Question 2: (40 marks)
Scenario 2 (Based on the notes you took whilst interviewing your client)
A small business commissions you to automate their process. Until now, they have operated their rebar straightening, cutting and bending machines, which are all installed level to a roller feeder system, manually.
It poses a bottleneck in their (largely manual) construction steel production process, so automating these machines are seen to be a required step in growth of this particular small business, to a point in future when they can afford to buy automated machines.
Picture source: http://www.ispc.com.sa/4.aspx
In the construction industry, steel is cast into concrete slabs, beams and other structures, to give it the required strength and rigidity.
The steel used most often, is called rebar and is a round, ribbed, heat-treated steel that comes in coils as shown.
Rebar are commonly available in diameters ranging from 6
mm to 46 mm.
As the rebar is uncoiled, it needs to be straightened by a machine. This 3-phase machine can be started / stopped by your PLC, but no speed control is fitted. The straightening machine was never designed to be automated and has no sensors or controllable devices.
After the rebars exits the machine at 0.5 meter per second, you introduced a length sensor which measures the length of the uncoiled rebar exiting the machine as well as a brake to forcefully stop the rebar un-coiling uncontrolled, due to the angular momentum of the de-coiler.
You did a few experiments with various rebar thicknesses and lengths and found that to control that rebar length and to prevent the rebar from wrinkling between the machine and the brake, we need to stop the straightening machine’s 3phase motor shy of our desired length and apply the double-acting 5/3 controlled pneumatic brake, once the full length is reached.
The following graph shows the results obtained for various diameters and lengths of rebar cut.
A similar rebar un-coiling/straightening machine is shown below.
Picture source: http://www.cosinmachine.com/sell–169837–scrap–steel–bar–straightener–my5–12.html
Picture source: http://ellsenrebarprocessingmachine.com/rebar–cuttingmachine/
Once the rebar is braked at the desired length, a guillotine (like the one shown to the left) is triggered by closing a dry relay contact for 500 ms, to execute one cut cycle to cut the rebar that was just uncoiled, to the desired length.
This guillotine is fitted 300 mm past the length sensor.
All rebar lengths are measured in mm. The output of the length sensors is sent to the PLC
as ten +24 V DC pulses per cm.
Sometimes, the brake slips or gabs pre-maturely on the ribs of the rebar. All rebar found to vary in length in excess of ±1”, should still be cut, but then rejected by two double-acting pneumatic cylinders, controlled by two 5/3 solenoid valves, fitted to the roller feeder table assembly.
These cylinders have a long push-bar fitted, that will ram the unwanted rebar off the feed table, to be salvaged and re-used in the manual process elsewhere.
A new rebar should then be cut to length in its place. You should keep count of successful and unsuccessful rebars produced.
After being cut and approved (length-wise), the roller-feed table advances the rebar till it is grabbed and forwarded by a pinch-roller feed, sitting 100 mm shy of the centre of bending on the bending machine, just before the support shoe/hydraulic brake combination. On its way, it flows through a second length measuring wheel, sitting 200 mm shy of the centre of bending,
By starting the bending pinch-roller feed, rebar is fed into the bender machine up to the length indicated by the recipe for the specific stirrup.
Briefly after activation the support shoe/hydraulic brake combination, your automation must start the bender motor. This involves a revolving roller that has sensing limit switches at 45o, 90o and 135o. To form a stirrup, the bending / feeding sequence is:
- No human body parts detected at bender
- Feed first ear length (3 x bending diameter, bending diameter is 5 x rebar diameter), forward bending roller motor to 135o, reverse bending roller motor.
- Feed short side, forward bending roller to 90o, reverse bending roller 4. Feed long side, forward bending roller to 90o, reverse bending roller
- Feed short side, forward bending roller to 90o, reverse bending roller
- Feed long side, forward bending roller to 135o to form second ear, reverse bending roller
- Remove formed stirrup manually.
- Only continue straightening and cutting a new length once detecting no human body parts at the bender
Typical stirrups and their use in casting pillars are shown next.
Picture source: http://debug.pi.gr/default.aspx?ch=34
Picture source: https://vasshaug.net/2014/06/04/revit–andreinforcement/
Below is a picture of a typical manual bending machine deck as well as some of the bends that can be achieved. These machines are currently operated by dual switches (one hand on each button required to start bend).
Picture source: http://stolendale.co/rebarbending–machines/
Shown next is a picture of automated bending machine that your customer wishes to buy in future.
Picture source: http://www.batterylisting.com/cnc–coiled–rod–stirrup–bending–machine–sale–now/
- Draw a picture (not to scale) of the complete production line, showing all machines, feed rollers, feed tables, air-operated equipment, sensors and their relative positions. (10 marks)
- If the control algorithm is written in Codesys and signed Integer (INT) variables are used to indicate the length of the rebar from the roller-wheel length-sensors discussed, calculate the longest square stirrup that can be formed theoretically. Show your calculations and
explain your thoughts used in calculating. (5 marks)
- Calculate the length of the feed roller table that is required to produce the theoretical
largest square stirrup calculated as above. (5 marks) d) Do you think that the bending equipment shown, will be able to automatically produce the largest square stirrup calculated above? Why or why not? (5 marks)
- What are the dimensions of the smallest square stirrup that can be manufacture by this production line? What constraints the dimension of the smallest stirrup most? (5 marks)
- Read the scenario and examine the experimental graph of empirical data found in experimenting with various diameters and lengths of rebar. What mechanical property of the rebar mostly influences the time the uncoiling machine should be stopped in advance,
before applying the brake and how is it calculated? (5 marks)
- What additional safety measures would you introduce to safeguard employees and
machines in this process? (5 marks)
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