Crane Intelligence with Remote Monitoring via Internet

 

Arnav Bansal¹, Gurpreet Singh², Harkamal Preet Singh^2*

¹Scholar, Panjab University SSG Regional Centre, Hoshiarpur

²Assistant Professor, UIET, Panjab University SSG Regional Centre, Hoshiarpur

 

ABSTRACT:

This paper briefly introduces the concept of Internet-of-Things being used in Industrial Applications. The intelligence feature is nothing but a set of interlocks established so as to extend crane’s structure life and reduce accidents. For intelligence feature, data from a VFD (Variable Frequency Drive) is fetched via MODBUS. Remote Monitoring is an extensive feature which allows the user to view data like SPEED (in RPM), TORQUE, and VOLTAGE in Real-Time via Internet. Apart from this, Physical Components Status like BRAKE Condition is also displayed on the Remote Monitor and if there is a fault, the Crane does not operate.

 

 

 

1. INTRODUCTION:

MODBUS is a communication protocol developed by Modicon in 1979[1]. This protocol operates in three modes i.e. RTU (Remote Terminal Unit), TCP/IP (Transfer Control Protocol/Internet Protocol) and ASCII (American Standard Code for Information Interchange). VFD uses RTU technology to transmit the relevant therefore MODBUS-RTU is used to fetch data.  The brain of the system is Raspberry Pi Model 2. Raspberry Pi is a credit card sized single board computer developed by Raspberry Pi Foundation. It’s powered by ARMv7 (Broadcom BCM2836) Quad Core Processor clocked at 900MHz, Video Core IV 3D graphics core and has 1GB of SDRAM (Synchronous Dynamic Random Access Memory). It is also equipped with 4 USB ports, 1 Ethernet Port, HDMI Output, Combined 3.5mm audio jack and composite video, CSI (Camera Interface), DSI (Display Interface). For general interfacing, it is equipped with 40 PIN GPIO (General Purpose Input/Output). Primary OS (Operating System) for Raspberry Pi is Raspbian (Based on Debian) and compiler used is Python IDE (Integrated Development Environment)[2].

 

MODBUS-RTU Communication is achieved via USB to RS485 Converter and the library used is Minimal Modbus developed by Jonas Berg[3]. For Internet Connectivity, WIFI Dongle is used (For Wireless Internet) but Ethernet Port (For Wired Connection) can also be used via LAN (Local Area Network) Connection.

 

2. SYSTEM DESIGN:

 

Fig – 1 – Pin Out of Raspberry Pi Model 2

The Input and Output Connections are made according to table - 1. At Input side (Fig – 2), generic Push Switch is connected and at Output side (Fig – 3), Inductive Relays are connected to Pi GPIO.

 

Fig – 2 – Input Side Connection to Pi

 

Fig – 3 – Output Side Connection to Pi

 

The next page  mentioned inputs are of Remote Control of the crane. There are few additional Hardwired Inputs which are listed in table -2.The crane is divided into 2 parts: Main Hoist and Cross Travel. Main Hoist is responsible for the lifting action whereas Cross Travel is responsible for movement actions along one defined axis.

 

Table – 1 – Input / Output List 

INPUT LIST	OUTPUT LIST
Emergency Switch	Main Contactor Relay
Main Contactor Switch	Up Relay – Main Hoist
Up Switch – Main Hoist	Down Relay – Main Hoist
Down Switch – Main Hoist	Speed Increase – Main Hoist (Relay)
Speed Increase – Main Hoist (Switch)	Speed Decrease – Main Hoist (Relay)
Right Switch – Cross Travel	Right Relay – Cross Travel
Left Switch – Cross Travel	Left Relay – Cross Travel
Speed Increase – Cross Travel (Switch)	Speed Increase – Cross Travel (Relay)
Shutdown Switch	-
Intelligent/Manual Switch	-
Inching ON/OFF Switch	-

Table – 2– Hardwired Inputs List 

 

Emergency Switch: It is a generic kill switch which when activated sends Emergency Kill Signal which disables the main contactor which in-turn cuts the power from drive thereby disabling any further action.

 

Main Contactor Switch: When activated by the operator, sends the Main Contactor ON Signal to the processor.

 

Up Switch – Main Hoist: When activated by the operator, sends the UP movement Signal to the processor.

Down Switch – Main Hoist: When activated by the operator, sends the DOWN movement Signal to the processor.

Speed Increase Switch – Main Hoist: When activated by the operator, sends the Speed Increase Signal for main hoist to the processor.

Right Switch – Cross Travel: When activated by the operator, sends the RIGHT movement Signal to the processor.

Left Switch – Cross Travel: When activated by the operator, sends the LEFT movement Signal to the processor.

Speed Increase Switch – Cross Travel: When activated by the operator, sends the Speed Increase Signal for cross travel hoist to the processor.

Main Contactor Relay: It activates main contactor provided Emergency Switch and Gravity Limit Switch are not activated.

Up Relay – Main Hoist: Initiates the UP movement of main hoist provided UP Limit Switch is not activated.

Down Relay – Main Hoist: Initiates the DOWN movement of main hoist provided DOWN Limit Switch is not activated.

Right Relay – Cross Travel: Initiates the RIGHT movements of cross travel hoist provided RIGHT Limit Switch is not activated.

Left Relay – Cross Travel: Initiates the LEFT movements of cross travel hoist provided LEFT Limit Switch is not activated.

Speed Increase – Main Hoist (Relay): Increases the speed of the current action of the main hoist.

Speed Decrease – Main Hoist (Relay): Decreases the speed of the current action of the main hoist.

Speed Increase – Cross Travel (Relay): Increases the speed of the current action of the cross travel hoist.

Gravity Limit Switch: It has the highest priority and is activated when Up Limit Switch fails. As a consequence, Main Contactor is shut off and it has to be realigned manually in order to start the crane functionality again.

Up Limit Switch – Main Hoist: When activated, sends a UP Maximum Position Signal to the processor indicating to stop up motion on main hoist.

 

Down Limit Switch – Main Hoist: When activated, sends a DOWN Maximum Position Signal to the processor indicating to stop down motion on main hoist.

Right Limit Switch – Cross Travel: When activated, sends a RIGHT Maximum Position Signal to the processor indicating to stop right motion on cross travel hoist.

 

Left Limit Switch – Cross Travel: When activated, sends a LEFT Maximum Position Signal to the processor indicating to stop left motion on cross travel hoist.

Shutdown Switch: When activated by the operator, gives the Power-off Signal to the processor which in-turn shuts down the processor.

 

Intelligent/Manual Switch: It is a selector switch which gives the operator the freedom to switch between Manual Hoist or Intelligent Hoist feature.

 

Inching ON/OFF Switch: It is a selector switch which gives the operator the freedom to engage/disengage the Inching Mode (Intelligent Hoist Feature). It does not operate under Intelligent Hoist Mode as Intelligent Interlocks defined in the script are not compatible with this mode thereby creating undefined states, hence can only be engaged if Intelligent Mode is deactivated, but is still referenced as an Intelligent Hoist function.

 

3. SYSTEM FRAMEWORK AND FLOW OF CONTROL:

There are two major processes involved during the operation of the crane as mentioned in Fig 4. The first one is the interlocks scripts which involves reading the data from VFD via MODBUS and subsequently performing the relevant functions. The second one is reading the data from the VFD via MODBUS-RTU and uploading it to the Remote SQL Server via database connectivity library known as MySQLdb developed by Andy Dustman. The first script or the main script, as presented in fig 5, run in foreground and second script or the data logging script runs in background. This multitasking feature of Raspberry Pi helps in achieving a glitch-free system. The main script allows the user to switch between two operating modes of the crane namely, Manual Hoist and Intelligent Hoist. Further user can engage/disengage Inching Function provided crane is operating under Manual Hoist mode.

 

Fig – 4 – Multiprocessing - Block Diagram

 

Fig – 5 – Main Script – Flowchart Initial Part

 

3.1  Manual Hoist Framework

Manual Hoist is nothing but non-monitored crane functionality. In this, there are basic interlocks which are independent of MODBUS readings but are dependent on physical input status of various Limit Switches. It also includes a special function Inching Feature which operates under this division. But still Remote Monitoring Script runs in the background so that data can still be viewed by the operator remotely.

 

Fig – 6 – Manual Hoist Script – Flowchart Initial Part

When inching mode is disengaged, as shown in fig 6, the hoist functions in a simple way that is it performs as per the function selected by the user through the push button until user releases the switch thereby disengaging the command or the extreme position is encountered which is sensed by Limit Switches installed in the hoist.

 

Fig – 7 – Main Hoist Flow Chart

 

The actual working of Main Hoist is described in above flowchart in fig 7. The Main Hoist is capable of only UP and DOWN movements. The Cross Travel Hoist is responsible for RIGHT and LEFT movements. Both of functions are in same script and work in parallel to give precise control to the operator. The flow chart of Cross Travel Hoist is given below in fig 8.

 

Fig – 8 – Cross travel Hoist Flow chart

3.2 Intelligent Hoist Framework:

Intelligent Hoist Script contains some interlocks based on TORQUE values and TIME, in which TORQUE values are fetched through VFD via MODBUS-RTU. The idea behind this is to make hoist intelligent so that it can sense situations which are difficult for the operator to recognize in time which leads to either damage in crane structure or even accidents in some cases. Despite additional interlocks, the original interlocks like ESP (Emergency Switch) and GRL (Gravity Limit) still come into play.

 

 

Table – 3 – Intelligent Hoist Features List

 

 

Inching Feature is referenced under Intelligent Hoist as it gives the operator to have precise lifting control, but still does not operate under Intelligent Hoist. It is because of the fact that other interlocks don’t work in inching feature due to timing interval issues, hence to avoid uncertainty it works under Manual Hoist.

 

 

3.2.1 Load Touch-Down Protection:

In a conventional hoist, when the load hits the ground and operator still sends the DOWN command, the rope on the pulley gets loose and eventually comes out and thereby damages the lifting structure of the hoist. To avoid this situation, Load Touch-Down protection interlock comes in handy. This interlock automatically stops the hoist when load hits the ground even if the DOWN command is activated. When the DOWN command is disengaged and then re-engaged, the pulley comes down so as to free the load from the hoist.

 

 

The concept behind this interlock is TORQUE variation in specified amount of TIME interval. As soon as TORQUE value drops by 12% or more (found experimentally for motor speed above 5%) in 1.5 seconds (found experimentally for all motor speeds), the load hits the ground.

 

Now in the starting, there are large TORQUE variations in the motor due to starting or stalling torque, hence first value is calculated after 2 second interval as shown in fig 9.This is applicable to one cycle that is until the motor is not stopped. As soon as motor is stopped, 2 second delay comes into play.

 

Fig – 9 – Induction Motor Graph

 

Fig – 10 – Load Touch-Down Flowchart

 

In fig 10 the flowchart for Load Touch-Down is for Constant Motor Speed that is 80%. The value of TORQUE difference varies with Motor Speed. Experimentally, the TORQUE difference for Motor Speeds less than 5% came out to be 2.2%.

3.2.2 Jerk Load Protection:

In a conventional hoist, when the load is lifted upwards and Motor Speed is large, the lifting mechanism tends to experience a jerk as soon as the load leaves the ground. This jerk damages the crane structural integrity and hence the safety of the equipment is compromised. Jerk Load Protection interlock helps in avoiding this sort of situation. This interlock automatically decreases the hoisting speed (to 10% Motor Speed) as soon as it senses that load has left the ground irrespective of the fact that the operator has sent the Speed Increase Command or not. The system decreases the hoisting speed for about 4 seconds and then increases the speed till the hoist has attained the original speed that is the motor speed before the interlock decreased it.

 

The concept behind this interval is the rise in TORQUE value in a specified amount of TIME frame. As soon as TORQUE value increases by 8% or more (found experimentally for motor speeds above 50%) in 2 seconds (found experimentally for all motor speeds), the load leaves the ground. Again there is a 2 second interval before the first set of values are recorded due to starting or stalling torque of induction motors as discussed above. Rests of the values are noted without the interval. This is applicable to one cycle that is until the motor is not stopped. As soon as motor is stopped, 2 second delay comes into play.

 

Fig – 11 – Jerk Load Flowchart

The fig 11 shows the flowchart for Jerk Load Protection is for Constant Motor Speed that is 80%. Also this interlock does not engage if the initial motor speed is less than 50% as the hoisting mechanism does not experience a jerk for motor speeds that are less than 50%.

 

3.2.3 Inching Feature:

When precise control over hoist’s lifting height is required, Inching feature is used. Jerk Load Protection and Load Touch-Down Protection Interlocks does not work in this feature but conventional hoist’s interlocks like GRL and ESP still operate in this feature. The lifting speed is set to 10% so as to have a precise control. In this feature, the hoist moves a specific amount of distance set by the operator as soon as movement command is received by the system. The hoist moves that specific distance regardless of the current command status and then stops until the command is resent to the system. The lifting height is resolved using the following formulae:

 

Let the RPM of Motor = 1250

And the gearbox of the hoist is designed in such a way that Hook Speed is 3 MPM (Hook of the Hoist covers 3 Meter height in 1 Minute at 100% Motor Speed).

 

So time ‘T’ (in seconds) required for covering ‘dis’ distance (in cm) at 10% motor speed:

 

                    T = (30*1250*dis)/(125*6)

 

According to the above equation, if the UP or DOWN relay is activated for ‘T’ seconds and motor speed is 10%, then the load moves for ‘dis’ distance. 10% Motor Speed is achieved by programming the VFD in such a way that when Speed Decrease Relay is activated, Motor Speed drops down to 10%.

 

Fig – 12 – Inching Feature Flowchart

The value of the distance ‘dis’ is fetched from the form and hence operator can assign any value. The Motor Speed is fixed and hence Speed Increase Input is disabled for this feature.

 

3.2.4 Data Logging or Remote Monitor Framework:

Remote Monitoring Framework consists of 2 parts that are Front-End Script and Back-End Script. The job of this feature is to display essential parameters of crane like TORQUE, BRAKE Condition, CURRENT and BUS VOLTAGE on a form application.

 

3.2.4.1 Back-End Script:

This script (fig – 13)  runs on the Raspberry Pi and the function of this script is to fetch data from VFD via MODBUS-RTU and store the relevant data on a Remote MySQL Database.

 

Fig – 13 – Back-End Script Flowchart

 

3.2.4.2  Front-End Script

In fig 14 the script runs on the operator’s machine. The function of this script is to connect to Remote MySQL Database and collect data and if required write relevant data in the Database. The script is nothing but a simple Visual Basic Form Application which uses dotNet (ADO.NET, developed by MySQL) connector to connect to Remote MySQL Database.

 

Fig – 14 – Front-End Script Flowchart

 

4. REFERENCES:

1.     Drury, Bill (2009). Control Techniques Drives and Controls Handbook (PDF) (2nd ed.). Institution of Engineering and Technology.

2.     Raspberry Pi FAQs - Frequently Asked Questions. Raspberry Pi. Retrieved 2017-04-08.

3.     Berg, Jonas (2016). Minimal Modbus Documentation (PDF) (Release 0.7).

4.     Dustman, Andy (2016). MySQLdb Documentation (PDF) (Release 1.2.4b4).

 

 

 

Received on 18.05.2017                             Accepted on 11.07.2017

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