicle will turn clockwise. If we want the direct movement of the vehicle, the turning radius must be infinite. In our program the limit of radius is R=9, 9e5 cm, higher values lead to direct movement.

Next important factor is to keep wheels velocity ratio in constant turning radius after changing vehicle velocity. Accelerating or slowing down is realized by adding or subtracting the constant of acceleration. If the speed of both motors is changed with same constant of acceleration, the turning radius is changed. This state is solved by counting difference between desired and actual value of speed for each wheel separately, by next equations:

(5)

Next, we compare these values and count a new constant of acceleration C_La- left wheel and C_Ra– right wheel from C_a – default constant of acceleration.

(6)

(7)

Function block FB3 “velocity sat” is used if the central point velocity is v_T= 100 % and turning radius is for example R =1000 cm, desired left wheel velocity is bigger than 100% and it is impossible to reach it. In this case the program will change central point velocity for reachable value for both wheels. In this case we use following equations.

(8)

(9)

(10)

(11)

The saturation of wheels velocities change the central point velocity and new recalculation is needed.

(12)

(13)

Function block FB4 “Acceleration” is used for increasing or decreasing the velocity by ramp curve. In case of acceleration, the constant of acceleration is added to current velocity in every step of calculation. In case of slowing down the constant is removed in every step. Each step is activated by “clock” bit. If the value exceeds limit, the saturation is activated.

Function block FB6 “absolute percentage direction” makes absolute value from velocities of wheels, which range from -100% to +100%, while the sign plus / minus is turned to logic value of output bits of 4DO module. If the sign is positive the logical value is 1(forward). Negative sign means logical 0 (backward).

The FB9 “twist ctrl” function block. Since control wheel on Panel 277F IWLAN mobile panel hasn’t zero position, the software calibration of this position is performed after start. This block also solves the direction of control wheel rotation. Pulses are generated by twisting of the control wheel. IB6 byte is used while the control wheel is turning clockwise and IB7 is used for counterclockwise movement. After the start both bytes are reset to zero. The value of impulses in bytes is always a positive number; bytes IB6 and IB7 are changed by rotation of the control wheel. The upper limit of impulses is 256 and after overshoot there is zero again. Minimum and maximum position of the wheel pos_wheelis programmed from -40 to 40 pulses. As we change turning radius R , individual movements like forward or turning to sides are described by following equations:

(14)

(15)

(16)

This block also processes signals from mobile panel MP 277F IWLAN, such as safety stop, normal run and stop as well as radius and central point velocity variable setting.

Measured transfer characteristic of motor, which is used in our work is on Figure 9. It is clear that in the change of motor power from 0 to 100%, the change of rotational speed is nonlinear.

Figure 9. Transfer characteristic of motor

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FB41 CONT_C ICONT block compensates this nonlinearity. FB41block consists of PID controller. We used only PI part in closed loop, derivation part is turned off. Constants of PI regulator are set experimentally. Firstly, PI control ensures half velocity of motors in 50% motor power. The controller also compensates the change of the load. If the load is changed, the controller changes the manipulated value. Controller tries to establish desired velocity for vehicle.

The function of the program is shown in the following figure.

Figure 10. Block schematic of SW solution

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4. Communication between AGV and HMI

Vehicle communication is based on wireless network IWLAN (IEEE 802.11i standard). The system uses two separate access points SCALANCE W788-2RR and SCALANCE W788-1RR. Client device SCALANCE W747-1RR is placed on vehicle base. This device is connected to access point with the strongest signal using Rapid Roaming function. CPU SIMATIC ET 200S device includes 3 ports PROFINET switch, which provides connection between the client module and PLC.

Figure 11. Network infrastructure

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The vehicle is controlled by touch panel SIMATIC MOBILE PANEL 277F IWLAN Figure 1. This panel consists of 8” color TFT display, 21 hardware buttons and one control wheel. The panel is wirelessly connected to one of the access points, which furthermore routes the communication to the client module installed on the vehicle.

The visualization for panel screen was designed in WINCC Flexible program. The velocity and orientation of vehicle are controlled by panel. The rotation of vehicle can be controlled by rotation of wheel or by value directly inserted into input field. The velocity can be set in three ways: the velocity can be changed by a slider, minimal and maximal values can be set by buttons or by entering into input field. The vehicle starts after pressing “Start” button. To stop the vehicle we use “Stop” button. In case of collision it is possible to use “Emergency Stop” button for fast stop of the vehicle.

Figure 12. SIMATIC mobile panel 277F IWLAN

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