How Do You Use Setpoints and Relays?

When designing systems for automation or maintenance monitoring, capturing high-accuracy force data with a load cell is only the first step. To act on that data, the sensor must be paired with instrumentation capable of processing the signal and triggering mechanical responses.

Interface’s Tech Talk breaks down the fundamentals of using setpoints and relays in force measurement programs and general testing. Understanding the distinct functions of setpoints and relays, and how they interact with sensor hardware, is critical for maintaining system safety, preventing overload, and achieving automation and testing objectives.

The Sensor Foundation from Force to Signal

Before an instrument can evaluate a setpoint, a strain gage load cell must convert physical force into an electrical process variable. When a load is applied to the sensor, the internal strain gages deform minutely, changing their electrical resistance. This change is measured as a millivolt-per-volt (mV/V) analog signal.

Because this raw sensor output is small, shielded cabling or wireless technologies route the signal to a paired instrument. The instrument provides a clean excitation voltage to power the sensor, amplifies the incoming mV/V analog signal, and converts it into measurement units such as pounds, kilograms, or Newtons. Once the data is digitized, the instrument can execute logic based on programmed parameters.

TIP: Review our post Raw Signals to Intelligent Force Sensing.

Mechanics of Setpoints

A setpoint is a digital or software-defined threshold value programmed into instrumentation that corresponds to a specific measurement target. The instrument continuously compares the live, processed load cell variable against this threshold to determine whether an action is needed based on predetermined criteria.

Configuration Styles

• High Setpoint activates when the force rises above the threshold. This is standard for structural overload protection, safety limits on cranes, or halting an assembly press before it damages a part.
• Low Setpoint activates when the force drops below the threshold. This is commonly deployed for slack-line detection in cable tensioning or identifying an empty material hopper.
• Band or Window Setpoint establishes an acceptable operating zone bounded by an upper and lower limit. The system tracks whether the active force remains within or exits this predefined window.

Hysteresis Boundaries

To prevent rapid, destructive cycling of external hardware when a load fluctuates minutely around the target threshold, instruments use a mathematical buffer called hysteresis. Hysteresis sets up a secondary reset point. For instance, if a high setpoint is set at 5000 pounds with a 50-pound hysteresis, the trigger activates exactly at 5000 pounds but will not deactivate until the force drops below 4950 pounds. This protects downstream electronics and mechanical switches from chatter caused by signal noise or physical vibration.

Mechanics of Relays

While the setpoint is the logical condition evaluated by the instrument’s mechanics or software, the relay is the physical or electronic switch that performs the mechanical work. When a setpoint condition is met, the instrumentation sends a signal to change the relay’s operational state, opening or closing an external electrical circuit. There are two basic relay technologies:

• Electromechanical Relays (EMR): These use an internal electromagnetic coil to slide mechanical contacts physically. EMRs provide excellent electrical isolation and handle high currents, making them ideal for switching heavy motors or large pneumatic valves. They have a fixed physical lifespan and switch on a millisecond time scale.
• Solid State Relays (SSR): These use semiconductor components to switch circuits electronically without moving parts. SSRs offer high-speed operation, long service lives, and excel in fast material testing or high-frequency batching environments.

Relays run in two primary states when unenergized:

• Normally Open (NO): The circuit stays disconnected until a setpoint condition forces the relay to close, completing the path to power an external device.
• Normally Closed (NC): The circuit passes current continuously until a setpoint condition forces the relay to open, breaking the path to shut down active machinery.

Instrumentation Integration and Selection

Pairing the correct instrumentation with your load cell ensures that setpoints are evaluated at the necessary speed and relays switch reliably under the application load. Interface provides a diverse set of instrumentation options for managing these requirements and control loops.

Interface 920i Programmable Weight Indicator and Controller

The Interface 920i Programmable Weight Indicator and Controller is designed for complex, multi-sensor industrial automation and batching. It supports multiple independent setpoints, allowing operators to program sequential ingredient drops, monitor weight-rate changes, and control physical relay outputs directly linked to industrial actuators. For a complete system, check out our ILMP 920i System.

Interface 480 Weight Indicator

For straightforward safety monitoring and limit tracking, the Interface 480 Bidirectional Weight Indicator provides robust setpoint configurations with standard digital outputs. Paired with a LowProfile strain gage load cell, the 480 monitors forces in real time and trips its internal relays to halt hydraulic or mechanical systems the instant an overload threshold is crossed.

Interface BSC4A Multi-Channel Signal Conditioner

In applications involving multi-axis load cells or multiple separate sensors, the BSC4A Multi-Channel Analog Output Bridge Amplifier conditions up to four channels simultaneously. It allows engineers to map custom setpoint logic across distinct axes, ensuring that an off-axis force anomaly instantly triggers a system-wide shutdown through its integrated digital outputs.

TIP: For additional information, watch our instrumentation webinar that highlights products and ideal pairings based on system requirements.

Instrumentation Selection Tips

Effectively implementing setpoints and relays requires matching your exact system requirements to the right instrumentation hardware. To streamline this engineering process, the Interface Instrumentation Selection Guide provides a structured framework to evaluate devices based on critical operational capabilities and functions.

When reviewing the Interface selection guides to find the ideal instrument for your sensor system, prioritize the following parameters:

• Output and control protocols to determine whether your system requires physical relay contacts (electromechanical or solid-state), standard digital outputs, or specific industrial Fieldbus protocols for direct communication with a PLC.
• The number of channels will determine whether the instrument needs to monitor a single load cell or synchronize inputs from multi-axis sensors, such as 2-, 3-, or 6-axis configurations.
• Speed and resolution are important considerations. High-speed material testing requires rapid analog-to-digital conversion rates to ensure that setpoints are triggered without system latency, whereas precision industrial weighing demands higher bit resolution.
• Programmability and software capabilities help evaluate whether the application requires advanced internal logic for automated batching sequences, which the 920i supports, or standard, streamlined limit tracking, which the 480 handles.
• Enclosure options ensure the physical instrument housing matches the operating environment, ranging from clean laboratory benchtop environments to harsh, contaminated industrial spaces requiring specific IP ratings.

By using the Interface guides, you can systematically review these technical variables to ensure that your load cells, cables, hardware accessories, and instrumentation operate as a reliable, safe, and highly accurate force measurement system. As a bonus, be sure to use our Instrumentation Cheat Sheet to find common terms, abbreviations, and references.