A linear position transducer on an injection molding machine is a position sensor that measures where a moving part is, continuously and along its whole stroke: the moving platen, the injection screw, the ejector and, on some machines, the injection carriage. The controller uses it to show positions on the operator panel, to change speed and pressure at the positions you set, and to measure shot size and cushion. On many hydraulic machines it is the main position sensor of each axis.
Where linear transducers sit
Each transducer is mounted parallel to the axis it measures. Its body is fixed to the frame or another stationary part, and its rod or slider is linked to the moving part, often through a ball joint or a flexible link that absorbs small misalignment. Typical places are along the clamp unit for the moving platen, along the injection unit for the screw position, and at the ejector.
The drawing beside this page shows the limit switch and the proximity sensor that detect single positions around the mold area. A linear transducer is not drawn: unlike those sensors, it reports every position along the stroke, not only whether one point has been reached.
Types of linear position transducer
| Potentiometric (resistive) | Magnetostrictive | |
|---|---|---|
| Principle | A wiper slides along a resistive track | A position magnet moves along a waveguide, and the sensor times a pulse along it |
| Contact | The wiper touches the track | No contact |
| Wear | The track and wiper wear, most at the positions used most | No wear of the measuring element |
| Output | A voltage proportional to position | Analog voltage or current, or a digital signal |
Potentiometric transducers, in rod or slider form, are common on hydraulic injection molding machines. Magnetostrictive transducers are used where a longer life or a finer resolution is needed. On all-electric machines, positions are usually calculated from the servo motor encoders instead.
How a potentiometric transducer works
The controller feeds a stable reference voltage across the resistive track. The wiper picks off a voltage proportional to its position, and the controller converts it to millimeters using the zero and full-stroke values stored during calibration. Dirt, wear or poor contact on the track shows up directly as a jump in the position reading.
Signs of a worn or faulty transducer
- The position on the panel jumps or flickers, often at the same point of the stroke.
- The reading drifts, or the zero position changes from day to day.
- A transducer or position alarm, or a reading that stays at one value.
- Speed and pressure changes happen in the wrong place, or the shot size, cushion or mold-open position vary although the process has not changed.
Causes
- Wear of the track and wiper at the positions passed every cycle, such as the mold-closed position or the cushion position.
- Side loads from a misaligned mounting or a seized ball joint, which wear the rod and its guide.
- Loose brackets that let the body or the rod move.
- A damaged cable or connector, oil inside the connector, or electrical noise picked up by a poorly shielded cable.
- An unstable reference voltage from the power supply board.
Checking and replacing a linear transducer
Wiring checks and replacement are for qualified electricians. Change a transducer only with the machine switched off at the main isolator and locked out, and with the axis in a safe position.
- In manual mode, move the axis slowly and watch the position on the panel. A jump at the same point every time points to the track.
- With the machine in a safe state, have the electrician measure the output signal along the stroke; it should change smoothly.
- Check the mounting, the brackets and the ball joints for play or binding.
- Isolate and lock out the machine, then fit a transducer with the same stroke, body length, mounting, output signal and connector.
- Align the rod with the axis so that it runs freely over the whole stroke.
- Calibrate the zero and full-stroke positions in the controller as the machine manual describes, then check the positions before running production.
What to send for a replacement quote
- The type (rod or slider, potentiometric or magnetostrictive) and a photo of the label.
- The electrical stroke and the overall body length.
- The mounting: clamps, rod-end thread or ball joint.
- The connector or cable, and the output signal.
- Which axis it measures, and the machine brand, model and serial number.
The proximity sensor guide covers the sensors that detect single positions, the pressure transducer guide covers another analog sensor of a hydraulic machine, and the electrical system guide gives the overview.
Updated by the IMSPARES technical team.