Day: July 11, 2019

Week 9 – 11 Jul: The secondary arm is primary

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Aside from thinking about how to code to control the linear actuators, we got our hands dirty with some work on the ground. We have made great progress for the lifting arm and base. 

Firstly, to strengthen the support tower, we have decided to install three 1.5m 3060 aluminum profiles in parallel. These three 3060 aluminum profiles will have 30mm bore diameter pillow blocks attached to them. Then, the primary arm which has 2 3060 aluminum profiles will be attached via two pillow blocks to the support tower.

We tried to attach the wheels onto the base (after tipping the whole frame onto its side). However, we didn’t have the right adapter for it. The holes for the wheels were huge and were wayyy bigger than the M6 screw size. We will have to buy and use some M6 adapters then.

To ensure that the new actuator can be mounted securely and properly onto the primary arm to control the secondary arm’s rotation, we had to rethink and readjust the configuration of the two actuators supporting the primary arm. We figured that the two actuators can be in parallel. We checked the extension length, speed of the 300mm actuators and estimated where the double actuators in parallel should be placed.

Here is the connection between the first arm and the second arm!

 

For increased safety, we decided to improve the sturdiness of the double actuator in parallel mount at the support tower. We went through 3 different types of mounts before settling on the 3rd design seen.

Here is our first design. We decided that better L-shape brackets can be used to improve the strength of the mount.

 

Here is our second design. The distance between the second and third clamp (from right) is too far to comfortably fit both actuators connected to first arm.

 

Here is our third and final design. We plan to use the sturdier L-shape brackets to support our actuators.

Close-up of the two actuators secured onto the common rod. Cable ties are also used as a placeholder for our trial purposes as we weren’t sure if this arrangement would work. Now that it does work, we will replace these cable ties with proper supports/mounts.

 

 

 

 

 

 

 

 

 

 

 

 

 

On the other end of these actuators, the two parallel actuators are secured onto the primary arm using two individual bracket-plates, which have a hole each. This allows another steel rod/screw to pass through them to secure the third linear actuator (not yet mounted) which will then support the secondary arm.

 

 

 

 

 

 

 

 

 

This video below shows the parallel actuator system in action with the secondary arm freely rotatable (actuator yet to be attached) about the pillow block. Some solid progress there!

Week 9 – 11 Jul: How to control dis?

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After looking at the specifications of each actuator, here is a summary of the actuator characteristics:

  1. 500mm actuator, 12V DC, 10mm/s, 750N load
  2. 300mm actuator, 12V DC, 5mm/s, 5000N load

Two main conclusions can be drawn from this:

  1. 300mm will extend more slowly than the 500mm actuator.
  2. Our chosen relay and programmable logic controller must be able to handle supplying enough current for 4x 12V DC

 

For the control of the linear actuators, we can choose between relays, motor controllers, or a combination of both. We probably need a combination of both so we procured relays and motor controllers.

 

For relays, we have procured the Jayoyi 16 Channel Relay Board Module Optocoupler. It is well-suited for our project as its power supply satisfies the DC 12V range. Also, it is one of the best PLC relay expansion modules available as it comes with 16 channels and has an optocoupler. The perk of an PLC relay expansion module is that it increases the number of safe outputs of another safety relay. These are suitable in applications where several devices need to be stopped. The optocoupler enables electrical signals to be passed between low and high voltage circuits via short optical transmission paths while keeping them electrically isolated. This removes electrical noise from signals, and isolate and protect-voltage devices from high-voltage circuits. Essentially, this ensures that our lifting arm can work properly without erratic operation, which is certainly important for safety!

 

For motor controller, we have procured the Dual Channel H Bridge Motor Drive Module Controller. It satisfies the DC 12V range, has reversible control and PWM speed control. This means that there can be finer control over our actuators. The actuators should be able to be locked in certain position with the motor controller. This will be helpful for the control of the double actuator configuration.

Week 9 – 10 Jul: Actuator Algorithm

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Before we can code anything to control the actuators in our lifting system, we first need to derive an algorithm for the process. As of now, we are not sure which actuator will take longer to fully extend as we have not completed our real-life motion study, so two key assumptions will be made:

  1.  500mm actuators take longer to extend as they are longer in length.
  2. The first actuator (closer to support tower) of double actuator configuration will take longer to extend as it will likely bear more weight.

 

Here is Seet Ynn’s draft summary for the actuator algorithm (if simplified to up and down motion, and forward and backward motion of arm is ignored):

  1. Press UP button, then all 4 actuators extend
  2. While length limit of 2x 300mm double actuator is reached, 2x 500mm actuator will continue to extend.
  3. When reach lifting arm height of 2.75m, first actuator of double actuator configuration will stop extension and second actuator of double actuator configuration will continue to extend.
  4. When reach lifting arm height of 2.8m, second actuator of double actuator configuration will stop extension.
  5. When the lifting arm stops moving, SMRT technicians can proceed with unit replacement.

 

Here is Seet Ynn’s draft, in full, with thinking process:

  1. Press UP button, then all 4 actuators extend (2x 300mm double actuator in parallel between support tower and first arm, 2x 500mm double actuator).
    1. The 2x 300mm double parallel actuators have to extend together as they both handle the movement of the first arm. If only one of the actuators extend, this will cause the first arm to be bent sideways and cause unintended strain on the first arm.
    2. The 2x 500mm double actuator configuration must extend with the 2x 300 double actuator to increase the rate at which the arm lifts, especially since the 2x 500mm double actuator will take longer to extend.
  2. While length limit of 2x 300mm double actuator is reached, 2x 300mm double actuator will stop extension and 2x 500mm actuator will continue to extend.
    1. 2x 300mm double actuator cannot extend beyond length limit to prevent damage to the actuator and programmable logic controller.
    2. 2x 500mm double actuator will continue to extend to increase height of lifting arm.
  3. When reach lifting arm height of 2.75m, first actuator of double actuator configuration will stop extension and second actuator of double actuator configuration will continue to extend.
    1. Lifting arm height of 2.75m is chosen, as the targeted height of the lifting arm (excluding clamp system) for it to be within the working space is about 2.8m. A vertical clearance of 5cm and slower motion should be sufficient to ensure safety and good condition of the parts.
    2. Movement of a single actuator to ensure greater precision when the unit is inside the train and close to the small working place. This will better ensure the safety of workers who may have to adjust the unit in the working space to better align it to its lock. This greater precision is also important to prevent the unit from being scuffed up against the LED screen behind the working space, and vice versa.
    3. Since first actuator is slower than the second actuator, first actuator will be stopped and locked so the lifting arm will be slowed down but not too slow
  4. When reach lifting arm height of 2.8m, second actuator of double actuator configuration will stop extension.
    1. Any future extension of the double actuator configuration at this point may cause the unit to hit the top of the working space or be too high to slot into lock, even with our highly mobile double ball joint configuration
  5. When the lifting arm stops moving, SMRT technicians can proceed with unit replacement.
    1. Safety first!!
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