Legacy servo systems have a particular failure pattern. They don’t suddenly stop working — they gradually become harder to support. Spares get scarce. The drive manufacturer discontinues the model. The engineer who commissioned it retired years ago and the parameter backup is nowhere to be found.
Then one day a drive faults out and the lead time on a replacement is six months.
If you’re running ageing brushless servo installations — SEM motors being a common example in UK manufacturing — you’re probably already aware of this creeping risk. The motors themselves are often mechanically sound. It’s the ecosystem around them that becomes the problem.
The Real Cost of Doing Nothing
Keeping legacy servo hardware running is a legitimate strategy, up to a point. But it’s worth being honest about what “keeping it running” actually costs.
Sourcing obsolete drives and spare encoder modules through the secondary market introduces its own risks — no warranty, unknown operating history, and no guarantee the firmware version matches what’s already installed. Every repair becomes a small project.
The hidden cost is engineering time. The hours spent diagnosing an intermittent fault on a drive that hasn’t been supported for a decade are rarely captured anywhere, but they add up fast.
What a Migration Actually Involves
Replacing a servo axis isn’t just a like-for-like swap of motor and drive. Done properly, it’s an opportunity to address problems that the original installation might have introduced — or that the machine has accumulated over years of running.
The starting point is the mechanical interface: motor frame size, shaft dimensions, mounting arrangement, and whether the existing gearbox or coupling is still fit for purpose. A modern replacement motor might have a slightly different inertia ratio to the load, which affects how the drive needs to be tuned.
Feedback is the next consideration. Older installations sometimes used resolvers where a modern replacement would use a high-resolution encoder. The drive needs to be matched to the feedback device — and parameterised correctly, not just wired in.
Getting the Drive Tuning Right
This is where migrations most commonly go wrong. A new drive installed on the same axis as the old one won’t behave identically out of the box — the control loop gains need to be set for the specific motor, load inertia, and mechanical characteristics of that axis.
PID tuning, current loop bandwidth, and velocity feed-forward all need to be dialled in. An axis driving a high-inertia load needs different gain settings to one driving a lightweight positioning stage. If the tuning isn’t right, the machine will feel sluggish, overshoot on positioning moves, or hunt under load — none of which builds confidence in the upgrade.
The mechanical system matters too. Backlash in worn gearboxes, flex in older couplings, and resonance in long ballscrews all interact with the servo loop. Sometimes a migration project reveals that the mechanical drivetrain needs attention alongside the electrical components.
Multi-Axis Systems and Network Architecture
Where multiple servo axes need to move in coordination, the communication network is as important as the drives themselves.
EtherCAT provides distributed clock synchronisation across the network, allowing multiple drives to execute motion profiles with sub-microsecond timing consistency. For applications like electronic gearing, coordinated pick-and-place, or flying shear, that level of synchronisation is what makes the difference between a machine that works and one that doesn’t.
CANopen is a well-proven alternative where the cycle time demands are less extreme — deterministic, standardised, and simpler to integrate in applications where raw speed isn’t the priority.
The network choice should be made at the architecture stage. Changing communication protocols after commissioning has started is an expensive exercise.
Three Things That Get Overlooked on Migration Projects
Drive parameter documentation. Before decommissioning the old drive, extract and document every parameter you can. Even if the new drive doesn’t use the same parameter set, having the original values is useful reference when tuning the replacement.
Cable screening and earthing. Encoder feedback cables need proper screening, connected at one end only to avoid ground loops. Interference on the feedback cable introduces noise into the position reading — the drive tries to correct it, which introduces vibration and instability that can look like a tuning problem when it isn’t.
Thermal management. Modern brushless servomotors running at high duty cycles in enclosed cabinets can run hot. Check the thermal derating figures for the new motor against the actual duty cycle of the application before signing off the specification.
When to Act
The honest answer is: before you’re forced to. A planned migration, with time to source components properly, test on a machine that isn’t in production, and commission carefully, produces a far better result than an emergency replacement driven by a production stoppage.
If you have legacy servo hardware that’s getting harder to support, a condition review now — looking at parts availability, feedback device condition, and drive health — gives you the information to make a planned decision rather than a reactive one.
Frequently Asked Questions
Can I replace a legacy SEM servo drive without replacing the motor? Sometimes — it depends on whether a modern drive can interface with the existing feedback device and motor winding characteristics. In some cases the motor remains serviceable and only the drive needs replacing; in others, a matched motor and drive replacement is the more reliable approach. A technical review of the existing installation is the right starting point.
What is the difference between EtherCAT and CANopen for servo motion control? EtherCAT offers very high-speed, deterministic communication with distributed clock synchronisation, suited to tight multi-axis coordination. CANopen is a well-established, standardised protocol better suited to applications where cycle time requirements are moderate and integration simplicity is a priority.
Why does servo drive tuning matter on a replacement project? A new drive installed on an existing axis won’t automatically replicate the performance of the old system. Control loop gains, current loop bandwidth, and velocity feed-forward all need to be configured for the specific motor and mechanical load — otherwise the machine may perform worse after the upgrade than before it.
How do I know if my legacy servo system is at risk? Key indicators include difficulty sourcing spare drives or encoder modules, no active manufacturer support for the drive model, loss of original commissioning documentation, and increasing frequency of unexplained faults. Any of these warrants a proactive review.
