Technical Insights > Benchtop

Troubleshoot a Sensor with a Digital Multimeter

2019-02-14  |  10 min read 

There are many types of sensors on the market – flow sensors, temperature sensors, pressure sensors, humidity sensors, accelerometer sensors, proximity sensors, and more. Most sensors generally provide electrical output signals as feedback to a display or control system. Figure 1 shows the many types of sensors that can be wired to a control system and display for specific applications.

Sensor wiring to control system
Figure 1. Sensor wiring to a control system and display

When a sensor fails, there are a few things that can happen; here is a snapshot:

  1. The sensor or control system is smart enough to detect a fault and provide a warning to the user. Some control systems have redundancies built-in to offer this feature.
  2. There is an intermittent failure, and the control system may or may not provide a warning. The application may be partially functional or non-functional in this scenario.
  3. There is a complete failure in the application, and the system does not know, and cannot provide feedback to the user.

The impact of the application failure depends on the critical nature of the feedback to the control system. If it is critical, normally redundancy or some form of intelligent sensor management needs to be implemented. There are intelligent sensors on the market that can self-test, self-validate, and self-adapt based on external environment.

No matter how intelligent the sensors or control systems are, when a sensor fails, someone has to troubleshoot what went wrong. In this blog post, we explain step-by-step how to troubleshoot a sensor with a digital multimeter (DMM). Here are the steps:

  • Prepare to troubleshoot the sensor
  • Assess considerations in troubleshooting the sensor
  • Determine how the sensor failed and the cause of the failure
  • Recommend a solution or corrective action

Prepare to troubleshoot the sensor

Troubleshooting a sensor measurement failure requires mechanical tools to uncover the protective shields or components so you can reach the sensor in question. For safety reasons, it is also important to wear appropriate personal protective equipment (PPE) such as insulated gloves, and safety glasses.

Next, you need an electrical probing instrument to validate the measurements detected by the control system. Since most sensors have electrical outputs, a digital multimeter (DMM) is typically the measurement tool of choice. It can measure voltage, current, resistance, capacitance, diode, temperature, and more.

Considerations for troubleshooting the sensor

There are three parts to the sensor path to isolate the measurement problem as shown in Figure 2.

Sensor path troubleshooting
Figure 2. Sensor path troubleshooting

The first part consists of the wiring and the connections between the sensor and the control system. You can use your DMM to perform a continuity or resistance check on the wiring. In some cases, the internal part of the cable may have ruptured or worn out insulation due to movements, water trapped inside, bending, or simply prolonged exposure to sunlight and rain. Be sure to check the wire connection interface because it is possible to get microcracks on solders or loose screws that create resistance or intermittent interface.

Next, either go straight to validate your sensor’s measurement or, if possible, check to confirm your control system is in proper order. Ensure your control system’s setup is correct and that proper calibration is in place and capable of measuring similar sensors. If the wiring, connections, and control system are in proper order, you have isolated the problem to the sensor. Troubleshooting the sensor depends on its output.

Sensors that output voltage

There are many types of sensors that output voltage. It is the simplest form of input for control systems. Some sensors are called transducers which have built-in signal conditioning circuits to provide a linear analog output or even digital output for the control systems to process easily.

Use a digital multimeter (DMM) to validate sensor output corresponding to physical inputs, whether it is light intensity, rotational speed, or humidity, then check the sensor’s characteristics on the data sheet.

Sensors that output current

There is a large installed base of 4-20 mA current loop systems throughout the world, including the well-established Highway Addressable Remote Transducer (HART) protocol sensors. There are two ways to measure the output current from HART sensors. The first method is to use a DMM current probe to measure the current. This is a good, non-intrusive method of measurement. The other method is to use a shunt resistor across the output of the transducer/sensor as represented in Figure 3.

A shunt resistor on HART current output sensors
Figure 3. A shunt resistor on HART current output sensors

Diode sensors (leaky sensors)

Diode sensors are used in many applications such as sensing light intensity, sensing object proximity, or radio frequency/microwave power. Some diode sensors are DC-biased, and some are not. DMMs typically have a test function called “diode test” where it injects current and measure the voltage across the diode under test. By changing the polarity during the diode test, you can troubleshoot the basic function of the diode sensor. If the diode’s P-N (Positive-Negative) junction is damaged, it may be electrically opened or shorted. That can be detected by the DMM.

Resistive/capacitive sensors

These sensors are used to measure many physical parameters such as position and displacement, force, human interface devices, and more. Temperature sensors such as thermistors and resistance temperature detector (RTD) are resistive sensors. These sensors can be damaged due to abuse, electrical overstress, or through normal wear and tear. DMMs can measure resistance accurately with wide enough range Ω to GΩ.

Some DMMs also have the capability to measure capacitance which can help troubleshoot a wide variety of capacitive sensors.

Temperature sensors

Digital multimeters (DMMs) with built-in signal conditioning hardware measure various types of temperature sensors such as thermocouple, resistance temperature detector (RTD), and thermistor. You can easily disconnect your temperature sensor from the control system and validate the temperature sensor directly with your DMM. Identify the sensor type and set it up accordingly with your DMM.

DMM temperature setting screen
Figure 4. Keysight’s 34470A DMM temperature probe selection setting

Determine how sensors fail and what causes the sensors to fail

When performing failure analysis, especially for critical sensors, you need to determine how sensor fail and the potential causes of sensor failure. This will help improve your overall system over time.

Here are some examples of how the sensor fails and its potential causes.

How the sensor fails

Potential causes of failure

Catastrophic failure
- no measurement or completely out of range

- a short or open circuit in the sensor or the wiring
- sensor failure

Offset error
- measurements are stable, but readings are slightly above or below the expected value

- poor wiring connections
 - ground loops
- solution contamination on sensor or wire

Unstable readings
- erratic measurements at times

- water damage to wire or sensor
- external or non-representative interference
- ground loops

Figure 5. Examples of how sensor fails and potential causes (source: https://www.arguscontrols.com/resources/Sensor-Troubleshooting.pdf)

Recommend solutions, identify corrective actions

It is important to analyze the causes of sensor failures and provide recommended solutions. Record corrective actions that you take to overcome sensor failures. Over time, you can analyze the effectiveness of these actions and create a list of best practices.

Summary

This blog post explains how to troubleshoot a sensor with a digital multimeter (DMM). It also details actions you can take before troubleshooting a sensor and suggests noting corrective actions to ensure continuous improvement.

To find out more about Keysight’s 34460A and 34470A Truevolt Series digital multimeters (DMM), please visit our website at www.keysight.com/find/truevolt.

To learn more, read 10 Things You Must Know Before Buying Your Next Benchtop Digital Multimeter.