Tip of the Day 168: Of Pinched Piezo Cables Signal Drift

At a recent customer visit we were testing the tool on the bench. We were pushing on pins while watching values on the Cycle Values screen and noticed one changing when we were not touching it. The value was continuously going down.

The cause? A pinched piezo wire.

Moral of the story: Be very careful of piezo cables. Do not pinch, crush or kink them

                                                   (or fold, staple or mutilate).

Any damage to a piezo cable can cause drift in the signal. It also can cause other weird pressure data behavior like plateaus on the graph or other behaviors not natural to plastic.

Some Technical Discussion

If you recall from tip #165 any dirt or water that can provide a path between the signal wire and the shield can cause drift. This is due to the natural behavior of the piezo amplifier circuit injecting electrons across the dirt.

In the case of a pinched wire the electrons can get in from outside. Any hole punctured in the shield provides a path. A very small hole or pinch may leave a tiny hole with a large resistance. Furthermore the insulation resistance of a crushed cable will decrease because the insulating material is squashed thinner.

But the resistance with the hole is not as large as a sealed cable. Thus any pinched point, especially while it remains pinched, can create another path from ground into the inside of the cable. Hence the drift.

Note: Our specification for drift is less than 0.2% in 1 minute. Usually the Lynx amplifiers and cables are much better than that.

Tip of the Day 165: Visual Fault Detection for Piezo Sensors

In tip 164 we mentioned how the electronics do not automatically detect sensor problems for piezo sensors like they do for strain gage. Howerver the data provide useful indications if a piezo sensor is having problems.

Zero pressure

If the sensor reads zero pressure throughout the cycle but the parts are full and packed then the cable between the piezo adapter and the sensor itself may be disconnected.

1. 1. Add “Plastic Pressure / sensor location” to the Cycle Values window or open Raw Data Viewer and watch it there.

1. 1. Open the mold and press on the sensor face or ejector pin with a brass pin. The readings should go up and down. If they do not the cable may be disconnected.

Drifting Pressure

If dirt gets into the connections it provides a path for the electrons to move around the sensor and influence the reading. You can see this in a steady rise in the pressure value while the mold is open like this:

The pressure can also be falling. But the normal scaling on the cycle graph does not show this. Add the “Plastic Pressure / sensor location” to the Cycle Values window to see if it is drifting down with the mold open. Or scale the low end of the curve on the Cycle Graph to something below zero (say -1000 psi) to see if it is drifting down.

If either a dramatic up or down drift is occurring then there could be dirt or water in a connection somewhere. This could be in the Fischer connector to the piezo adapter or the threaded connection between the sensor and its cable.

The solution is to disconnect the connections, clean them, let them dry and re-connect them. The document for cleaning these connections is located on the RJG web site here.

Tip of the Day 164: Piezo Sensors Do Not Detect Disconnects

It’s all in the title.

Some of our customers have thought that because our Lynx Piezo sensors have the integrated case electronics that they would behave the same as the strain gage sensors in detecting disconnections or broken wires. This is not true. When a piezo sensor becomes disconnected or its wire cut it simply reads zero pressure. Here is why.

A piezo sensor is made with a crystal sandwiched between two wires. It puts out charge (electrons) in one direction or the other during change in pressure. When the pressure is not changing it just sits there, inert and doing nothing.

The crystal itself is a superb electrical insulator. The schematic symbol looks like this:

The crystal, when not being pressurized or depressurized, looks exactly like air. Disconnecting the wire like this…

…appears to the Lynx electronics just the same as the sensor when connected. Thus if you disconnect a piezo sensor or do not connect the cable from the piezo adapter to the sensor then the electronics sees no change and reports zero pressure.

The strain gage sensors, on the other hand, will drive the electronics to the limit when a wire is cut, shorted or broken. This is how the strain gage electronics can detect that kind of damage and report to the eDART that the sensor reading is not good.

Tip of the Day 159: Be Kind to Your Sensors: Load ’em True

In recent weeks we have had some reports of sensor installations not up to standards. Specifically these were cases where the sensor was not supported or loaded on its center line. Like the three-legged stool, sensors will not work if you don’t have symmetrical support behind them.

Example 1: Pin Off Center

Using a force (ejector pin) sensor, the pocket holding the sensor was not centered under the pin. This example shows this problem in exaggerated form to make the point.

This causes the load to “twist” the front of the sensor creating inaccurate readings and eventual sensor damage. Also having the sensor tilted (non-parallel pocket floor) will cause the load to be off of the center axis.

Here is same example shown with a centered pin:

Example 2: Sleeve Cut Asymmetrically

While most customers use the threaded retainer nut sometimes a sleeve is used. In this example  the support sleeve for the sensor was sliced off on one side to make room for some mold component as shown here:

The force on one side caused the sensor to read innacurately.

Here is the same example with the sleeve with the proper small wire slot left at the back.

Conclusion:

Always install sensors so that the load is on the center axis of the sensor. Also ensure that the support behind the sensor is symetrical around the center. These requirements are called out on the sensor installation documents.

Tip of the Day 156: No negatives on stroke scaling, please

“Always with the negative waves, Moriarty…”        ( from…? )

A customer found a way to confuse an eDART™ by entering a negative number in the scaling for the stroke analog input (from the machine). If you are taking injection stroke from the machine instead of the LER-30, always enter a positive number for the stroke full scale like this:

We will probably lock out negative values for stroke on the next release.

The eDART figures out the correct stroke direction using the Screw Run signal and watching the direction of motion^†. It will invert the signal automatically if necessary to get a positive volume and stroke curves. That way we can analyze flow of material from the plastic’s point of view: How much went IN to the mold rather than how much is left in the barrel.

Also, the wiring should be such that the (+) terminal on the analog input always goes above the (-) terminal and not the reverse. The analog input module cannot see negative voltages.

^† If there is no Seq. Module Input / Screw Run going into the eDART then you still need to use positive numbers for full scale. Then set the direction correctly in “Sequence Settings”. The correct direction is that the stroke signal should go positive while the material is being injected into the mold. You also need to set zero at the point just before suckback.

Tip of the Day 141: Using Cycle Values to Reduce Cursor Handling

I have observed that some customers spend time moving the Cycle Graph cursor around to find different values on the screen. It is true that you can find many useful numbers that way. But this requires you to access the eDART™ using the mouse, move it around and read some small numbers. Thus it requires some skill in hand-eye coordination and curve interpretation.


The Cycle Values window can display many of the values that people use the cursor to try to find. It displays them in a larger font easier to read from a distance. Furthermore it does not require that you precisely position the cursor, especially if one cursor pixel is too coarse to find the number.

Here is a graphic that shows some common values that you can add to the Cycle Values instead of using the cursor.




With many curves displayed we also hear questions about how we could annotate the cursor values or graphs on the cycle graph to make them easier to distinguish. Even if we put the labels on they are still hard to read.

Here is an example with 32 sensors for which we want to know the peak for each sensor. Placing the peaks on the Cycle Values can be easier than trying to read the values at the cursor for each.

You can also keep the cycle graph up but just show the “#High” and “#Low” sensors and remove the rest (as in tip # 23 ). This gives you an “envelope” to which you can match templates while the Cycle Values show you the curves that are pushing the envelope.

 

Per tip # 80, remember that once you have a long list of sensors on the Cycle Values a single click on the “Value” header puts them all in value order. That way you can find the highest or lowest of reading quickly.

The eDART‘s optional “Balance Viewer” tool can simplify cavity-to-cavity comparison even further, especially for large numbers of sensors.

[youtube=UdEpaqtiqIE]

Tip of the Day 136: Positive Volume and the Zero Line

Over the years we have been challenged to explain why we show “Volume / Injection” as a value that goes up as the material is injected into the mold. The simplest answer is that “Systematic Molding” means thinking from the plastic’s point of view inside the mold rather than inside the machine. We want to show how much material you have put into the mold (a positive number) rather than how much is left in the barrel.

This means that we start the volume curve at the machine’s shot size setting: the point where the screw was when the screw motor stopped. Thus the decompression after recovery (“suck-back”) shows up as a negative number.

As an attempt to help visualize this I have created a video that shows the screw position in relation to the Cycle Graph volume curve. The cycle graph is rotated so that the tip of the screw lines up with the volume curve.

A second video shows the same thing mirrored so that the screw traverse is in the direction it is when viewed from the operator side of the press. In both videos the plastic is going into the mold above the dashed zero volume line.

Tip of the Day 138: Redundant Id Names

We have found that in the naming of sensors some people are duplication the location into the “Identifier” column of Sensor Locations. For example, thus:




This duplicates the information that the sensors are located at the end of the cavity (end of flow). Or sometimes people put “Cavity 1”, “Cavity 2” etc.


In the Alarm Settings and other tools the locations will show up like this:




We recommend that you simply place number or letters in the identifier column because the location is already indicated under the “Sensor Location” column. The simpler version looks like this:

Tip of the Day 132: The “Save Notes” tool for the eDART™

 

Clicking in the white area (or on “New Note”) allows you to begin typing a note.




Once typed, click “Save Note” and it is added to the last finished shot in the summary data.


The simple instructions go like this:

1. Clear the previous note by clicking “New Note”. This places the cursor in the white area.
   
    


2. Type your note.
   
    


3. Click “Save Note”

• You could you set up security for saving notes to put the operators name into the change log.
  
   


• Or you could just request that they put their initials into the note.
  
   


• If you intend to use the tool across the plant you could set up a view with the notes tool in a convenient location. Here is the view used by the “Conx” (machine-only) eDART showing the Save Notes tool circled in yellow:

• When using the Analyzer you can export all notes for a given period by clicking “Export” and adding the type “Text” and the location “Summary Note” as shown here:

Once opened in Excel sort on the “Note” column and all of the actual notes will rise to the top.

Tip of the Day 131: Naming Temperature Sensors

I recently discovered that a customer had made an error in naming the locations of in-cavity temperature sensors. The rules are these:

• 
  
  If the plastic contacts the sensor or the sensor is just behind the cavity wall use one of the in-cavity locations:
  
  Post Gate, Mid Cavity or End of Cavity. Then add an id for each cavity in multi-cavity applications.
  
   
  
  


• 
  
  For in-cavity sensors pick a location name relative to the flow path.
  
   
  
  


• 
  
  If the sensors are on the mold steel elsewhere you can use Parting Line or Mold Surface.