---
publication_id: "OT-2026-000000033"
title: "Dual Actuator Lift Table Synchronization System"
published_at: "2026-09-15T19:01:35.136963Z"
canonical_url: "https://opentechnical.org/p/OT-2026-000000033"
categories:
  - "lifting-positioning-manipulation/lift-tables-and-positioners/scissor-lifts"
  - "industrial-motion-power-transmission/actuators/hydraulic-actuators"
keywords:
  - "scissor lift"
  - "actuator synchronization"
  - "position feedback"
  - "hydraulic cylinder"
  - "electric linear actuator"
  - "control system"
  - "rack prevention"
  - "fault detection"
  - "position error"
  - "lift table"
---

# Dual Actuator Lift Table Synchronization System

## Abstract

This disclosure describes a system for synchronizing the vertical position of a lift table actuated by two independent linear actuators. The system employs position feedback from each actuator to a control unit that independently adjusts actuator commands. A primary objective is to prevent racking of the lift table scissor structure by maintaining a defined positional tolerance between the two sides. The system includes logic for detecting actuator seizure and limits the authority of correctional adjustments to prevent overcorrection or damage. Safe states are defined for conditions such as the loss of a feedback channel or actuator failure.

## 1. Technical Field

[0001] The present disclosure relates to lift tables, and more particularly to systems and methods for synchronizing the extension and retraction of multiple linear actuators employed in parallel to raise and lower a common platform.

## 2. Technical Context

[0002] Lift tables often utilize a scissor mechanism to achieve vertical motion. For wider or heavier platforms, it is common to employ two or more linear actuators operating in parallel to provide the necessary lifting force and stability. These actuators, frequently hydraulic cylinders or electromechanical screw jacks, are typically mounted on opposite sides of the scissor structure. A common issue arises when these independent actuators do not extend or retract at precisely the same rate. This differential movement can lead to a condition known as "racking," where one side of the lift table platform rises or lowers faster than the other. Racking induces torsional stresses on the scissor linkages, potentially leading to premature wear, structural deformation, or mechanical binding. Conventional solutions sometimes involve mechanical linkages, such as torsion bars or gears, to force synchronization, but these add complexity, cost, and potential points of failure, and may not be suitable for all applications. The problem addressed is the need for accurate and reliable synchronization of independent actuators without a direct mechanical tie, to prevent racking and ensure smooth, safe operation.

## 3. System Overview

[0003] The synchronization system operates by continuously monitoring the position of each independent linear actuator and making real-time adjustments to their commanded movements. As shown in FIG. 1, a central control unit (10) receives position feedback signals from a first position sensor (16) associated with a first actuator (12) and a second position sensor (18) associated with a second actuator (14). The control unit (10) compares these sensed positions against a target position and also against each other to calculate any positional deviation, referred to as an error. Based on these error values, the control unit (10) generates independent control signals for each actuator (12, 14). These control signals modulate the speed or direction of the actuators, for example, by controlling hydraulic flow through proportional valves from a hydraulic power unit (20) or by adjusting motor speed for electromechanical actuators. The system is designed to maintain the positional difference between the two actuators within a predefined tolerance. It also incorporates logic to detect fault conditions, such as a seized actuator or loss of a feedback signal, and to transition the system to a safe state.

## 4. Components

[0004] The system comprises several key components:

- **Control Unit (10):** This unit contains processing logic, memory, and input/output interfaces. It receives position data, executes the synchronization algorithm, calculates error signals, and outputs actuator command signals. It may be implemented using a programmable logic controller (PLC), a dedicated microcontroller, or an industrial PC. The control unit (10) also manages fault detection and safety protocols.
- **First Actuator (12) and Second Actuator (14):** These are independent linear actuators responsible for raising and lowering the lift table platform (32). They can be hydraulic cylinders, where fluid is supplied by a hydraulic power unit (20), or electromechanical linear actuators, such as ball screw or acme screw drives powered by electric motors. Each actuator is mechanically coupled to the scissor mechanism, as depicted in FIG. 2.
- **First Position Sensor (16) and Second Position Sensor (18):** Each actuator (12, 14) is equipped with a dedicated position sensor. These sensors provide continuous feedback on the absolute or relative extension of their respective actuators. Common sensor types include linear potentiometers, magnetostrictive transducers, LVDTs (Linear Variable Differential Transformers), or optical encoders integrated with the actuator.
- **Hydraulic Power Unit (20):** (If hydraulic actuators are used) This unit supplies pressurized hydraulic fluid to the actuators (12, 14). It typically includes a pump, reservoir, filter, and directional/proportional control valves. The control unit (10) interfaces with these valves to regulate fluid flow to each actuator independently.
- **Scissor Mechanism:** This mechanical linkage structure connects the base frame (22) to the lift table platform (32) and translates linear actuator motion into vertical platform movement. It consists of lower scissor arms (24, 28) and upper scissor arms (26, 30) arranged in a crisscross pattern, typically with pivot points and rollers.
- **Base Frame (22):** The stationary foundation upon which the scissor mechanism rests and from which the actuators extend.
- **Lift Table Platform (32):** The movable surface that is raised and lowered by the scissor mechanism.

## 5. Operation

[0006] The operation of the synchronization system proceeds through several phases:

[0007] **Start-up:**

- Upon system power-up, the control unit (10) performs a self-diagnostic check of its internal components and verifies communication with the position sensors (16, 18) and actuators (12, 14).
- The current positions of both actuators (12, 14) are read from their respective sensors (16, 18).
- If the lift table is at its fully retracted position, the control unit (10) may calibrate or home the position sensors to a known reference point.

[0009] **Steady Running (Movement):**

- When a command to move the lift table platform (32) is issued (e.g., raise or lower), the control unit (10) calculates the target position or velocity profile for the movement.
- The control unit (10) continuously reads the actual positions, P1 and P2, from the first position sensor (16) and second position sensor (18), respectively.
- It calculates the positional error for each actuator relative to the target trajectory.
- Crucially, it calculates the differential position error, ΔP = |P1 - P2|.
- If ΔP exceeds a predefined synchronization tolerance (e.g., 5 mm), the control unit (10) adjusts the command signals to the actuators. The actuator that is leading (further extended or retracted) is commanded to slow down or briefly pause, while the lagging actuator is commanded to maintain or increase its speed, until ΔP is brought back within tolerance.
- The authority of these correctional adjustments is limited to a maximum differential speed or a maximum allowed positional correction rate. For example, an actuator may be commanded to move at 10% faster or slower than its counterpart for correction, or its speed may be reduced by a maximum of 20% to allow the other to catch up. This limit prevents aggressive overcorrection that could induce oscillations or excessive mechanical stress.
- This feedback and adjustment loop operates at a control frequency, for example, between 10 Hz and 100 Hz, ensuring continuous synchronization.

[0011] **Fault or Interruption:**

- **Actuator Seizure Detection:** If one actuator (e.g., actuator (12)) is commanded to move but its corresponding position sensor (16) reports no significant change in position over a defined time interval, while the other actuator (14) is moving, the control unit (10) detects a potential seizure or blockage. Similarly, if an actuator is commanded to stop but continues to move, it indicates a control valve or motor issue.
- **Loss of One Channel:** If communication with one position sensor (e.g., sensor (16)) is lost, or if its reported data becomes erratic or falls outside expected ranges, the control unit (10) identifies this as a fault.
- **Safe State:** Upon detection of any critical fault (actuator seizure, loss of feedback, excessive differential position error that cannot be corrected within a timeout), the system transitions to a safe state. This typically involves:
- Immediately stopping all movement commands to both actuators (12, 14).
- Activating safety brakes (if equipped, especially for electromechanical actuators) or closing hydraulic lock valves to prevent further movement.
- Alerting an operator through an alarm or indicator.
- Logging the fault condition for diagnostic purposes.
- The system remains in this stopped, safe state until the fault is acknowledged and resolved, or until a specific manual override procedure is performed.

## 6. Example Embodiments

[0013] **Embodiment 1: Hydraulic Cylinder System with Proportional Valves** In this embodiment, the first actuator (12) and second actuator (14) are double-acting hydraulic cylinders. The hydraulic power unit (20) supplies pressurized fluid. The control unit (10) sends analog or digital commands to two independent proportional directional control valves, one for each cylinder. These valves precisely meter the flow of hydraulic fluid into and out of each cylinder, thereby controlling its extension or retraction speed. Position feedback is provided by magnetostrictive linear position sensors (16, 18) integrated within each cylinder rod. The control unit (10) implements a PID (Proportional-Integral-Derivative) control loop for each actuator, with an additional supervisory loop that adjusts the setpoints of the individual PID controllers to minimize the differential position error.

[0014] **Embodiment 2: Electromechanical Screw Jack System with Servo Drives** This embodiment utilizes two electromechanical linear actuators (12, 14), each comprising an electric motor driving a ball screw or acme screw mechanism. Each motor is controlled by a dedicated servo drive that receives commands from the control unit (10). Position feedback is provided by high-resolution rotary encoders (16, 18) mounted on the motor shafts or directly on the screw mechanisms. The control unit (10) sends velocity or position commands to each servo drive. The servo drives manage their respective motor's speed and position accuracy locally, while the control unit (10) monitors the absolute positions from the encoders and issues correctional speed adjustments to one or both servo drives to maintain synchronization.

[0015] **Embodiment 3: Hybrid Hydraulic/Electric System with Load Sensing** This configuration employs hydraulic cylinders (12, 14) but integrates load cells at critical points within the scissor mechanism or directly in line with the actuator forces. The control unit (10) not only monitors actuator positions via sensors (16, 18) but also receives load data. If a significant differential load is detected between the two sides, even if positional error is within tolerance, the control unit (10) can preemptively adjust actuator speeds to prevent future racking or to balance the load more effectively. This is particularly useful in applications where the load distribution on the platform (32) can be uneven. The hydraulic proportional valves are also equipped with pressure transducers, allowing the control unit to monitor actuator force output and detect binding or overpressure conditions.

## 7. Alternative Configurations

- **Master-Slave Control:** Instead of a fully distributed control logic, one actuator (e.g., actuator (12)) can be designated as the "master," operating according to the primary movement command. The other actuator (14) then becomes the "slave," whose position is continuously adjusted by the control unit (10) to precisely match that of the master actuator. This simplifies the control algorithm by focusing correction on a single actuator.
- **Decentralized Control with Peer-to-Peer Communication:** Each actuator could have its own local controller capable of reading its own position sensor and communicating with the other actuator's controller. A higher-level supervisor would provide overall movement commands, but the fine-tuning for synchronization could occur directly between the two local controllers, potentially reducing latency in correction.
- **Variable Speed Pump Control:** For hydraulic systems, instead of using proportional valves, the hydraulic power unit (20) could employ individual variable speed pumps for each actuator. The control unit (10) would then directly control the speed of each pump to regulate fluid flow and thus actuator speed. This can offer higher energy efficiency and finer control.

## 8. Parameter Variations

| Parameter | Realistic Range | Effect of Increasing Value | Effect of Decreasing Value |
| --- | --- | --- | --- |
| Synchronization Tolerance (ΔP) | 1 mm to 20 mm | Allows greater differential movement, potentially reducing control effort but increasing risk of racking stress. | Requires tighter synchronization, increasing control system activity and potentially increasing system instability if too small. |
| Maximum Correctional Authority | 5% to 50% differential speed | Allows for faster correction of large positional errors, but risks overshooting and oscillatory behavior. | Reduces the rate at which errors can be corrected, potentially leading to sustained positional drift during movement. |
| Control Loop Frequency | 10 Hz to 200 Hz | Provides more frequent position updates and quicker response to deviations, improving accuracy but increasing computational load. | Slower response to deviations, potentially leading to larger transient position errors before correction is applied. |
| Fault Detection Time Threshold | 0.1 s to 5 s | System reacts faster to actuator seizure or sensor failure, reducing potential damage but increasing false positive risk. | System takes longer to detect faults, potentially allowing more damage or unsafe operation before stopping. |
| Actuator Speed | 5 mm/s to 200 mm/s | Faster lift/lower cycles, but requires higher power output and more responsive control for synchronization. | Slower operation, potentially easier to synchronize but reduces throughput. |
| Position Sensor Resolution | 0.01 mm to 1 mm | Finer measurement allows for more precise control and smaller synchronization tolerances. | Coarser measurement limits achievable synchronization accuracy and may lead to dithering around the target. |

## 9. Additional Implementations

- **Pre-emptive Load Compensation:** In applications where the load distribution is known to change in a predictable manner (e.g., loading from one side), the control unit (10) can incorporate a pre-emptive adjustment to the actuator commands to anticipate and counteract the expected differential movement before it occurs, rather than solely reacting to positional errors.
- **Redundant Feedback Channels:** For enhanced reliability, each actuator (12, 14) could be equipped with two independent position sensors (e.g., primary sensor (16) and a secondary redundant sensor). The control unit (10) would continuously compare readings from both sensors for a given actuator. Discrepancies beyond a specified threshold would trigger a fault, providing robustness against single sensor failures.
- **External Reference Synchronization:** For very long travel or highly critical applications, an external, independent position reference system (e.g., laser distance sensor measuring platform height) could be used as a tertiary check. This reference could periodically recalibrate or fine-tune the synchronization of the two internal actuator position feedback loops.
- **Predictive Maintenance Integration:** The control unit (10) can monitor parameters such as the frequency and magnitude of correctional adjustments required for each actuator. A consistent increase in correction for one actuator over time, or a persistent offset in its commanded speed to maintain synchronization, could indicate impending mechanical wear or hydraulic degradation in that actuator, triggering a maintenance alert.

## 10. Figures

![FIG. 1](figures/fig-001.svg) A block diagram illustrating the main components and data flow for the lift table synchronization system.

![FIG. 2](figures/fig-002.svg) An elevation view of a scissor lift table showing the dual actuator arrangement.