Exteroceptive and proprioceptive sensors for robotics
Proprioception refers to the perception, conscious or not, of the position of the different parts of the body. Exteroception, as opposed to proprioception, groups together the sensations caused by external stimuli (sight, smell, hearing, etc.). It is these phenomena that enable humans to move around in an environment and to be aware of their location.
What are the different types of sensors?
According to the same principle, mobile robots (i.e. capable of locomotion, as opposed to manipulative robots) carry different types of proprioceptive and exteroceptive sensors to perform different functions.
In order to differentiate between these two categories of sensors, it is first necessary to define the general characteristics that are important for their use in mobile robotics.
All sensors are characterised by an acquisition frequency (response time), a measurement resolution, a noise on the measured physical quantity (defined by its repeatability) … Also, the measurement principle defines whether the sensor performs an absolute measurement (always with respect to a known reference frame) or a relative one.
The above characteristics are generally associated with the intrinsic measurement principle of each sensor and/or the acceptable cost of these sensors. For example, there are different technologies for distance measurement sensors (laser, ultrasound, etc.) with different properties (resolution, reflectivity, etc.) and different costs.
Proprioceptive and exteroceptive sensors
Proprioceptive sensors measure the state of the robot itself (wheel position or speed, battery charge, etc.) while exteroceptive sensors measure the state of the environment (mapping, temperature, etc.).
From the roboticist’s point of view, exteroceptive sensors are mainly absolute measurement sensors, which generally have a lower acquisition frequency than proprioceptive sensors. The best known example is probably the GPS.
However, this rule is not a general truth but may also depend on the use of the sensor.
The exteroceptive and proprioceptive sensors in practice
Let’s take the case of the accelerometer (measurement of an acceleration); it can be defined as exteroceptive if it measures the acceleration due to gravity, while it will be defined as proprioceptive if it measures the acceleration of a robot.
The difference between absolute and relative measurements is fundamental for a robot. Absolute measurements have a fixed and bounded measurement uncertainty (often larger than that of relative sensors) whereas relative measurements (often with small measurement uncertainties) are used cumulatively, as in odometry. As a result, relative sensors sooner or later end up providing information that drifts in an increasing and unbounded manner.
One solution to this problem is to combine these different types of sensors (this is called sensor fusion) in order to obtain the most reliable (with a limited uncertainty) and accurate data possible. This objective can be illustrated by the use of inertial measurement units (IMUs).
Example of IMU
This is usually a combination of accelerometers, gyrometers (relative, proprioceptive, measuring angular velocity) and a magnetometer (absolute, exteroceptive, measuring angle to magnetic north). The magnetometer (also called magnetic compass) compensates for the drift of the gyrometer (along the yaw axis) but is not able to detect angular movements as finely as the gyrometer. Accelerometers, on the other hand, compensate for gyro drift along the pitch and roll axes by deducing the direction of the earth’s gravity (absolute measurement). In this case, the fusion of proprioceptive and exteroceptive sensors makes it possible to obtain very frequent, stable and precise orientation data.
External and proprioceptive sensors at INNOWTECH
Our robot-sensors evolve in constraining environments thanks to a judicious fusion of proprioceptive and exteroceptive sensors, combining the advantages of each and allowing to answer the constraints of severe environments and the metrological needs (related to the precision of measurement).
Given the constraints of harsh environments, some sensors are prohibited (e.g.: impossibility of using a GPS inside a nuclear power plant). Each case must therefore be studied to define the appropriate combination of sensors in terms of desired accuracy, use in a constrained environment, and cost.