The actuator either traverses the required 5.5-inch move too quickly or cannot complete it at the slow setting; the target average speed is about 0.057 to 0.229 inches per second, not simply a matter of choosing a 12-inch actuator.
What travel time and speed must the lift deliver?
Convert the requested distance and time into average velocity before selecting a motor or changing a controller. For a 5.5-inch move, average speed is distance divided by elapsed time:
- 5.5 in ÷ 24 s = 0.229 in/s.
- 5.5 in ÷ 96 s = 0.057 in/s.
Those results closely match the stated approximate range of 0.05 to 0.25 inches per second. Treat them as average move speeds unless the process specifically defines the required velocity at every point in travel. A 12-inch actuator stroke is the available range; the stated timed move uses only 5.5 inches. Confirm that the remaining travel is intentional and that both endpoints are within the actuator's usable mechanical stroke.
A commanded move generally accelerates, travels, then decelerates. If it must cover 5.5 inches in a given time, the average speed is fixed by that time, but the cruise speed must be higher than the average whenever the profile includes nonzero acceleration and deceleration. The controller's profile and actual measured travel time—not a nominal motor speed—determine whether the target is met.
Why does changing supply voltage fail to regulate speed?
A variable-voltage supply changes motor voltage, but that does not directly regulate shaft speed. A DC motor's speed changes with load as well as applied voltage. In a vertical lift, gravity creates a persistent load, so a voltage setting that produces one speed unloaded may produce a different speed with the payload. Near the low end of the speed range, available torque and smooth motion can also become limiting.
PWM varies the fraction of time the motor receives supply voltage. It can provide useful low-speed control for some DC motor setups, but duty cycle alone is still an open-loop command: it does not measure actual RPM or correct speed error as load changes. A regulated supply may compensate for voltage droop, but voltage regulation is not the same as motor-speed regulation. For repeatable speed under changing load, use motor-speed feedback, such as an encoder or tachometer, with a controller that uses feedback to adjust drive output.
The alternative discussed for this motion is a stepper motor, lead screw, and programmable indexer/drive. The indexer can command acceleration, travel speed, and deceleration as a single profile. That architecture can generate a continuous move without a commanded pause, but the motor and screw still must be sized for the vertical load and required acceleration. A stepper command without feedback does not prove that the motor actually followed the commanded motion.
Which motion architecture matches the control requirement?
| Requirement | Suitable control approach | What it does not prove |
|---|---|---|
| Repeat the move in a specified time despite load variation | Closed-loop speed control using motor feedback | Endpoint position, unless position is also measured or controlled |
| Move between defined endpoints with a programmed ramp and travel speed | Motion controller or indexer executing a motion profile | That an open-loop motor did not stall or lose steps |
| Confirm arrival at one endpoint | Endpoint proximity switch or other discrete position sensor | Velocity regulation during the move or precise position throughout travel |
Choose the control objective explicitly. If process quality depends on velocity throughout the move, specify speed control and define acceptable speed variation under load. If only arrival time and endpoint matter, a timed profile plus endpoint sensing may be sufficient, but verify the process tolerates its changing velocity during acceleration and deceleration. A proximity switch at the destination closes an endpoint-detection loop; it does not close the motor's velocity loop.
When both a motor, drive, and indexer are selected, using components designed to work together can simplify configuration and support. Evaluate software usability, availability of replacement parts, documentation, wiring conventions, and maintainability as part of the design, rather than choosing solely on initial cost.
How should the motor, screw, and drive be sized?
Start with the actual moving mass and the actuator's mechanical arrangement. The approximate 100-pound load is not enough by itself to select a motor. Determine whether that figure includes the moving carriage and fixtures, and obtain the actuator's load, friction, screw-pitch, and efficiency data. For a vertical axis, account for gravity throughout the travel and for the acceleration required by the selected profile.
- Confirm the maximum moving load and the direction of travel that requires the greatest motor effort.
- Use the screw lead and mechanical efficiency to calculate the motor torque needed to raise the load at the required speed; add the torque needed for acceleration and account for friction using actuator data.
- Check required peak torque against the motor's torque-versus-speed curve at the operating speed. Do not size from holding torque alone.
- Confirm that the drive and motor can operate smoothly across the required low-speed range and that their continuous and peak ratings suit the actual motion duty.
- Check how the vertical axis behaves when power is removed. Select a load-retention method appropriate to the actuator and risk; do not assume the screw or motor will hold the load safely without power.
If a supplier provides force rather than mass, use the supplier's units and mechanical data consistently. The approximate load description does not establish actuator force capacity, torque, or a safe holding method. Read those values from the selected actuator and motor documentation before commissioning.
How should the continuous motion profile be configured?
Program one move covering the requested 5.5-inch distance, with acceleration, travel, and deceleration segments and no dwell between segments. A continuous profile may change speed during the move while remaining one uninterrupted motion; if the process requires nearly constant velocity, keep ramps short relative to the move where the mechanics and drive permit, then validate actual motion. Do not use a pause or stop as a substitute for low-speed control when the process prohibits one.
- Enter the required distance and target elapsed time for each operating condition.
- Set acceleration and deceleration so the axis can reach and leave its travel speed without exceeding the motor's torque-speed capability.
- Set travel speed above the calculated average when ramp time consumes part of the move; tune the profile using measured end-to-end time and speed feedback where required.
- Set the drive's speed loop, if provided, to use encoder or tachometer feedback. A PWM duty command or voltage command without speed feedback remains open loop.
- Configure endpoint sensing and travel limits separately from speed regulation. Confirm the sensor state and controller response before allowing production motion.
Use the stepper/indexer approach only after confirming its profile supports the required timing and motion behavior, and that the selected motor/screw combination can meet torque demand throughout the move. If velocity control is the key requirement, make the control loop's feedback source part of the specification rather than assuming that a position switch supplies it.
What symptoms distinguish a speed-control fault from a position fault?
| Observed behavior | Likely issue to investigate | Check |
|---|---|---|
| Travel time changes when load changes | Open-loop voltage or PWM command; insufficient torque reserve | Compare actual speed under load and inspect the motor torque-speed data |
| Motor command runs, but the lift stalls or moves unevenly at the slow setting | Insufficient low-speed torque, excessive friction, or unsuitable drive/motor pairing | Observe motor feedback and inspect actuator load and mechanical data |
| Lift reaches the endpoint, but the move time varies | Position sensing is working, but velocity is not regulated | Measure elapsed time and determine whether the controller has motor-speed feedback |
| Controller reports arrival but the axis is not at the intended point | Endpoint sensor placement, wiring, or interpretation fault | Check the sensor state at the physical endpoint and compare it with controller input status |
| Commanded stepper move differs from actual travel | Lost steps or mechanical mismatch in an open-loop setup | Compare commanded movement with measured position; add feedback if position confirmation is required |
Separate the two questions during diagnosis: did the motor maintain the required speed, and did the mechanism reach the required position? A single endpoint prox answers only the second question at that location. It cannot show whether the lift moved at the specified velocity between endpoints.
How do you prove the lift meets its operating requirement?
Commission with the actual payload and the final mechanical arrangement. Record commanded settings alongside observed travel time, speed behavior, and endpoint state; repeat at both ends of the requested time range. Test the slowest condition carefully because inadequate torque or friction effects are most visible at low speed. Confirm that the faster move does not exceed the available acceleration and deceleration capability.
Before releasing the axis for normal use, verify that the lift completes the full commanded 5.5-inch travel without a stop or dwell, reaches the intended endpoint, and meets the selected 24- to 96-second travel-time requirement under load. Confirm the motor feedback reports speed if closed-loop speed regulation is required, and confirm the load-retention behavior when drive power is removed. Correct discrepancies at the responsible layer: the speed command/profile, motor feedback and drive, actuator mechanics, or endpoint sensor.
FAQ
Why does my linear actuator move too fast at low voltage?
Voltage sets a motor operating condition, not a guaranteed speed. Load and friction affect actual speed; use feedback-based speed regulation if the travel time must remain repeatable under load.
Why does PWM not hold a constant actuator speed?
PWM changes duty cycle, but an open-loop PWM command does not measure RPM or correct load-related speed changes. A motor encoder or tachometer and a feedback controller provide speed regulation.
Can a prox switch close the speed-control loop?
No. A prox at the endpoint confirms a discrete position condition there; it does not measure velocity through the move. Use motor-speed feedback for closed-loop velocity control.
What speed is needed to move 5.5 inches in 24 seconds?
The average speed is 5.5 ÷ 24 = about 0.229 inches per second. The required travel speed will be higher if acceleration and deceleration take a meaningful part of the move.
Why can a stepper miss the commanded lift position?
An open-loop stepper system commands steps without confirming that the motor followed them. Verify actual position under load or use position feedback when missed steps cannot be tolerated.
Final verification: Run the 5.5-inch move with the actual load at each required timing, confirm continuous motion and correct endpoint detection, and verify the configured motor feedback and power-off load retention before returning the lift to service.