PG Position-Measuring Series — Technical Reference
A PG guide carries a magnetic scale inside the rail and a read head in the block, so the measuring system sits where the load is instead of on a separate glass scale bolted beside it. This page collects the facts that decide whether PG is the right answer for an axis: the sizes, the resolution options, what the controller has to accept, and what has to be protected in service. Block dimensions and the interface specification are issued on the signed drawing for the size.
Sizes and travel
| Size | Rail | Travel | Resolution options |
|---|---|---|---|
| 20 / 25 / 30 / 35 / 45 | PG rail with integrated scale | up to 30 m with joined rails | 5 µm or 10 µm, depending on the head |
Long travel is not one piece: the rail is supplied in lengths that are joined on the machine, and the scale is written so it continues across the joints. That is why the joint positions are set on the drawing rather than left to the fitter — a joint in the wrong place is a position error the controller cannot see.
Resolution is not accuracy
The 5 µm figure is the smallest step the head can report. The accuracy the axis actually holds also depends on rail straightness, the mounting surface, the temperature of the structure and the controller's own loop. A machine that quotes 5 µm resolution and bolts the rail to a welded frame holds neither. Decide the resolution from the control loop, then check the structure can support it.
The controller interface decides the order
A read head outputs a specific signal — sinusoidal, TTL or a serial protocol — and the drive has to accept exactly that. This is where PG projects stall: a rail is ordered, arrives, and the drive cannot read the head. Send us the drive model and the interface before the rail is ordered; matching the head afterwards is a new head and a new lead time.
What PG is not
It is not a drop-in for an HGR or EGR rail: the profile carries the scale, so the rail is specific to the system. It is also not a substitute for a good ball screw on a short axis — where the screw is accurate and the axis is stiff, the motor encoder is usually enough, and the extra cost buys nothing. PG earns its place on long axes, on structures that move with temperature, and where the customer has already decided they need direct feedback at the carriage.
Protection in service
The scale side is sensitive to swarf and coolant: the wiper and the sealed head are part of the system, not an accessory. Protect the rail on the scale side during assembly. Keep magnetic chucks and strong magnets away from the rail — the scale is written into the rail, and a strong external field is the one thing that can damage it after installation.
What to send with an enquiry
Size, travel, the drive or controller model and the interface it accepts, the resolution the loop needs, and whether the rail will be one piece or joined. Those five answers are enough to quote the head and rail as a matched set, which is how PG is supplied.
How the loop closes
With a scale at the carriage, the drive closes its position loop on the machine rather than on the motor. That changes how the axis behaves: pitch error of the screw, thermal growth of the frame and backlash in the thrust bearing are all inside the loop, so the controller corrects them instead of living with them. It also changes the tuning — a loop fed by the carriage is stiffer and less tolerant of a loose mechanical assembly, so the machine has to be mechanically sound before the feedback helps.
That is the practical test for whether PG is worth the cost: if the axis already holds position with the motor encoder and the parts are good, PG adds little. If the machine is long, or grows with temperature, or the customer measures the part and finds a drift that the controller never saw, the scale at the carriage is the fix.
Mounting practice
The rail mounts like any HG-standard profile, but the scale side sets two extra rules: keep the wiper and head sealing intact during assembly, and never let a magnet or a magnetic chuck come near the rail. Rails are joined on the machine for long travel, and the scale continues across the joint, so the joint position is set on the drawing. Check the mounting face along its full length before bolting — the head reads the rail, so a bow in the rail becomes a position error that no tuning removes.
What PG does not fix
It does not correct a flexible frame, a loose bearing or a screw that whips at speed: those are mechanical problems, and putting a scale on them only makes the controller chase them. It also does not replace the wiper. The most common failure in service is contamination on the scale side, not the electronics.
FAQ
| Is the encoder in the block or the rail? | The scale is in the rail, the read head in the block. They are supplied as a pair. |
| Can I retrofit PG to an existing machine? | Only if the controller accepts the head output and the mounting is changed to the PG profile. It is not a drop-in for an HGR rail. |
| Why is there no dimension table here? | Block dimensions and the interface specification are issued on the signed drawing for the size. We do not publish a figure we have not verified for that size. |
| What resolution can I have? | 5 µm or 10 µm depending on the head; the accuracy of the axis is set by the whole system. |
Series overview: PG series · rail length and joints · preload bands · request a quote.