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3.45 TouchSensor
Derived from Solid.
TouchSensor { |
3.45.1 Description
A TouchSensor node is used to model a bumper or a force sensor. The TouchSensor comes in three different types. The "bumper" type simply detects collisions and returns a boolean status. The "force" type measures the force exerted on the sensor's body on one axis (z-axis). The "force-3d" type measures the 3d force vector exerted by external object on the sensor's body.
Examples of using the TouchSensor are provided by the hoap2_sumo.wbt and hoap2_walk.wbt worlds (located in the projects/robots/hoap2/worlds directory of Webots) and by the force_sensor.wbt and bumper.wbt worlds (located in the projects/samples/devices/worlds directory of Webots).
3.45.2 Field Summary
-
type: allows the user to select the type of sensor: "bumper", "force", or "force-3d", described below.
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lookupTable: similar to the one used by the DistanceSensor node.
3.45.3 Description
"bumper" Sensors
A "bumper" TouchSensor returns a boolean value that indicates whether or not there is a collision with another object. More precisely, it returns 1.0 if a collision is detected and 0.0 otherwise. A collision is detected when the boundingObject of the TouchSensor intersects the boundingObject of any other Solid object. The lookupTable field of a "bumper" sensor is ignored. The Physics node of a "bumper" sensor is not required.
"force" Sensors
A "force" TouchSensor computes the (scalar) amount of force currently exerted on the sensor's body along the z-axis. The sensor uses this equation: r=|f|*|z|*cos(alpha), where r is the return value, f is the cumulative force currently exerted on the sensor's body, z is the sensor's z-axis and alpha is the angle between f and z. So the "force" sensor returns the projection of the force on its z-axis; a force perpendicular to the z-axis yields zero. For this reason, a "force" sensor must be oriented such that its positive z-axis points outside of the robot, in the direction where the force needs to me measured. For example if the TouchSensor is used as foot sensor then the z-axis should be oriented downwards. The scalar force value must be read using the wb_touch_sensor_get_value() function.
"force-3d" Sensors
A "force-3d" TouchSensor returns a 3d-vector that represents the cumulative force currently applied to its body. This 3d-vector is expressed in the coordinate system of the TouchSensor. The length of the vector reflects the magnitude of the force. The force vector must be read using the wb_touch_sensor_get_values() function.
| "bumper" | "force" | "force-3d" | |
| boundingObject | required | required | required |
| Physics node | not required | required | required |
| lookupTable | ignored | used | used |
| return value | 0 or 1 | scalar | vector |
| API function | wb_touch_sensor_get_value() | wb_touch_sensor_get_value() | wb_touch_sensor_get_values() |
Table 3.6: TouchSensor types
Lookup Table
A "force" and "force-3d" sensors can optionally specify a lookupTable to simulate the possible non-linearity (and saturation) of the real device. The lookupTable allows the user to map the simulated force measured in N (Newtons) to an output value that will be returned by the wb_touch_sensor_get_value() function. The value returned by the force sensor is first computed by the ODE physics engine, then interpolated using the lookupTable, and finally noise is added (if specified in the lookupTable). Each line of the lookupTable contains three numbers: (1) an input force in Newtons, (2) the corresponding output value, and (3) a noise level between 0.0 and 1.0 (see DistanceSensor for more info). Note that the default lookupTable of the TouchSensor node is:
[ 0 0 0 |
Collision detection
TouchSensors detects collisions based on the 3D geometry of its boundingObject. So the boundingObject must be specified for every type of TouchSensor. Because the actual 3d intersection of the sensors boundingObjects with other boundingObjects is used in the calculation, it is very important that the sensors' boundingObjects are correctly positioned; they should be able to collide other objects, otherwise they would be ineffective. For that reason, the boundingObjects of TouchSensors should always extend beyond the other boundingObjects of the robot in the area where the collision is expected.
For example, let's assume that you want to add a TouchSensor under the foot of a humanoid robot. In this case, it is critical that the boundingObject of this sensor (and not any other boundingObject of the robot) makes the actual contact with the floor. Therefore, it is necessary that the sensor's boundingObject extend below any other boundingObject of the robot (e.g., foot, ankle, etc.).
Coordinate System
It is easy to check the orientation of the coordinate system of a TouchSensor: if you select the TouchSensor object in the Scene Tree, then only the bounding object of this TouchSensor should be shown in the 3d window. If you zoom in on this bounding object, you should see the red/green/blue depiction of the TouchSensor's coordinate system (the color coding is: x/y/z = red/green/blue). For a "force" sensor, the blue (z) component should point in the direction where the collision is expected.
Accuracy
The force measured by the ODE physics engine is only a rough approximation of real physical force. Usually the approximation of reality improves as the basicTimeStep (WorldInfo node) decreases.
3.45.4 TouchSensor Functions
NAME
wb_touch_sensor_enable, wb_touch_sensor_disable - enable and disable touch sensor measurementsSYNOPSIS [C++] [Java] [Python] [Matlab]
#include <webots/touch_sensor.h>void wb_touch_sensor_enable(WbDeviceTag tag, int ms);
void wb_touch_sensor_disable(WbDeviceTag tag);
DESCRIPTION
wb_touch_sensor_enable() allows the user to enable a touch sensor measurement every ms milliseconds.
wb_touch_sensor_disable() turns the touch sensor off, saving computation time.
NAME
wb_touch_sensor_get_value, wb_touch_sensor_get_values - get the latest touch sensor measurementSYNOPSIS [C++] [Java] [Python] [Matlab]
#include <webots/touch_sensor.h>double wb_touch_sensor_get_value(WbDeviceTag tag);
const double *wb_touch_sensor_get_values(WbDeviceTag tag);
DESCRIPTION
wb_touch_sensor_get_value() returns the last value measured by a "bumper" or "force" TouchSensor. This function can be used with a sensor of type "bumper" or "force". For a "force" sensor, the value may be altered by an optional lookup table. For a "bumper" sensor, the value can be 0.0 or 1.0.
wb_touch_sensor_get_values() returns the last force vector measured by a "force-3d" TouchSensor. This function can be used with a sensor of type "force-3d" exclusively.
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