From e340fac2e0fd994b00446ada74d64c3619cce254 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Wed, 25 Oct 2017 14:01:26 +1100
Subject: [PATCH] AP_Motors: avoid float exception for small expo

very small values of the thrust expo cause a floating point
exception. As zero is a documented value meaning linear we need to
handle this.
---
 libraries/AP_Motors/AP_MotorsMulticopter.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libraries/AP_Motors/AP_MotorsMulticopter.cpp b/libraries/AP_Motors/AP_MotorsMulticopter.cpp
index 090bc85a1a..0e22b1e38d 100644
--- a/libraries/AP_Motors/AP_MotorsMulticopter.cpp
+++ b/libraries/AP_Motors/AP_MotorsMulticopter.cpp
@@ -298,6 +298,10 @@ float AP_MotorsMulticopter::apply_thrust_curve_and_volt_scaling(float thrust) co
     float throttle_ratio = thrust;
     // apply thrust curve - domain 0.0 to 1.0, range 0.0 to 1.0
     float thrust_curve_expo = constrain_float(_thrust_curve_expo, -1.0f, 1.0f);
+    if (fabsf(thrust_curve_expo) < 0.001) {
+        // zero expo means linear, avoid floating point exception for small values
+        return thrust;
+    }
     if(!is_zero(_batt_voltage_filt.get())) {
         throttle_ratio = ((thrust_curve_expo-1.0f) + safe_sqrt((1.0f-thrust_curve_expo)*(1.0f-thrust_curve_expo) + 4.0f*thrust_curve_expo*_lift_max*thrust))/(2.0f*thrust_curve_expo*_batt_voltage_filt.get());
     } else {