Mechatronics Volume 8 Issue 1

They need to avoid undershoot where the mold is not completely filled as well as overshoot which can cause flash—resin flowing out beyond the mold cavity where the mold halves meet.

Current machine technology is pushing to and past 800 mm/sec (31.5 in/sec). This means the speed that the position of the rods is delivered to a controller and the precision of that position value are critical in achieving the goals mentioned above. The trick is to very accurately control the speed, stopping point and time-to-stop from shot to shot. Especially enabling a higher speed, then decelerating extremely quickly for the shortest stopping distance. To achieve this level of control portions of motion during this process have to be monitored at a much faster rate than the screw is traveling to push resin into the mold.

Closed loop control systems can apply information to make very minute adjustments for each shot based on information from the previous shot if it is available to the controller soon enough.

It turns out that "soon enough" means taking position measurements that are accurate to < 1mm while moving at a high rate of speed and < 0.1 mm positioning precision at low speed. This position needs to be acquired between every 0.05 and 0.20 msec while delivering that position information to the controller in 0.25 msec.

These time values translate to having position sensors that are capable of sample rate of 0.1 msec and propagate the signal from the mechanical input to the signal output significantly less than 0.5 msec so that the controller has time to update the information to the hydraulic controls that move the screw and open/close the mold.

But there's a problem. Magnetostrictive technology can only acquire position information at a theoretical sample rate of about 0.2 to 0.25 msec. The physics of it reveals the echos, from the magnetic marker, can only travel so fast to the sensor. Current magnetostrictive devices also have a sample rate of 0.25 to 0.5 msec or longer. The way injection molding machine manufacturers are getting past this problem is to use advanced inductive technology position sensors. Instead of moving a magnet, they move an epoxy-sealed, precision inductive marker along the position sensor length. The result is sensors that are able to deliver about 1μm of precision to the output for the controller to read in 0.25 msec. And the rate of operation is between 10 and 15 kHz.

Some of the benefits and features of advanced inductive technology sensors are:

Not Magnetic—avoid errors from magnetized metal flakes or filings. TF1 Series' technology does not incorporate a magnet in the moving position marker that can trap flakes between the marker and sensor.

EMI Immunity—our advanced inductive sensing is immune to spurious errors from electro-magnetic interference (EMI) generated by high-powered machinery.

Speed—with an update rate of 100 μsec, it is much faster than virtually any magnetostrictive sensor, and it maintains its full accuracy up to its 10 kHz cycle rate.

Robust—TF1 Series of sensors maintain critical specifications, including absolute linearity of ≤ ± 0.025%, while operating—even when subjected to their specified 20 g of vibration and 100 g shock. Many magnetostrictive sensors do not come close.

Here's an in-depth look at one of these advanced inductive sensors and its unique capabilities and advantages.